Sedimentology of a “nonactualistic” Middle Ordovician tidal-influenced reservoir in the Murzuq Basin (Libya)

ABSTRACT

The subsurface of the highly productive Murzuq Basin in southwest Libya remains poorly understood. As a consequence, a need exists for detailed sedimentological studies of both the oil-prone Mamuniyat Formation and Hawaz Formation reservoirs in this area. Of particular interest in this case is the Middle Ordovician Hawaz Formation, interpreted as an excellent example of a “nonactualistic,” tidally influenced clastic reservoir that appears to extend hundreds of kilometers across much of the North African or Saharan craton. The Hawaz Formation comprises 15 characteristic lithofacies grouped into 7 correlatable facies associations distributed in broad and laterally extensive facies belts deposited in a shallow marine, intertidal to subtidal environment. Three main depositional sequences and their respective systems tracts have also been identified. On this basis, a genetic-based stratigraphic zonation scheme has been proposed as a tool to improve subsurface management of this reservoir unit. A nonactualistic sedimentary model is proposed in this work with new ideas presented for marginal to shallow marine depositional environments during the Middle Ordovician in the northern margin of Gondwana.

INTRODUCTION

For many years, the main Libyan petroleum province was the prolific Sirte Basin with a limited contribution from the Ghadames Basin (Berkine Basin in Algeria) (Hallet, 2002; Figure 1). However, since the mid-1990s, the Murzuq Basin has developed into a major oil- and gas-producing province. The Hawaz Formation constitutes one of the most important reservoirs in several producing fields in the central and northern part of the basin. The generally high reservoir quality (average 5%–15% porosity and 0.1–150 md permeability) and lateral continuity, characteristic of the Hawaz, are key factors in the development and production of these accumulations. However, despite the well-documented potential of the Hawaz Formation, its subsurface character remains poorly understood.

Figure 1. Geological map of Libya showing the main sedimentary basins. The Murzuq Basin is bounded by the Atshan arch to the northwest, the Gargaf high to the north, the Tihemboka high to the southwest and the Tibesti high to the southeast. The area of interest represented in Figure 3A is highlighted in the red box. Modified from M. Marzo and E. Ramos (2003, personal communication).

To date, only a few sedimentological studies of this formation have been carried out, and all are exclusively based on surface geology (Vos, 1981; Anfray and Rubino, 2003; M. Marzo and E. Ramos, 2003, personal communication; Ramos et al., 2006; Gibert et al., 2011). Other published works have focused on diagenesis (Abouessa and Morad, 2009; Abouessa, 2012) and trapping mechanisms (Franco et al., 2012). In addition, subsurface interpretations of the formation are based on inconsistent lithostratigraphic correlations unconstrained by a consistent sequence stratigraphic framework. As such, no genetic or sequence stratigraphy–based zonation exists. This limited database highlights the necessity of providing a sequence stratigraphic framework based on a robust sedimentological model of the transitional to shallow marine Hawaz Formation.

As Dalrymple and Choi (2007) have highlighted, transitional tide-dominated and deltaic facies reflect the interaction of numerous terrestrial and marine processes in a very complex depositional environment. Any paleoenvironmental or stratigraphic interpretation of such transition zone successions requires a comprehensive understanding of the facies and facies associations. Hence, a comprehensive understanding of the facies changes through this transition zone is necessary to make proper paleoenvironmental and sequence-stratigraphic interpretations of the sedimentary successions. However, is it actually possible to compare these paleoenvironments with any “actualistic” sedimentary model?

The limitations of the approach become apparent when the uniformitarian principle is extended to depositional environments in the most ancient geological record. In particular, the assumption that modern environments can provide analogs for all geological successions must be questioned (Nichols, 2017). It is broadly accepted that Earth’s dynamics have changed considerably throughout geological history, and accordingly, factors controlling sedimentation have changed also, such as a lack of flora stabilizing river banks, greenhouse versus icehouse periods defining coastal geomorphology, tidal ranges controlling facies belts or characteristic ichnofacies during a particular period of geological time. The analysis of some of these factors suggests that the facies succession of the Hawaz Formation reflects different depositional processes from those observed in modern environments. From this point forward, we will use the term “nonactualistic” to describe those processes affecting the geological signature of the Hawaz Formation that are difficult to compare with any modern depositional environment analog.

Consequently, the main aim of this paper is to present a sedimentological characterization of the Hawaz Formation based on a detailed lithofacies description and interpretation together with the development of a facies association classification. This forms the basis for an appropriate depositional model in accordance with plausible physical and chemical processes during the Middle Ordovician. In addition, the overall analysis aims to build a genetically based zonation through sequence stratigraphy that will improve reservoir management and provide tools for maximizing hydrocarbon recovery efficiency. Finally, it is intended that these sedimentological and stratigraphic models should be a well-documented subsurface analog for clastic reservoirs in similar settings.

GEOLOGICAL SETTING

The Structure and Stratigraphy of the Murzuq Basin

The Paleozoic succession of the Murzuq Basin is an erosional remnant of a much more extensive regional succession extending along the northern margin of the Gondwana supercontinent (Davidson et al., 2000; Shalbak, 2015). Its present extent reflects several periods of uplift and unroofing during the late Paleozoic, Mesozoic, and Cenozoic, which, together, are responsible for its modern architecture. As a consequence, the present-day basin geometry bears little relation to the broader and larger preexisting sedimentary basin. The current basin is composed of a central Cretaceous depression bounded to the northwest by the Atshan arch, the Gargaf high to the north, and the Tibesti and Tihemboka highs on the southeast and southwest, respectively (Figure 1). These structural highs were formed by multiphase tectonic uplifts from the middle Paleozoic to Cenozoic, although the main periods of uplift and erosion occurred during the Pennsylvanian (late Carboniferous; Hercynian) and early Cenozoic (Alpine) orogenic cycles.

A series of geological events can be recognized in the stratigraphic record of the Murzuq Basin, some represented by basin-scale unconformities within the sedimentary infill reflecting the Pan-African, Caledonian, and Hercynian orogenesis and the short Late Ordovician glacial event responsible for the Taconic or basal glacial erosional surface (Figure 2). Other unconformities that may be recognized within the sedimentary record are minor or belong to the younger Austrian or Alpine cycles, and consequently, they do not strongly affect the Paleozoic section directly in the central Murzuq Basin; although, they may have had strong implications in terms of overburden removal, source rock maturity, and reservoir quality because of uplift and unroofing of Mesozoic series on the Paleozoic section (Boote et al., 2008).

Figure 2. (A) Stratigraphic chart summarizing the stratigraphic column for the Murzuq Basin highlighting the main stratigraphic units and major basin-scale unconformities. 1 = Cambrian–Ordovician; 2 = Silurian (Silur.); 3 = Devonian (Dev.)–Carboniferous (Carbonif.); 4 = Mesozoic. (B) Wheeler diagram showing lithostratigraphic to chronostratigraphic relationships of the Ordovician and Lower Silurian succession in the area of study. (C) Seismic line showing the typical geomorphological signature of the Ordovician succession in form of paleo-highs (“buried hills”) and paleovalleys. The main petroleum systems elements are also represented in Figure 2A, B. Cretac. = Cretaceous; Dev. = Devonian; Fm. = Formation; Pale-Neo = Paleogene–Neogene; Perm. = Permian; Q = Quaternary.

The maximum sedimentary thickness in the present-day Murzuq Basin is approximately 4000 m (∼13,000 ft). Despite successive erosive episodes during several phases of uplift throughout the history of the basin, the maximum sedimentary thickness most probably never exceeded 5000 m (16,400 ft) (Davidson et al., 2000). The age of the infill ranges from Cambrian to Cretaceous, commonly covered by large Quaternary sand dunes in the central part of the basin. The sedimentary infill can be subdivided into four main units: (1) Cambrian–Ordovician, (2) Silurian, (3) Devonian–Carboniferous, and (4) Mesozoic (Figure 2).

The lower Paleozoic succession comprises the terrigenous Cambrian–Ordovician Gargaf Group consisting of at least five formations; from bottom to top, they are the following: Hasawnah, Ash Shabiyat, Hawaz, Melaz Shuqran, and Mamuniyat Formations (Figure 2). The lowermost Hasawnah Formation rests unconformably on the Precambrian basement and is composed of Cambrian to Lower Ordovician conglomeratic to sandy continental and shallow marine littoral deposits. The Hasawnah Formation is overlain, above a transgressive surface of erosion, by the shallow marine and preglacial Ash Shabiyat and Hawaz Formations, attributed respectively to the Lower and Middle Ordovician (Tremadocian–Sandbian). The Upper Ordovician succession, associated with a major glaciation, principally comprises the Melaz Shuqran and Mamuniyat Formations, locally overlain by a thin and somewhat enigmatic package known as the Bir Tlacsin. The former is most probably lower Hirnantian and predominantly mud-prone, representing the period of the highest relative sea level during the Late Ordovician (McDougall and Martin, 2000), whereas the Mamuniyat Formation is a major Hirnantian sand-prone package.

The Petroleum Systems and the Hydrocarbon Production History of the Murzuq Basin

Early exploration in the Murzuq Basin focused upon surface structures. The first exploratory well was drilled in the northern Murzuq in 1955–1956. Subsequently, several successful discoveries in the neighboring Illizi Basin (southeastern Algeria) encouraged further exploration across the border. Three years later, Exxon discovered gas at the Atshan region and Gulf tested oil at low rates from Ordovician sandstones. However, in 1958, industry attention shifted east with the discovery of a major oil accumulation in the Sirte Rift province, and there was little further exploration of the Murzuq Basin for the next 20 yr. During the late 1980s to 1990s, Rompetrol, and later Repsol, drilled up to 57 exploratory wells in the basin, all of which targeted Ordovician prospects. This exploratory activity resulted in many significant oil discoveries, highlighting the rapidly growing potential of the basin.

The most recent hydrocarbons-in-place estimation for the Murzuq Basin is approximately 6 billion bbl (∼9.5 × 108 m3) of oil and approximately 35 tcf (∼1 trillion m3) of gas, which represent approximately 6.5% of Libya’s resources and 30% of Libya’s current oil production (Shalbak, 2015).

The main petroleum system in the Murzuq Basin comprises a basal Silurian (Tanezzuft) hot-shale source rock, Ordovician sandstone reservoirs, and a thick Tanezzuft shale seal (Figure 2). A secondary petroleum system in the basin (noncommercial to date) is composed of the basal Devonian sandstones as reservoirs and the intra-Devonian shales as the seal (Hallet, 2002; Shalbak, 2015), which also involves the basal Silurian hot-shale source rock (Fello et al., 2006; Hall et al., 2012).

The Ordovician sandstone reservoirs, associated with the primary petroleum system, are the Middle Ordovician Hawaz Formation and the Upper Ordovician Mamuniyat Formation, separated by a deeply incised unconformity related to the Late Ordovician glaciation. This succession was cut by north-northwest– to west-flowing Hirnantian glaciers (Ghienne et al., 2003; Le Heron et al., 2004), eroding down into the Hawaz Formation to create a rugged landscape of paleovalleys and highs (“buried hills”). The valleys were partially infilled by the periglacial to subglacial Melaz Shuqran, Mamuniyat and Bir Tlacsin clastics, and the residual topography subsequently buried by Tanezzuft shales. This sometimes sealed the Hawaz erosional highs to form paleotopographic traps with now reservoir-significant volume of hydrocarbons (Figure 2).

The Hawaz Formation

In the subsurface of the northern Murzuq Basin, the Hawaz Formation is represented by a detrital succession slightly more than 200 m (>650 ft) thick, composed of fine-grained quartz arenites and subarkosic arenites with subordinate sublithic arenites similar to the equivalent succession exposed on the Gargaf high (Ramos et al., 2006).

Trace fossils are frequent and locally abundant enough to overprint most primary sedimentary structures (Ramos et al., 2006). Gibert et al. (2011) identify 11 ichnogenera that exhibit a close relationship with both lithofacies and depositional paleoenvironments (facies associations). In broad terms, nearshore to shoreface facies are dominated by dense “pipe rock” fabric formed by Skolithos and Siphonichnus. In contrast, storm-dominated heterolithic facies are characterized by horizontal deposit-feeding Cruziana bioturbation.

Two main paleocurrent trends have been identified by Ramos et al. (2006): (1) small-scale sedimentary structures including ripples and small sigmoidal cross-bedded sets, indicative of widely dispersed flow directions; and (2) large-scale sedimentary structures suggesting a dominant flow toward the northeast and northwest but locally with bidirectional currents.

Several sedimentary models have been proposed for the Hawaz Formation, but all within transitional to shallow marine setting. Vos (1981) suggested the outcrop succession represented a fan-delta complex. Other authors (i.e., Anfray and Rubino, 2003; Ramos et al., 2006) identified sedimentary structures indicative of strong tidal influence, and the latter proposed a tide-dominated model with deposition in a megaestuary or gulf where the morphology of the paleocoastline enhanced tidal action, especially during transgressive episodes, when the coastal embayment was flooded.

Measured porosity can reach up to 25.7%, although values approximately 15% to 16% are the most frequent. Pore connectivity is good, with pore-throat diameters ranging from 0.1 to 64 μm (average 14.6 μm). Measured horizontal permeability values from core plugs may reach 900–1000 md (Shalbak, 2015), although most commonly average values in wells are approximately 0.2–150 md. However, diagenetic alterations have also had an impact on reservoir quality, as noted by Abouessa and Morad (2009). Specifically, the presence of higher amounts of feldspar, illite, a higher dickite to kaolinite ratio, and more abundant quartz cement, compared with those sampled in outcrops, is possibly because of the longer residence time under deep burial conditions.

DATABASE AND METHODOLOGY

The present study was based on data from 36 wells located across the northcentral sector of the Murzuq Basin (Figure 3). These data included core descriptions, high-resolution image logs (formation microimager [FMI]), gamma-ray (GR), sonic, neutron porosity, and density wire-line logs. The methodology that followed consisted of the following.

  1. Well data synthesis and standardization from the 36 wells by means of building well composite charts with the wire-line logs available for each well.
  2. Description and interpretation of the sedimentary facies based on 14 cored wells and FMI data. Conventional wire-line logs were not used to define lithofacies at this stage because the typical thickness of most lithofacies units is below the vertical resolution of these tools. The resultant facies analysis was compared with previous outcrop descriptions from the northern Gargaf high by M. Marzo and E. Ramos (2003, personal communication), Ramos et al. (2006), and Gibert et al. (2011), and was used as an analog for subsurface correlations.
  3. Grouping the resultant lithofacies into facies associations, defined by cores and FMI logs, each with distinct wire-line log profiles and stacking patterns. These log profiles were then used to identify facies associations in wells lacking core or FMI data.
  4. Construction of a comprehensive depositional model defined by the lithofacies and facies associations identified in cores, FMI logs, and conventional wire-line log profiles.
  5. Sequence stratigraphic analysis of the Hawaz Formation. Vertical changes in facies associations and their stacking patterns were used to identify correlatable stratigraphic genetic units. These units were then preliminary traced throughout the study area and used to define the sedimentary architecture of the Hawaz succession (M. Gil-Ortiz et al., personal communication).

Figure 3. (A) Satellite image of the northern Murzuq Basin highlighting the study area (red box). (B) Study area showing the position of the wells. Find highlighted the wells with core data available and, in white, the wells from Figures 4, 6, 7, 9, and 10. Note the distance between the studied area in the subsurface and the western Gargaf high where the outcrops studied by Ramos et al. (2006), referred to in this paper, are located.

SEDIMENTOLOGY OF THE HAWAZ FORMATION

Lithofacies

Fifteen Hawaz lithofacies were defined in the subsurface of the central Murzuq Basin based upon their lithology and internal fabric, including sedimentary structures and bioturbation (Table 1). These include sandstones (S), muddy sandstones (MS), heterolithic sandstones (HS), and heterolithic mudstones (HM). These lithofacies have been compared with those outcropping in the Gargaf high, as described by M. Marzo and E. Ramos (2003, personal communication) and Ramos et al. (2006), and complemented with valuable ichnofacies observations from outcrops described by Gibert et al. (2011). Each of the lithofacies is described and interpreted as follows.

Large-Scale Cross-Bedded Sandstones

Lithofacies Sx1 are fine-grained, well-sorted, and cross-bedded sandstones with high-angle foresets (>15°) (Figure 4) characterized by a north-northwest–directed paleoflow derived from image log dip picking. Locally, mud drapes, and rare mudstone intraclasts line set bases and foresets. No evidence of bioturbation exists. Typically, these sandstones form sets more than 50 cm (>20 in.) thick and cosets up to 10 m (33 ft) thick. The cross-bedding is interpreted as a response to the migration of dune bedforms under conditions of net sedimentation. The mud-draped foresets reflect alternating periods of slack water in a tidal regime. The lack of detrital clays and bioturbation suggests moderate- to high-energy conditions, under which the fines were carried off in suspension. Equivalent lithofacies have been described by Ramos et al. (2006) in outcrops as large-scale, sigmoidal cross-bedded sandstones with occasional horizontal trace fossils (Cruziana ichnofacies).

Figure 4. Core sections (∼90-cm [∼35-in.] length) of the main lithofacies identified in this study. See the location of the corresponding wells (B–G) in Figure 3B. HM = muddy heterolithic; HMb = burrowed muddy heterolithic; HS = sandy heterolithic; HSb = burrowed sandy heterolithic; MSb = burrowed sandstone with feeding ichnofauna; Pl = Planolites; Sb = burrowed sandstone with Siphonichnus; Si = Siphonichnus; Sk = Skolithos; Sl = parallel-laminated sandstone; Sr = ripple cross-laminated sandstone; Srb = burrowed ripple cross-laminated sandstone; Sv = massive sandstone; Sx1 = large-scale cross-bedded sandstone; Sx2 = small- to medium-scale cross-bedded sandstone; Sxb = burrowed cross-bedded sandstone; Sxl = cross-laminated sandstone; Sxlb = burrowed cross-laminated sandstone; Th = Thalassionides.

Small- to Medium-Scale Cross-Bedded Sandstones

Lithofacies Sx2 are fine- to medium-grained, well-sorted, and cross-bedded sandstones characterized by low-angle (5°–15°) foresets (Figure 4) again characterized by a north-northwest–directed paleoflow as suggested by image log interpretation. Planar lamination, current ripple cross-lamination, mud drapes, and mudstone intraclasts also occur locally. The degree of bioturbation ranges from absent to weak with rare Planolites. It forms sets up to 50 cm (20 in.) thick. The cross stratification and cross-lamination record the migration of medium-scale dunes and ripples and megaripples, respectively, under the influence of unidirectional current flow. This lithofacies could also be interpreted as corresponding to toesets of the previously described large-scale cross-bedded sandstones (i.e., lithofacies Sx1). Most probably, deposition occurred within a high-energy, tidally influenced environment. Equivalent lithofacies have been described by Ramos et al. (2006) outcropping as medium-scale, sigmoidal cross-bedded sandstones with occasional horizontal trace fossils (Cruziana ichnofacies).

Parallel-Laminated Sandstones

Lithofacies Sl are fine-grained standstones with parallel lamination (<5°) (Figure 4). Bioturbation was not recognized (Figure 4). The lithofacies is organized in sets 10–100 cm (4–39 in.) thick. It is interpreted to record sand deposition from nearshore currents under a moderate- to high-energy, upper-flow regime. A similar lithofacies has been described by Ramos et al. (2006) in outcrops as parallel-laminated sandstones with occasional parting lineation and very scarce bioturbation.

Cross-Laminated Sandstones

Lithofacies Sxl are fine-grained sandstones with low-angle cross-lamination (Figure 4). Climbing ripple lamination and mud drapes are also occasionally present. In general, it is a nonbioturbated lithofacies, although sparse Skolithos were occasionally observed. The set thicknesses range from 10 to 140 cm (4 to 55 in.). This lithofacies is interpreted as the deposits of storm events in a nearshore environment. When climbing ripples are present, a high rate of sedimentation under unidirectional flows is inferred. Similar lithofacies are described by Ramos et al. (2006) outcropping in the Gargaf high as low-angle, swaley to hummocky cross-stratified sandstones.

Ripple Cross-Laminated Sandstones

Lithofacies Sr are fine-grained, very well-sorted sandstones with ripple cross-lamination and locally intraclasts. Locally, the current ripples display bimodal foreset directions. Bedset or coset thickness does not exceed 50 cm (20 in.), whereas individual sets are up to 3 cm (1 in.) thick, typically associated with very thin clay drapes (Figure 4). This is an unbioturbated lithofacies. The cross-lamination records the migration of current ripples under low to moderate velocity currents. The presence of clay drapes and the bimodal foreset directions, observed in some sets, would suggest deposition in a subtidal setting. Equivalent ripple cross-laminated sandstones with occasional horizontal trace fossils (Cruziana ichnofacies) have also been identified in outcrop by Ramos et al. (2006) characterized by a dominantly north-northwest paleoflow direction, locally bimodal toward south-southeast.

Massive Sandstones

Lithofacies Sv are fine-grained, clean, generally well-sorted sandstones with poorly defined planar lamination and cross-bedding (Figure 4). Locally, mud intraclasts and basal erosive surfaces were identified. This lithofacies is characterized by the absence of bioturbation. It is organized, forming sets of 30–100 cm (10–39 in.) thick. The massive appearance of this facies could be interpreted as the result of early postdepositional processes involving dewatering and partial fluidization suggestive of a high sedimentation rate in the depositional system. This lithofacies can be easily misinterpreted as Sx1 in cores when the clean nature of the sandstones, reflecting the lack of micas and fine-grained sedimentary layers obscures the limits between cross-bed sets. The lack of detrital clays and micas in these sandstones suggests deposition in a relatively high-energy environment where fines were carried off in suspension. Equivalent lithofacies have been observed by Ramos et al. (2006) outcropping in the northern margin of the basin as apparently massive sandstones.

Burrowed Cross-Bedded Sandstones

Lithofacies Sxb are clean, fine-grained sandstones displaying small- to medium-scale cross-bedding with local mudstone intraclasts. Moderate degree of bioturbation with Skolithos and Siphonichnus burrows (Figure 4). It is typically organized in 30- to 200-cm (10- to 79-in.)-thick beds. The clean nature of the sandstones and the presence of mudstone intraclasts suggest moderate- to high-energy conditions in which fines were carried off in suspension. The cross-bedding records the migration of dune and bar bedforms, whereas the vertical to oblique burrows suggest a shallow, high-energy marine environment.

Burrowed Cross-Laminated Sandstones

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ABSTRACT

The subsurface of the highly productive Murzuq Basin in southwest Libya remains poorly understood. As a consequence, a need exists for detailed sedimentological studies of both the oil-prone Mamuniyat Formation and Hawaz Formation reservoirs in this area. Of particular interest in this case is the Middle Ordovician Hawaz Formation, interpreted as an excellent example of a “nonactualistic,” tidally influenced clastic reservoir that appears to extend hundreds of kilometers across much of the North African or Saharan craton. The Hawaz Formation comprises 15 characteristic lithofacies grouped into 7 correlatable facies associations distributed in broad and laterally extensive facies belts deposited in a shallow marine, intertidal to subtidal environment. Three main depositional sequences and their respective systems tracts have also been identified. On this basis, a genetic-based stratigraphic zonation scheme has been proposed as a tool to improve subsurface management of this reservoir unit. A nonactualistic sedimentary model is proposed in this work with new ideas presented for marginal to shallow marine depositional environments during the Middle Ordovician in the northern margin of Gondwana.

INTRODUCTION

For many years, the main Libyan petroleum province was the prolific Sirte Basin with a limited contribution from the Ghadames Basin (Berkine Basin in Algeria) (Hallet, 2002; Figure 1). However, since the mid-1990s, the Murzuq Basin has developed into a major oil- and gas-producing province. The Hawaz Formation constitutes one of the most important reservoirs in several producing fields in the central and northern part of the basin. The generally high reservoir quality (average 5%–15% porosity and 0.1–150 md permeability) and lateral continuity, characteristic of the Hawaz, are key factors in the development and production of these accumulations. However, despite the well-documented potential of the Hawaz Formation, its subsurface character remains poorly understood.

yfyqdyfbwuewwbcbrw Figure 1. Geological map of Libya showing the main sedimentary basins. The Murzuq Basin is bounded by the Atshan arch to the northwest, the Gargaf high to the north, the Tihemboka high to the southwest and the Tibesti high to the southeast. The area of interest represented in Figure 3A is highlighted in the red box. Modified from M. Marzo and E. Ramos (2003, personal communication).

To date, only a few sedimentological studies of this formation have been carried out, and all are exclusively based on surface geology (Vos, 1981; Anfray and Rubino, 2003; M. Marzo and E. Ramos, 2003, personal communication; Ramos et al., 2006; Gibert et al., 2011). Other published works have focused on diagenesis (Abouessa and Morad, 2009; Abouessa, 2012) and trapping mechanisms (Franco et al., 2012). In addition, subsurface interpretations of the formation are based on inconsistent lithostratigraphic correlations unconstrained by a consistent sequence stratigraphic framework. As such, no genetic or sequence stratigraphy–based zonation exists. This limited database highlights the necessity of providing a sequence stratigraphic framework based on a robust sedimentological model of the transitional to shallow marine Hawaz Formation.

As Dalrymple and Choi (2007) have highlighted, transitional tide-dominated and deltaic facies reflect the interaction of numerous terrestrial and marine processes in a very complex depositional environment. Any paleoenvironmental or stratigraphic interpretation of such transition zone successions requires a comprehensive understanding of the facies and facies associations. Hence, a comprehensive understanding of the facies changes through this transition zone is necessary to make proper paleoenvironmental and sequence-stratigraphic interpretations of the sedimentary successions. However, is it actually possible to compare these paleoenvironments with any “actualistic” sedimentary model?

The limitations of the approach become apparent when the uniformitarian principle is extended to depositional environments in the most ancient geological record. In particular, the assumption that modern environments can provide analogs for all geological successions must be questioned (Nichols, 2017). It is broadly accepted that Earth’s dynamics have changed considerably throughout geological history, and accordingly, factors controlling sedimentation have changed also, such as a lack of flora stabilizing river banks, greenhouse versus icehouse periods defining coastal geomorphology, tidal ranges controlling facies belts or characteristic ichnofacies during a particular period of geological time. The analysis of some of these factors suggests that the facies succession of the Hawaz Formation reflects different depositional processes from those observed in modern environments. From this point forward, we will use the term “nonactualistic” to describe those processes affecting the geological signature of the Hawaz Formation that are difficult to compare with any modern depositional environment analog.

Consequently, the main aim of this paper is to present a sedimentological characterization of the Hawaz Formation based on a detailed lithofacies description and interpretation together with the development of a facies association classification. This forms the basis for an appropriate depositional model in accordance with plausible physical and chemical processes during the Middle Ordovician. In addition, the overall analysis aims to build a genetically based zonation through sequence stratigraphy that will improve reservoir management and provide tools for maximizing hydrocarbon recovery efficiency. Finally, it is intended that these sedimentological and stratigraphic models should be a well-documented subsurface analog for clastic reservoirs in similar settings.

GEOLOGICAL SETTING

The Structure and Stratigraphy of the Murzuq Basin

The Paleozoic succession of the Murzuq Basin is an erosional remnant of a much more extensive regional succession extending along the northern margin of the Gondwana supercontinent (Davidson et al., 2000; Shalbak, 2015). Its present extent reflects several periods of uplift and unroofing during the late Paleozoic, Mesozoic, and Cenozoic, which, together, are responsible for its modern architecture. As a consequence, the present-day basin geometry bears little relation to the broader and larger preexisting sedimentary basin. The current basin is composed of a central Cretaceous depression bounded to the northwest by the Atshan arch, the Gargaf high to the north, and the Tibesti and Tihemboka highs on the southeast and southwest, respectively (Figure 1). These structural highs were formed by multiphase tectonic uplifts from the middle Paleozoic to Cenozoic, although the main periods of uplift and erosion occurred during the Pennsylvanian (late Carboniferous; Hercynian) and early Cenozoic (Alpine) orogenic cycles.

A series of geological events can be recognized in the stratigraphic record of the Murzuq Basin, some represented by basin-scale unconformities within the sedimentary infill reflecting the Pan-African, Caledonian, and Hercynian orogenesis and the short Late Ordovician glacial event responsible for the Taconic or basal glacial erosional surface (Figure 2). Other unconformities that may be recognized within the sedimentary record are minor or belong to the younger Austrian or Alpine cycles, and consequently, they do not strongly affect the Paleozoic section directly in the central Murzuq Basin; although, they may have had strong implications in terms of overburden removal, source rock maturity, and reservoir quality because of uplift and unroofing of Mesozoic series on the Paleozoic section (Boote et al., 2008).

Figure 2. (A) Stratigraphic chart summarizing the stratigraphic column for the Murzuq Basin highlighting the main stratigraphic units and major basin-scale unconformities. 1 = Cambrian–Ordovician; 2 = Silurian (Silur.); 3 = Devonian (Dev.)–Carboniferous (Carbonif.); 4 = Mesozoic. (B) Wheeler diagram showing lithostratigraphic to chronostratigraphic relationships of the Ordovician and Lower Silurian succession in the area of study. (C) Seismic line showing the typical geomorphological signature of the Ordovician succession in form of paleo-highs (“buried hills”) and paleovalleys. The main petroleum systems elements are also represented in Figure 2A, B. Cretac. = Cretaceous; Dev. = Devonian; Fm. = Formation; Pale-Neo = Paleogene–Neogene; Perm. = Permian; Q = Quaternary.

The maximum sedimentary thickness in the present-day Murzuq Basin is approximately 4000 m (∼13,000 ft). Despite successive erosive episodes during several phases of uplift throughout the history of the basin, the maximum sedimentary thickness most probably never exceeded 5000 m (16,400 ft) (Davidson et al., 2000). The age of the infill ranges from Cambrian to Cretaceous, commonly covered by large Quaternary sand dunes in the central part of the basin. The sedimentary infill can be subdivided into four main units: (1) Cambrian–Ordovician, (2) Silurian, (3) Devonian–Carboniferous, and (4) Mesozoic (Figure 2).

The lower Paleozoic succession comprises the terrigenous Cambrian–Ordovician Gargaf Group consisting of at least five formations; from bottom to top, they are the following: Hasawnah, Ash Shabiyat, Hawaz, Melaz Shuqran, and Mamuniyat Formations (Figure 2). The lowermost Hasawnah Formation rests unconformably on the Precambrian basement and is composed of Cambrian to Lower Ordovician conglomeratic to sandy continental and shallow marine littoral deposits. The Hasawnah Formation is overlain, above a transgressive surface of erosion, by the shallow marine and preglacial Ash Shabiyat and Hawaz Formations, attributed respectively to the Lower and Middle Ordovician (Tremadocian–Sandbian). The Upper Ordovician succession, associated with a major glaciation, principally comprises the Melaz Shuqran and Mamuniyat Formations, locally overlain by a thin and somewhat enigmatic package known as the Bir Tlacsin. The former is most probably lower Hirnantian and predominantly mud-prone, representing the period of the highest relative sea level during the Late Ordovician (McDougall and Martin, 2000), whereas the Mamuniyat Formation is a major Hirnantian sand-prone package.

The Petroleum Systems and the Hydrocarbon Production History of the Murzuq Basin

Early exploration in the Murzuq Basin focused upon surface structures. The first exploratory well was drilled in the northern Murzuq in 1955–1956. Subsequently, several successful discoveries in the neighboring Illizi Basin (southeastern Algeria) encouraged further exploration across the border. Three years later, Exxon discovered gas at the Atshan region and Gulf tested oil at low rates from Ordovician sandstones. However, in 1958, industry attention shifted east with the discovery of a major oil accumulation in the Sirte Rift province, and there was little further exploration of the Murzuq Basin for the next 20 yr. During the late 1980s to 1990s, Rompetrol, and later Repsol, drilled up to 57 exploratory wells in the basin, all of which targeted Ordovician prospects. This exploratory activity resulted in many significant oil discoveries, highlighting the rapidly growing potential of the basin.

The most recent hydrocarbons-in-place estimation for the Murzuq Basin is approximately 6 billion bbl (∼9.5 × 108 m3) of oil and approximately 35 tcf (∼1 trillion m3) of gas, which represent approximately 6.5% of Libya’s resources and 30% of Libya’s current oil production (Shalbak, 2015).

The main petroleum system in the Murzuq Basin comprises a basal Silurian (Tanezzuft) hot-shale source rock, Ordovician sandstone reservoirs, and a thick Tanezzuft shale seal (Figure 2). A secondary petroleum system in the basin (noncommercial to date) is composed of the basal Devonian sandstones as reservoirs and the intra-Devonian shales as the seal (Hallet, 2002; Shalbak, 2015), which also involves the basal Silurian hot-shale source rock (Fello et al., 2006; Hall et al., 2012).

The Ordovician sandstone reservoirs, associated with the primary petroleum system, are the Middle Ordovician Hawaz Formation and the Upper Ordovician Mamuniyat Formation, separated by a deeply incised unconformity related to the Late Ordovician glaciation. This succession was cut by north-northwest– to west-flowing Hirnantian glaciers (Ghienne et al., 2003; Le Heron et al., 2004), eroding down into the Hawaz Formation to create a rugged landscape of paleovalleys and highs (“buried hills”). The valleys were partially infilled by the periglacial to subglacial Melaz Shuqran, Mamuniyat and Bir Tlacsin clastics, and the residual topography subsequently buried by Tanezzuft shales. This sometimes sealed the Hawaz erosional highs to form paleotopographic traps with now reservoir-significant volume of hydrocarbons (Figure 2).

The Hawaz Formation

In the subsurface of the northern Murzuq Basin, the Hawaz Formation is represented by a detrital succession slightly more than 200 m (>650 ft) thick, composed of fine-grained quartz arenites and subarkosic arenites with subordinate sublithic arenites similar to the equivalent succession exposed on the Gargaf high (Ramos et al., 2006).

Trace fossils are frequent and locally abundant enough to overprint most primary sedimentary structures (Ramos et al., 2006). Gibert et al. (2011) identify 11 ichnogenera that exhibit a close relationship with both lithofacies and depositional paleoenvironments (facies associations). In broad terms, nearshore to shoreface facies are dominated by dense “pipe rock” fabric formed by Skolithos and Siphonichnus. In contrast, storm-dominated heterolithic facies are characterized by horizontal deposit-feeding Cruziana bioturbation.

Two main paleocurrent trends have been identified by Ramos et al. (2006): (1) small-scale sedimentary structures including ripples and small sigmoidal cross-bedded sets, indicative of widely dispersed flow directions; and (2) large-scale sedimentary structures suggesting a dominant flow toward the northeast and northwest but locally with bidirectional currents.

Several sedimentary models have been proposed for the Hawaz Formation, but all within transitional to shallow marine setting. Vos (1981) suggested the outcrop succession represented a fan-delta complex. Other authors (i.e., Anfray and Rubino, 2003; Ramos et al., 2006) identified sedimentary structures indicative of strong tidal influence, and the latter proposed a tide-dominated model with deposition in a megaestuary or gulf where the morphology of the paleocoastline enhanced tidal action, especially during transgressive episodes, when the coastal embayment was flooded.

Measured porosity can reach up to 25.7%, although values approximately 15% to 16% are the most frequent. Pore connectivity is good, with pore-throat diameters ranging from 0.1 to 64 μm (average 14.6 μm). Measured horizontal permeability values from core plugs may reach 900–1000 md (Shalbak, 2015), although most commonly average values in wells are approximately 0.2–150 md. However, diagenetic alterations have also had an impact on reservoir quality, as noted by Abouessa and Morad (2009). Specifically, the presence of higher amounts of feldspar, illite, a higher dickite to kaolinite ratio, and more abundant quartz cement, compared with those sampled in outcrops, is possibly because of the longer residence time under deep burial conditions.

DATABASE AND METHODOLOGY

The present study was based on data from 36 wells located across the northcentral sector of the Murzuq Basin (Figure 3). These data included core descriptions, high-resolution image logs (formation microimager [FMI]), gamma-ray (GR), sonic, neutron porosity, and density wire-line logs. The methodology that followed consisted of the following.

  1. Well data synthesis and standardization from the 36 wells by means of building well composite charts with the wire-line logs available for each well.
  2. Description and interpretation of the sedimentary facies based on 14 cored wells and FMI data. Conventional wire-line logs were not used to define lithofacies at this stage because the typical thickness of most lithofacies units is below the vertical resolution of these tools. The resultant facies analysis was compared with previous outcrop descriptions from the northern Gargaf high by M. Marzo and E. Ramos (2003, personal communication), Ramos et al. (2006), and Gibert et al. (2011), and was used as an analog for subsurface correlations.
  3. Grouping the resultant lithofacies into facies associations, defined by cores and FMI logs, each with distinct wire-line log profiles and stacking patterns. These log profiles were then used to identify facies associations in wells lacking core or FMI data.
  4. Construction of a comprehensive depositional model defined by the lithofacies and facies associations identified in cores, FMI logs, and conventional wire-line log profiles.
  5. Sequence stratigraphic analysis of the Hawaz Formation. Vertical changes in facies associations and their stacking patterns were used to identify correlatable stratigraphic genetic units. These units were then preliminary traced throughout the study area and used to define the sedimentary architecture of the Hawaz succession (M. Gil-Ortiz et al., personal communication).

Figure 3. (A) Satellite image of the northern Murzuq Basin highlighting the study area (red box). (B) Study area showing the position of the wells. Find highlighted the wells with core data available and, in white, the wells from Figures 4, 6, 7, 9, and 10. Note the distance between the studied area in the subsurface and the western Gargaf high where the outcrops studied by Ramos et al. (2006), referred to in this paper, are located.

SEDIMENTOLOGY OF THE HAWAZ FORMATION

Lithofacies

Fifteen Hawaz lithofacies were defined in the subsurface of the central Murzuq Basin based upon their lithology and internal fabric, including sedimentary structures and bioturbation (Table 1). These include sandstones (S), muddy sandstones (MS), heterolithic sandstones (HS), and heterolithic mudstones (HM). These lithofacies have been compared with those outcropping in the Gargaf high, as described by M. Marzo and E. Ramos (2003, personal communication) and Ramos et al. (2006), and complemented with valuable ichnofacies observations from outcrops described by Gibert et al. (2011). Each of the lithofacies is described and interpreted as follows.

Large-Scale Cross-Bedded Sandstones

Lithofacies Sx1 are fine-grained, well-sorted, and cross-bedded sandstones with high-angle foresets (>15°) (Figure 4) characterized by a north-northwest–directed paleoflow derived from image log dip picking. Locally, mud drapes, and rare mudstone intraclasts line set bases and foresets. No evidence of bioturbation exists. Typically, these sandstones form sets more than 50 cm (>20 in.) thick and cosets up to 10 m (33 ft) thick. The cross-bedding is interpreted as a response to the migration of dune bedforms under conditions of net sedimentation. The mud-draped foresets reflect alternating periods of slack water in a tidal regime. The lack of detrital clays and bioturbation suggests moderate- to high-energy conditions, under which the fines were carried off in suspension. Equivalent lithofacies have been described by Ramos et al. (2006) in outcrops as large-scale, sigmoidal cross-bedded sandstones with occasional horizontal trace fossils (Cruziana ichnofacies).

Figure 4. Core sections (∼90-cm [∼35-in.] length) of the main lithofacies identified in this study. See the location of the corresponding wells (B–G) in Figure 3B. HM = muddy heterolithic; HMb = burrowed muddy heterolithic; HS = sandy heterolithic; HSb = burrowed sandy heterolithic; MSb = burrowed sandstone with feeding ichnofauna; Pl = Planolites; Sb = burrowed sandstone with Siphonichnus; Si = Siphonichnus; Sk = Skolithos; Sl = parallel-laminated sandstone; Sr = ripple cross-laminated sandstone; Srb = burrowed ripple cross-laminated sandstone; Sv = massive sandstone; Sx1 = large-scale cross-bedded sandstone; Sx2 = small- to medium-scale cross-bedded sandstone; Sxb = burrowed cross-bedded sandstone; Sxl = cross-laminated sandstone; Sxlb = burrowed cross-laminated sandstone; Th = Thalassionides.

Small- to Medium-Scale Cross-Bedded Sandstones

Lithofacies Sx2 are fine- to medium-grained, well-sorted, and cross-bedded sandstones characterized by low-angle (5°–15°) foresets (Figure 4) again characterized by a north-northwest–directed paleoflow as suggested by image log interpretation. Planar lamination, current ripple cross-lamination, mud drapes, and mudstone intraclasts also occur locally. The degree of bioturbation ranges from absent to weak with rare Planolites. It forms sets up to 50 cm (20 in.) thick. The cross stratification and cross-lamination record the migration of medium-scale dunes and ripples and megaripples, respectively, under the influence of unidirectional current flow. This lithofacies could also be interpreted as corresponding to toesets of the previously described large-scale cross-bedded sandstones (i.e., lithofacies Sx1). Most probably, deposition occurred within a high-energy, tidally influenced environment. Equivalent lithofacies have been described by Ramos et al. (2006) outcropping as medium-scale, sigmoidal cross-bedded sandstones with occasional horizontal trace fossils (Cruziana ichnofacies).

Parallel-Laminated Sandstones

Lithofacies Sl are fine-grained standstones with parallel lamination (<5°) (Figure 4). Bioturbation was not recognized (Figure 4). The lithofacies is organized in sets 10–100 cm (4–39 in.) thick. It is interpreted to record sand deposition from nearshore currents under a moderate- to high-energy, upper-flow regime. A similar lithofacies has been described by Ramos et al. (2006) in outcrops as parallel-laminated sandstones with occasional parting lineation and very scarce bioturbation.

Cross-Laminated Sandstones

Lithofacies Sxl are fine-grained sandstones with low-angle cross-lamination (Figure 4). Climbing ripple lamination and mud drapes are also occasionally present. In general, it is a nonbioturbated lithofacies, although sparse Skolithos were occasionally observed. The set thicknesses range from 10 to 140 cm (4 to 55 in.). This lithofacies is interpreted as the deposits of storm events in a nearshore environment. When climbing ripples are present, a high rate of sedimentation under unidirectional flows is inferred. Similar lithofacies are described by Ramos et al. (2006) outcropping in the Gargaf high as low-angle, swaley to hummocky cross-stratified sandstones.

Ripple Cross-Laminated Sandstones

Lithofacies Sr are fine-grained, very well-sorted sandstones with ripple cross-lamination and locally intraclasts. Locally, the current ripples display bimodal foreset directions. Bedset or coset thickness does not exceed 50 cm (20 in.), whereas individual sets are up to 3 cm (1 in.) thick, typically associated with very thin clay drapes (Figure 4). This is an unbioturbated lithofacies. The cross-lamination records the migration of current ripples under low to moderate velocity currents. The presence of clay drapes and the bimodal foreset directions, observed in some sets, would suggest deposition in a subtidal setting. Equivalent ripple cross-laminated sandstones with occasional horizontal trace fossils (Cruziana ichnofacies) have also been identified in outcrop by Ramos et al. (2006) characterized by a dominantly north-northwest paleoflow direction, locally bimodal toward south-southeast.

Massive Sandstones

Lithofacies Sv are fine-grained, clean, generally well-sorted sandstones with poorly defined planar lamination and cross-bedding (Figure 4). Locally, mud intraclasts and basal erosive surfaces were identified. This lithofacies is characterized by the absence of bioturbation. It is organized, forming sets of 30–100 cm (10–39 in.) thick. The massive appearance of this facies could be interpreted as the result of early postdepositional processes involving dewatering and partial fluidization suggestive of a high sedimentation rate in the depositional system. This lithofacies can be easily misinterpreted as Sx1 in cores when the clean nature of the sandstones, reflecting the lack of micas and fine-grained sedimentary layers obscures the limits between cross-bed sets. The lack of detrital clays and micas in these sandstones suggests deposition in a relatively high-energy environment where fines were carried off in suspension. Equivalent lithofacies have been observed by Ramos et al. (2006) outcropping in the northern margin of the basin as apparently massive sandstones.

Burrowed Cross-Bedded Sandstones

Lithofacies Sxb are clean, fine-grained sandstones displaying small- to medium-scale cross-bedding with local mudstone intraclasts. Moderate degree of bioturbation with Skolithos and Siphonichnus burrows (Figure 4). It is typically organized in 30- to 200-cm (10- to 79-in.)-thick beds. The clean nature of the sandstones and the presence of mudstone intraclasts suggest moderate- to high-energy conditions in which fines were carried off in suspension. The cross-bedding records the migration of dune and bar bedforms, whereas the vertical to oblique burrows suggest a shallow, high-energy marine environment.

Burrowed Cross-Laminated Sandstones

Lithofacies Sxlb are fine-grained, variably argillaceous, and micaceous sandstones with low-angle cross-lamination and local mud laminae and mudstone intraclasts. This lithofacies is moderately bioturbated with an ichnofabric dominated by Skolithos and Siphonichnus, indeterminate burrows and meniscate-backfilled burrows (Figure 4). The minimum thickness observed of this lithofacies is 70 cm (28 in.). The moderately intense bioturbation, dominated by mainly vertical, suspension-feeding burrows suggests a shallow, high-energy subtidal environment. However, the mud laminae also reflect low-energy conditions. Thus, depending on the context, this lithofacies may have different interpretations ranging from a lower shoreface to an intertidal environment. The low-angle cross-lamination is interpreted as reflecting deposition from subtidal sand sheets or low relief sand bars.

Burrowed Ripple Cross-Laminated Sandstones

Lithofacies Srb are very fine– to fine-grained sandstones, locally argillaceous, and micaceous characterized by current ripple cross-lamination and planar lamination. A moderate degree of bioturbation characterizes this lithofacies (Figure 4), with an ichnofabric dominated by Skolithos (6–8-mm [0.24–0.31-in.] diameter and maximum length of 30 cm [12 in.]), Siphonichnus, and local indeterminate burrows. This lithofacies forms packages 15–170 cm (6–67 in.) thick. The fine grain size and the locally argillaceous composition of this lithofacies imply deposition in a relatively low-energy environment. The cross-lamination records the migration of current ripples under conditions of net sedimentation and implies that the sand was transported by a unidirectional current of low to moderate velocity. The ichnofauna (mostly represented by vertical burrows) suggests a shallow marine environment dominated by suspension-feeding benthonic fauna.

Burrowed Sandstones with Siphonichnus

Lithofacies Sb are fine-grained, well-sorted sandstones locally with mud laminae. This lithofacies is highly bioturbated, with an ichnofauna dominated by Siphonichnus burrows, locally up to 100 cm (39 in.) in length, giving rise to a distinctive pipe rock fabric. The minimum bed thickness appears to be approximately 20 cm (∼8 in.), although bed boundaries are typically obscured by bioturbation (Figure 4). This lithofacies is volumetrically very abundant, and continuous sections of up to 20 m (66 ft) have been identified in some wells. The occurrence of vertical burrows (Skolithos ichnofacies) suggests a moderate- to low-energy restricted to shallow marine environment, and the presence of mud laminae (mud drapes) implies fluctuating energy levels. Equivalent lithofacies have been described by Ramos et al. (2006) in outcrops as thick-bedded, massive, bioturbated sandstones.

Burrowed Sandstones with Feeding Ichnofauna

Lithofacies MSb are argillaceous fine-grained sandstones characterized by moderately intense bioturbation dominated by horizontal, deposit-feeding burrows (Figure 4), notably Teichichnus and Thalassinoides. Individual beds range in thickness from 10 to 270 cm (4 to 106 in.). The moderately high detrital clay content of these sandstones and the characteristic low-energy ichnofauna suggests a relatively protected depositional setting or open-marine conditions.

Sandy Heterolithics

Lithofacies HS are interbedded very fine– to fine-grained sandstones and argillaceous siltstones (>50% sand content). This lithofacies displays flaser structures together with combined current and wave ripple cross-lamination and also planar lamination (Figure 4). Only a limited amount of bioturbation with rare Chondrites and Planolites burrows exists. The thickness of this lithofacies ranges between 1 cm (0.4 in.) sets up to an accumulated bedset thickness of 5 m (16 ft). The interbedding of sandstone and argillaceous siltstone implies fluctuating energy levels. Sands were transported and deposited by both unidirectional and oscillatory (wave-generated) flows. Unidirectional current flow was mostly of low to moderate velocity, resulting in the formation of current ripples. By contrast, the presence of cross-bedding (because of the migration of dune and bar bedforms) and mudstone intraclasts indicates higher current velocities. The presence of Chondrites indicates that burrowing took place under marine conditions; the remaining burrows, Planolites and indeterminate horizontal tubes, also suggest a marine environment. The low bioturbation index together with the local occurrence of Chondrites (generally considered to be characteristic of low oxygen conditions), suggests that oxygenation levels were low. Wave, current, and combined flow cross-lamination suggests sands were deposited during storm events below fair-weather wave base (FWWB).

Burrowed Sandy Heterolithics

Lithofacies HSb are thinly interbedded, very fine–grained, micaceous, argillaceous sandstone and micaceous, argillaceous siltstone (>50% sand content). Locally, the argillaceous siltstones display planar lamination and the sandstones’ current and wave ripple cross-lamination. Bioturbation is moderately intense characterized by overprinted Skolithos and Cruziana ichnofacies (Siphonichnus burrows, with subordinate Planolites and indeterminate burrows) (Figure 4). Minimum bed thickness is 1 cm (0.4 in.), whereas accumulated bedset thickness can reach 4 m (13 ft). The interbedding of sandstone and siltstone suggests fluctuating energy conditions, with the sandstones representing higher energy levels. The cross-lamination within the sandstones records the migration of combined current and wave ripples under conditions of net sedimentation and low to moderate current velocities. The mixed assemblage of ichnofauna suggests the transition from a high-energy to a low-energy setting, from an open-marine inner shelf up to a lower shoreface setting. A variation of this lithofacies in the upper part of the Hawaz Formation exists, in which the base of the sandy intervals locally displays rip-up mudstone clasts and a rhythmic alternation of thin, inclined mud drapes and sandstones. In this case, the interpretation given to this lithofacies corresponds to inclined heterolithic stratification (IHS) associated with minor channels or tidal creeks in a restricted, sandy to mixed intertidal subenvironment.

Muddy Heterolithics

Lithofacies HM are mudstones interbedded with micaceous argillaceous siltstone and very fine–grained sandstone (>50% clay content). The mudstone and argillaceous siltstone display planar lamination and lenticular bedding (current and wave-rippled sand lenses). The sandstone contains current ripples and rare wave ripples (Figure 4). Individual lithofacies packages have a minimum thickness of 5 cm (2 in.) but may reach an accumulated bedset thickness up to 3.5 m (11.5 ft). The sandstone beds and lenses represent energetic pulses in an overall low-energy setting where mud settled out of suspension. During the higher-energy pulses, sand was moved by both unidirectional and oscillatory (wave-generated) flows. The lack of burrows indicates anoxic conditions in a fairly distal marine setting or a restricted and stressed subenvironment, such as a tidal mudflat or lagoon.

Burrowed Muddy Heterolithics

Lithofacies HMb are argillaceous siltstone interbedded with minor fine-grained sandstone layers and sandstone laminae (>50% clay content). It is characterized by a variable degree of bioturbation with Siphonichnus, Skolithos, Planolites, and indeterminate vertical burrows (Figure 4). Shrinkage cracks may occur locally. The minimum thickness of individual facies units is 7 cm (3 in.), whereas the accumulated bedset thickness is up to 3.8 m (12.5 ft). The interbedding of argillaceous siltstone and very fine– to fine-grained sandstone suggests fluctuating energy conditions in an overall low-energy setting. The shrinkage cracks are probably related to variations in salinity and temperature when present. The depositional setting of this lithofacies ranges from a relatively distal, inner shelf subenvironment to a restricted intertidal flat subenvironment.

Facies Associations

The proposed scheme based on the previously described lithofacies establishes seven facies associations designated as Hawaz facies association 1 (HWFA1) to HWFA7 assigned to proximal and increasingly distal environments (Figure 5).

Figure 5. Summary of facies associations and interpreted depositional settings. Description includes typical core sections and thickness ranges. See also the main lithofacies composing each facies association and the location of detailed features shown in Figure 6. Interpretation in terms of depositional environment is also included. In addition, summary conventional core analysis (CCA), porosity (Ø), and permeability (K) and data for every facies association and average gamma ray values are also shown. The last column shows the sequence stratigraphic interpretation plus the location of each association within the depositional model of the Figure 8. Cl = clay; fs = fine sandstone; HM = muddy heterolithic; HMb = burrowed muddy heterolithic; HS = sandy heterolithic; HSb = burrowed sandy heterolithic; HST = highstand systems tract; HWFA = Hawaz facies association; MFWB = mean fairweather wave base; ms = medium sandstone; MSb = burrowed sandstone with feeding ichnofauna; MSWB = mean stormweather wave base; Sb = burrowed sandstone with Siphonichnus; si = silt; Sl = parallel-laminated sandstone; Sr = ripple cross-laminated sandstone; Srb = burrowed ripple cross-laminated sandstone; Sv = massive sandstone; Sx1 = large-scale cross-bedded sandstone; Sx2 = small- to medium-scale cross-bedded sandstone; Sxb = burrowed cross-bedded sandstone; Sxl = cross-laminated sandstones; Sxlb = burrowed cross-laminated sandstone; TST = transgressive systems tract; vfs = very fine sandstone.

Hawaz Facies Association 1: Tidal Flat

Facies association HWFA1 mainly consists of lithofacies Sxlb, MSb, Sb, HMb, and HSb with subordinate Srb and Sv (Figure 5). The thickness of individual packages of this facies association is very variable, ranging from 30 to 60 m (100 to 200 ft) as a direct consequence of the downcutting associated with the Upper Ordovician glaciogenic unconformities. The GR log response ranges significantly from 30 to 140 API units in a characteristic fining-upward succession. The intensity of bioturbation is moderate to very high, characterized by a mixed low diversity Skolithos and Cruziana ichnofacies assemblage indicative of a relatively high-energy environment grading toward a more protected and restricted low-energy setting. It is also characterized by an upward-increasing detrital clay content typical of tidal flat environments. Furthermore, the low diversity of acritarch assemblages and the strong predominance of leiospheres, characteristic of a marginal marine setting, identified in palynological studies of some wells, suggests a relatively protected tidal sand to mixed flat environment grading normally from the underlying HWFA3 or HWFA2 (see below). Some ichnogenera identified as Planolites, Siphonichnus, and Thalassinoides strongly associated with tidal flat deposits (Gingras et al., 2012) also support this hypothesis together with the common occurrence of clay drapes and flaser lenticular bedding (Figure 6A). The sporadic occurrences of individual massive to rippled sandstone levels (Sv and Srb) and the presence of rip-up mudstone clasts at the base of these units in the heterolithic intervals (locally associated with small synsedimentary faults) are interpreted in terms of bank collapse in tidal creeks on the sand flat. The same package in the Gargaf high was described as an upper shoreface wave dominated facies assemblage by Ramos et al. (2006), which probably would represent a beach to barrier island setting laterally equivalent to this facies association HWFA1.

Figure 6. Detailed close-up views of some characteristic sedimentary structures and fabrics of the Hawaz Formation in core. (A) Mud-draped (flaser) lamination (arrows) in Hawaz facies association (HWFA1) tidal flat facies association from well E. (B) Mudstone rip-up clasts (arrows) from fluviotidal to subtidal channels of HWFA2 subtidal complex in well F. (C) Clay-draped current ripples (small arrows) from HWFA2 subtidal complex from well C. Notice the direction of the paleocurrent flow leftward (horizontal arrow). (D) Burrowed sandstones with characteristic Skolithos ichnofacies of the HWFA5 burrowed shelfal and lower shoreface facies association in well E. (E) Characteristic view of the HWFA6 burrowed inner shelf deposits from well D. (F) Clay-draped combined flow ripples (small arrows) from Hawaz facies association 7 shelfal storm sheets in well D. Notice the direction of the paleocurrent flow rightward in the upper part and bidirectional in the lower part of the image (horizontal arrows). See the location of the corresponding wells (C, D, E, and F) in Figure 3B.

Hawaz Facies Association 2: Subtidal Complex

Facies association HWFA2 is mainly composed of lithofacies Sx2, Sx1, Sxl, Sr, Sl, and Sv with subordinate HM (Figure 5). It is organized into stacked packages 0.3–40 m (1–131 ft) thick. The basal contact of these packages is typically erosive, locally marked by the presence of mud clasts (Figure 6B), and the GR response is both clean and blocky (GR values ∼25 API units) locally marked by peaks (up to 65 API units) related to the presence of thin mud drapes or concentrations of mica. These values are within the established range for micaceous sandstones, which could have values of up to 80 API units (Rider, 2004). Bioturbation is scarce to absent, probably related to a very high sediment supply in a relatively short period of time. Paleocurrents, measured in this facies association from image log data, indicate a dominant trend toward the north-northwest with some bidirectionality, probably related to tidal effects as indicated by the mud drapes in lithofacies Sx1, Sx2, and Sr (Figure 6C). However, an additional secondary trend has also been identified, indicating flow toward the northeast. The reservoir quality of this facies association is the best of the entire Hawaz Formation, with an average porosity of 11% and an average horizontal permeability of 125 md.

Facies association HWFA2 is interpreted as an amalgamated complex of sand bars and dunes (slightly coarsening-upward profile with Sx1, Sx2, and Sr lithofacies) and channel deposits (slightly finning-upward profile with Sv, Sl, and Sr lithofacies) influenced by the action of the tides. The interpretation is a laterally extensive fluviotidal to subtidal complex. Subordinate heterolithic intervals are also found intercalated with the cross-stratified sand bars, possibly related to periods of slack water and deposition in relatively protected lagoonal or interbar subenvironments. The features of this facies association are very similar to those described by Ramos et al. (2006) from the Gargaf high 100 km (62 mi) to the north. They are almost equivalent in depositional environment; although in the subsurface of the northern Murzuq Basin, HWFA2 would represent a shallower lateral equivalent with higher fluvial influence because of the general absence of bioturbation reflecting higher energy and sedimentation rates.

Hawaz Facies Association 3: Abandoned Subtidal Complex

Facies association HWFA3 is primarily characterized by lithofacies Sxlb, Sxb, Srb, Sxl, Sv, and Sx2 (Figure 5). It forms packages ranging in thickness from 0.6 to 12 m (2 to 40 ft). Facies packages are distinguished by a fining-upward succession of fine-grained sandstones represented by a distinctive upward increase in the GR characterized by API values between 25 and 70. Bioturbation is moderate, typically becoming more abundant toward the upper part of these successions with common Skolithos and Siphonichnus burrows.

This facies association is interpreted to represent the abandonment of the associated subtidal complex (HWFA2) after a general rise in relative sea level and a cessation or major decrease in sediment supply, promoting colonization in a subtidal setting. It is quite common to find this facies association gradationally intercalated with the subtidal complex, reflecting a transgressional trend in a relatively protected environment.

Hawaz Facies Association 4: Middle to Lower Shoreface

Facies association HWFA4 is mainly composed of lithofacies Sr, Srb, Sxlb, Sxb, Sv, and HSb (Figure 5). The thickness of individual packages ranges between 0.6 and 14 m (2 and 46 ft). The GR response is typically a serrate, coarsening-upward succession with values ranging between 30 and 80 API units (Figure 9). Bioturbation ranges from scarce to moderate. Overall packages of this facies association form clear, coarsening-upward successions with a characteristic Skolithos ichnofacies related to regressive sand belts prograding during highstand sea-level conditions (Gibert et al., 2011). On this basis, the interpretation proposed is of a low- to moderate-energy, middle to lower shoreface setting prograding in a relatively high-energy subtidal environment.

Hawaz Facies Association 5: Burrowed Shelfal and Lower Shoreface

Facies association HWFA5 mainly consists of lithofacies Sb, MSb, and Sxlb (Figure 5). Thickness of individual packages ranges between 0.6 and 33 m (2 and 108 ft). The typical GR log response of this facies association is irregularly serrate, with values between 30 and 80 API units, reflecting a relative increase in the detrital clay content. Bioturbation is moderate to very abundant, tending to overprint and obscure all primary sedimentary structures (Figure 6D).

This facies association is interpreted to have been deposited in a lower shoreface to shelf environment as suggested by the variably clean to argillaceous nature of the sandstones and ubiquitous bioturbation with a well-developed Skolithos ichnofacies.

Hawaz Facies Association 6: Burrowed Inner Shelf

Facies association HWFA6 comprises lithofacies HMb and HSb (Figure 5). The minimum thickness of individual packages is approximately 30 cm (∼1 ft), whereas the maximum value is 15.8 m (52 ft). It may be considered as the distal equivalent of HWFA5 characterized by a spiky GR response characterized by notably higher values ranging from 60 to 120 API units. Bioturbation intensity is moderate, with an ichnofaunal assemblage dominated by the Cruziana ichnofacies.

This facies association is interpreted as having been deposited in a distal burrowed lower shoreface to inner shelf setting based on its heterolithic lithology, Cruziana ichnofacies (Figure 6E), and the occurrence of combined current and wave ripples. This suggests a low-energy, open-marine environment in moderate water depths above storm wave base (SWB).

Hawaz Facies Association 7: Shelfal Storm Sheets

Facies association HWFA7 is mostly composed of lithofacies HS and HM (Figure 5). The thickness of these facies packages ranges from 0.3 to 18 m (1 to 59 ft). It is characterized by a continuously high GR response with values of up to 150 API units or even higher. Where notably high GR peaks occur, these may represent local flooding events interrupting a shallower depositional sequence. This facies association has the lowest reservoir quality in the formation with an average porosity of approximately 5% and an average horizontal permeability of 0.2 md.

It is interpreted to have been deposited in a distal shelf environment on the basis of a high detrital clay content and the occurrence of combined wave and current ripples (Figure 6F). These suggest fluctuating energy levels in broadly very low-energy environment between the FWWB and SWB. This is supported by the generally very low intensity of bioturbation, the occasional occurrence of Chondrites burrows, and shrinkage cracks indicating deposition in a fairly distal, poorly oxygenated setting, perhaps associated with distal waning storm events capable of transporting sand to the open-marine shelf.

When core data were not available for several sections in the studied wells, image log data were key to characterize the seven facies associations previously mentioned (Figure 7).

Figure 7. Representative sections of slabbed cores (40 cm [16 in.]) for each facies association and a typical high-resolution formation microimager (FMI) image (3 m [10 ft] long) showing their main characteristics. From top left to bottom right: Hawaz facies association 1 (HWFA1) tidal flat, HWFA2 subtidal complex, HWFA3 abandoned subtidal complex, HWFA4 middle to lower shoreface, HWFA5 burrowed shelfal and lower shoreface, HWFA6 burrowed inner shelf, and HWFA7 shelfal storm sheets corresponding wells (A, C, D, and E) in Figure 3B.

NONACTUALISTIC SEDIMENTARY MODEL

Ever since James Hutton’s key observations in the late eighteenth century (modified by the work of John Playfair) and, critically, Lyell’s (1832) development of the concept of uniformitarianism in his Principles of Geology, geologists have sought to explain ancient processes by reference to actualistic processes to better understand the sedimentary record.

However, Earth has changed significantly through geological history. Indeed, even from the early Paleozoic until present day, some processes and depositional environments simply cannot be directly compared, because conditions were significantly different. As Nichols (2017, p. 4) certainly points out, if choosing a “present” to be the “key of the past,” probably choosing the most recent present is not the best idea.

After careful study of the Hawaz Formation and the sedimentary processes involved in its deposition, several significant concepts have been developed which require further discussion in this respect (Table 2). First, the lack of fauna and, specifically, flora in subaerial conditions during the Middle Ordovician and more ancient times must have constituted a key controlling factor on depositional processes operating in marginal marine and coastal environments (Kenrick et al., 2015, 2016; Bradley et al., 2018). First, vegetation constitutes a fixing element within the substrate, allowing the stabilization of floodplains and the control of lateral river channel migration (Davies and Gibling, 2010; Davies et al., 2011; Gibling and Davies, 2012), generally lowering the energy and net sediment throughput of the environment. Whereas fluvial meandering systems can be considered a general pattern in continental to marine transitional zones for most present-day cases (with the notable exception of glacial-influenced settings or proximity to high-relief source areas), the lack of vegetation in the Middle Ordovician would have almost certainly contributed to maintaining high-energy levels in the sedimentary system as far as the coastal plain, characterized by laterally extensive braided floodplains (Table 2).

The other remarkable aspect worthy of note is the effect of vegetation on the generation of clay minerals (Table 2). Many Precambrian to Ordovician clastic deposits are characterized by their low claystone or detrital clay content. One of the reasons for this may be the absence of vegetation and the resultant enhanced chemical weathering on land surfaces. The generation of clays by weathering was significantly less than at the present time, and therefore, the availability of clays in the source areas, including potentially erodible rocks, was also less for the same reason. Other mechanisms for inputting a clay fraction into the depositional environment may be associated with hydrothermal processes, diagenesis, or volcanic ash deposits; the latter has been identified by M. Marzo and E. Ramos (2003, personal communication) and Ramos et al. (2006).

This is indeed what we see in the upper part of the Hawaz Formation, typically comprising a package of sand-prone tidal flat deposits with very few clear claystone intervals, accumulating in a restricted low-energy environment where, in a modern system, vegetation would fix finer sediments at the very top of this kind of depositional succession. Furthermore, the possibility of a clay input of volcanoclastic origin should not be ruled out, because Ramos et al. (2006) highlight the presence of K-bentonite layers within the Hawaz Formation as observed in outcrops.

Second, in line with Nichols (2017), the climate factor related to periods of greenhouse and icehouse is also key in understanding how coastal environments have evolved. Given that the last few million years of geological history are considered as an icehouse period, some processes related to the characteristic low relative sea levels are clearly not equivalent to those produced during greenhouse periods, because much of the Cambrian–Ordovician actually was. The relative sea level, during much of the Ordovician (at least until the onset of the Hirnantian glaciation), was probably tens of meters higher than at present time, which, in the case study, would represent a very extensive area of land flooded across a very low relief cratonic margin (Table 2). Thus, confined estuary systems produced by incised valleys during sea-level drop are not expected in this setting. This discussion can be applied to the depositional model of the Hawaz Formation. As such, classical estuarine environments are inherently unlikely. Indeed, conventional lowstand systems tracts would be, in any case, extremely difficult to identify, because major erosive features related to sea-level drop would not be produced in this low-gradient, cratonic transitional setting.

Third, it is also relevant to our study that tidal range has not been constant through the whole of Earth’s history. Tides are largely controlled by differential gravitational forces exerted between Earth and the moon, but the distance between both bodies has changed through time at a currently calculated rate of 3.8 cm/yr (1.5 in./yr) (Odenwald, 2018), entailing an average Earth–moon distance of 367,000 km (228,000 mi) as opposed to 384,000 km (238,000 mi) today. Tidal-energy dissipation over time is thus a well-established process reflected in the increasing length of the day and thus number of days per year. This appears to be a purely linear process reflecting the progressive slowing of Earth’s rotation and the associated outward spiraling of the moon. Thus, a day in the Ordovician is calculated to have been 21 hr long and the year 414 days long. For our purposes, it is also true that the potential sediment load of nearshore tidal currents together with their depositional effectiveness are related directly to the tidal range or maximum tidal height (Williams, 2000), itself, controlled by global tidal forces, water depths, and local topography. In general, therefore, we can assume notably higher tidal ranges and more powerful tidal currents during the deposition of the Hawaz Formation. Going further, we may also assume that, in the case of the upper Hawaz Formation, for example, even very small variations in tidal range in such low-gradient depositional environment would result in a significant increase in the areal extension of marginal or paralic, tidally influenced environments (Table 2).

Fourth, ichnofacies are commonly related to sedimentary environments and, particularly in tidal settings, specific parameters exist such as salinity, depositional energy, sediment grain size, and sedimentation rates that control fauna colonization (Gingras, et al., 2012). However, some ichnological assemblages exist that may also have a chronostratigraphic value when looked at on the basis of bioturbation intensity and lateral extent. A very good example is the lower part of the Hawaz Formation and the underlying Lower Ordovician Ash Shabiyat Formation, which are characterized by their distinctive “pipe rock” or high-density burrowed Skolithos ichnofabric. Similarly, the association of this suspension-feeding fabric, commonly overprinting a deposit feeding burrowing characterized by common trilobite traces, and thus a “true” Cruziana ichnofacies is distinctive. Some, if not many or even all, of the organisms responsible for these ichnofabrics are already extinct (Table 2). Thus, the occurrence of these ichnofacies in such a very low gradient, cratonic platform is highly unlikely in the present day.

After these comments, it is also worthwhile considering that the geomorphology of clastic coastal depositional environments is closely linked to the relative influence of waves and tides along the coastline (Harris and Heap, 2003); their evolution is controlled by three main factors: sediment supply, physical processes (river currents, tidal currents, and waves), and relative sea-level variation (Boyd et al., 1992; Dalrymple, 1992, Dalrymple et al., 1992; Harris et al., 2002).

Thus, taking all of this into account with and applying it to the study data set in the area, a nonactualistic depositional model is proposed for the Hawaz Formation based upon modern sedimentological criteria but constrained and adapted to Middle Ordovician environmental conditions (Figure 8).

Figure 8. Evolutionary sedimentological model for the deposition of the Hawaz Formation. (A) Early transgressive systems tract highlighting embayments. (B) Late transgressive systems tract. (C) Highstand systems tract. The main facies associations are represented in the sketches. The sketches are purely conceptual but consistent with observed trends in the study area, but not geographically tied to well data. msl = mean sea level.

It was a constantly evolving tide-dominated environment, evolving from a relatively open-marine setting characterized by mixed storm tide–dominated deposition toward a more protected subtidal to intertidal setting on an embayed coastline. This promoted tides as the dominant controlling factor on sedimentation process, supported by the vertical arrangement or stacking of facies associations. It shows a lower shoreface to shelf environment with sandy storm sheet deposits present across much of the basin. Above this lower interval, a laterally extensive and fluviotidal to subtidal complex comprising of tidal channels and bars developed across the study area (Figure 8A). The distal part of this subtidal complex eventually became abandoned as sea level rose, creating a system of lagoons and barrier islands (not clearly identified in the subsurface) (Figure 8B). Finally, prograding tidal flats developed during a relative high-sea-level stage (Figure 8C).

From subsurface paleocurrent data, it is apparent that the depositional system evolved from a coastal environment in the south-southeast to fully marine environments toward the north-northwest. The data show only limited dispersion defining a clear depositional trend from southeast to northwest with strong ebb current indicators. These data are in accordance with those of Ramos et al. (2006) from outcrops in the Gargaf high. Evidence of bidirectional current indicators in primary sedimentary structures is, however, hard to observe. Although the presence of this kind of feature would strongly support an important tidal influence, it is not always present in many tidal deposits. However, no evidence for a seasonally controlled river have so far been found in the succession, which would help to preserve this type of reverse flow structure during periods of low fluvial regime (Dalrymple and Choi, 2007). However, the presence of clay drapes in most of the lithofacies described does strongly support an important tidal effect throughout the depositional system.

SEQUENCE STRATIGRAPHY AND ZONATION OF THE HAWAZ FORMATION

The purpose of this section is to recognize and correlate stratigraphic surfaces representing changes in depositional trends and to interpret the resulting stratigraphic units bounded by these surfaces.

The key bounding surfaces splitting genetic sedimentary packages were recognized using a material-based sequence stratigraphic approach (Embry, 2009). The defined surfaces are the following.

  • Maximum regressive surface, in which a conformable horizon marks a change from coarsening and shallowing upward to fining and deepening upward.
  • Maximum flooding surface, in which a conformable horizon marks a change from fining and deepening upward to coarsening and shallowing upward and is normally represented by the highest clay content in the succession.
  • Shoreline ravinement unconformity, in which a clear erosive surface is overlain by brackish marine deposits and which represents erosion in the stratigraphic unit produced by wave and tidal currents during an early transgressive stage just after a base level fall.
  • Regressive surface of marine erosion, in which, in an overall regressive succession, a clear change exists in depositional trend with shelfal deposits abruptly overlain by prograding shoreface deposits. As suggested by Embry (2009), this last surface is not a suitable surface for correlation because of its highly diachronous nature, so it has not been used as a main bounding surface for our sequence stratigraphic framework. However, locally, it may be of use in explaining trend changes in the facies succession observed in some wells.

Several low-order and numerous high-order sequences can be recognized in the stratigraphic record of the Hawaz Formation (Figure 9); but, after analyzing the evolution or stacking of the facies associations in each well, it is possible to erect a simplified scheme with three major depositional sequences (DS1–DS3) and five Hawaz reservoir zones (HWZ1–HWZ5), each defined by key, correlatable, genetic, material-based surfaces (Figure 9).

Figure 9. Composite section of a well showing a synthetic stratigraphic column of the Hawaz Formation, the wire-line log responses, the suggested zonation for the reservoir based on the facies associations, and sequence stratigraphic framework. The transgressive and regressive stacking patterns are represented on the figure together with the three main depositional sequences. See the location of the corresponding well (well B) in Figure 3B.

The top of the Ash Shabiyat Formation is marked by a sharp or slightly more gradational shift from the blocky, low GR response, characteristic of this formation, to a notably more spiky or serrate GR response typical of much of the lower Hawaz. This shift is interpreted not only as a maximum regressive surface, but also as a sequence boundary. As such, it is a compound surface and might be considered in terms of marine erosion as a ravinement, which marks the base of DS1 (Figure 9).

The overlying HWZ1 is broadly transgressive in character, comprising stacked fining-upward parasequences (including a regionally distinctive and extensive abandoned subtidal complex) capped by a regional flooding surface (Figure 9), and, finally, a cleaning-upward, progradational parasequence or parasequence set.

The boundary between HWZ1 and HWZ2 is marked in all the wells by an abrupt change in lithology to more argillaceous facies, recording a marked deepening in the basin. This is an excellent and consistent correlatable surface but is not fully genetic, because the maximum flooding surface of the DS1 only rarely coincides with the lithological change and is, instead, typically picked a short distance above the shift at the highest GR peak in the well (Figure 9).

The maximum flooding surface defines the onset of the highstand systems tract (HST) of DS1, which coincides completely with the zone HWZ2. This can commonly be divided into two subzones (HWZ2a and HWZ2b) separated by a regressive surface of marine erosion (Figure 9), created by the cut of waves and tides in the lower shoreface during the regression of the shoreline. This surface separates a dirty sandy package from a cleaner sandy package within a coarsening-upward parasequence or parasequence set, as suggested by the GR response and facies analysis. However, this surface is not easily recognizable in all wells and has not been used as a regional correlative surface because of its probable diachronous nature.

The HST of DS1 is truncated by an erosive surface interpreted as a shoreline ravinement unconformity (Figure 9) generated by the action of wave and tidal currents during an early transgressive stage just after a base level fall and probably enhanced by an allocyclic trigger mechanism, perhaps tectonics related. This surface would also be a sequence boundary and would correspond with the onset of the DS2 and the base of zone HWZ3, the main reservoir section of the Hawaz Formation. The facies association immediately overlying this key boundary is commonly HWFA2 (subtidal complex), considered to represent an early transgressive systems tract (TST) equivalent to zone HWZ3. Locally, this zone shows minor higher-frequency flooding surfaces mostly composed of heterolithics (Figure 9). These flooding surfaces could be interpreted as condensed lagoonal deposits, but the lack of biostratigraphic data in this sand-prone package suggests we should treat this hypothesis with caution, although the presence of these subenvironments should not be rejected. Tidal inlet storm deposits or IHS could also be a plausible option, considering the broad general subtidal setting of this zone.

The boundary between zones HWZ3 and HWZ4 is marked by a change in depositional environment from a subtidal to intertidal setting. This boundary would be close to the maximum flooding surface; after which, the tidal flat would prograde infilling the available space (bay infilling) under a forced regression pattern, whereas farther to the north barrier island deposits (observed in Gargaf outcrops by Ramos et al. [2006]) would most likely have limited the connection to the open sea.

Zone HWZ4 comprises stacked fining-upward parasequences mainly formed by tidal sand to mixed flat deposits cut by tidal creeks (Figure 9). Similar processes have been highlighted by Desjardins et al. (2012) in the lower Cambrian Gog Group of the Canadian Rocky Mountains where tidal flats are forced to regress in response to falling sea level in tide-dominated settings.

Above zone HWZ4, the depositional trend changes again, and GR values begin to decrease in response to increasingly abundant, cleaner sand deposits. No evidence of sharp changes exists either in lithology or in conventional log responses, suggesting no major unconformity exists. However, some subtidal packages are preserved sometimes at the very top of the Hawaz Formation, which would denote a new transgression. Thus, the boundary between HWZ4 and HWZ5 is considered to be a compound maximum regressive surface and sequence boundary that would constitute the beginning of a rarely preserved DS3 (Figure 9). Zone HWZ5 is commonly eroded and overlain by the Upper Ordovician formations or the base of the Silurian.

DISCUSSION

Following Boyd et al. (1992) and Dalrymple et al. (1992), clastic coastal depositional environments are classified on a ternary diagram, summarizing the main factors (rivers, waves, and tides) controlling the geomorphology of linear shorelines, deltas, or estuaries. This is a very useful and powerful tool in actualistic or near-actualistic systems; but in many cases, it might be hard to apply to very ancient coastal to shallow marine depositional systems, notably those of the Precambrian to lower Paleozoic because of major differences in Earth surface dynamics. Nevertheless, whereas some of these ancient depositional systems lack obvious modern analogs, some features remain comparable with modern environments. A detailed interpretation from subsurface cores and logs highlights the major depositional and paleogeographic factors responsible for the Middle Ordovician Hawaz Formation of the northern Murzuq Basin. The resultant seven correlatable facies associations (HWFA1–HWFA7) and the robust sequence stratigraphic framework suggest that the Hawaz Formation was deposited in an intertidal to subtidal environment prograding from south to north. The facies associations and their linked ichnogenera suggest that water depths are unlikely to have exceeded several tens of meters, with the sea floor above SWB at most locations.

Considering the significant areal extent, not only of the Hawaz Formation across the Murzuq Basin, but also its lateral equivalents in both Kufra and Illizi Basins, which lack the key unburrowed cross-bedded sandstones (McDougall et al., 2008, 2011) typical of the subtidal complex described in this work, it is clear that deposition occurred in and on the margins of an epeiric sea characterized by a very low bathymetric relief and very broad facies belts tracts. Dalrymple and Choi (2007) suggest fluviotidal transition zones may range in width up to hundreds of kilometers (hundreds of miles) in low-gradient settings, as would indeed be the case for the northern margin of Gondwana during the Middle Ordovician. In such environments, small changes in relative sea level would be sufficient to cause major lateral shifts in facies belts. These small changes occurred during a greenhouse period with relatively high global sea levels. No evidence exists of incised valley systems within the Hawaz succession, suggesting global sea level remained relatively high through its deposition. As such, lowstand systems tract facies could not be observed either in the Gargaf high outcrops (Anfray and Rubino, 2003) or in the subsurface of the Murzuq Basin.

During the initial stages of sea-level rise (TST), coastal areas were slowly flooded, producing subtidal sedimentation associated with fluvial discharge along embayed coastlines, presumably because of flooding of braided fluviotidal systems, whereas during stages of high sea levels (HST), the shoreline migrated seaward, resulting in the progradation of tidal-wave influenced strand plains, beaches, or deltas associated with gentle lobate to linear coasts. The embayed morphology of coastal areas was probably enhanced by tectonism, which controlled the size and subsidence of the basin, generating a large-scale depressed area elongated in an approximately north-south direction (Klitzsch, 2000). Such a large-scale embayment characterized by a very low gradient probably increased tidal power (Ramos et al., 2006).

The vertical stacking of the facies association packages was principally controlled by eustasy, as suggested by the presented zonation. However, other secondary factors exist that almost certainly acted to control the evolution of sedimentation in these coastal and shallow marine environments, notably subsidence and sediment supply (Dalrymple, 1992; Dalrymple et al., 1992; Walker and Plint, 1992; Johnson and Baldwin, 1996).

Given that this environment was characterized by a very low gradient, it is possible that sedimentation was controlled by a preexisting paleorelief expressed as complex lobate to linear shoreline. The low gradient of this depositional system impeded the development and identification of well-defined clinoforms both in outcrops and in seismic images. What is evident is the significant influence of tidal processes in these deposits with a preferential paleocurrent direction toward the north-northwest according to both outcrop (Ramos et al., 2006) and FMI data from wells showing some bidirectional current indicators in some cases. In addition, strong evidence also exists for a secondary paleocurrent dispersal system flowing toward the northeast which requires further study.

Several depositional models have been proposed for the Hawaz Formation. Vos (1981) suggested a fan delta complex as the more likely setting, whereas other scientists, including Ramos et al. (2006), have argued for deposition within a megaestuary or tidal gulf setting. The current study strongly suggests that the Hawaz Formation cannot be compared with any present-day coastal environment. The clear tidal influence observed in the system and the vertical stacking of facies associations highlight the evolution of a shallow marine environment from a subtidal to an intertidal setting accompanied by parallel evolution of ichnofacies and fossil content (Figure 10).

Figure 10. Three-dimensional conceptual sketch of a coastal, tidal-influenced environment analog to the Hawaz Formation deposition during a highstand systems tract stage, grading from a braided coastal plain environment in the most proximal part of the sedimentary system to intertidal and subtidal environments and lower shoreface to inner shelf settings. Note the clear relationship between the ichnofacies assemblage and the energy of the depositional environment. From left to right: (A) mixed Cruziana and Skolithos (Sk) ichnofacies assemblage with characteristic vertical suspension feeder burrows of Sk overprinting an ichnofabric comprising horizontal deposit feeders and miners such as Thalassionides (Th) and Planolites (Pl) associated with tidal flat deposits. (B) Characteristic Sk pipe rock ichnofacies with typical Siphonichnus (Si) burrows from lower shoreface to burrowed shelfal deposits. (C) Mixed Cruziana and Sk ichnofacies assemblage, from burrowed inner shelf sediments with characteristic Teichichnus (Te), Th, and Sk burrows. (D) Heterolithic mudstones belonging to the most distal storm deposits with Chondrites (Ch) burrows characteristic of the distal Cruziana ichnofacies. See the location of the corresponding well (wells E and D) in Figure 3B.

The presence of some ichnogenera, such as Chondrites, in heterolithics from the most distal facies associations HWFA6 and HWFA7, compared with those deposited in the most proximal association HWFA1, suggests that a different setting for the lower (DS1; HWZ1 and HWZ2) and upper (DS2 and DS3; HWZ3–HWZ5) parts of the Hawaz Formation should be considered. Gibert et al. (2011) concluded that the restricted and uncommon ichnofacies assemblage in the upper part of the Hawaz was not clear. A mixed Cruziana and Skolithos ichnofacies has been observed both in the subsurface and in outcrops; the latter showing many excellent examples of trilobite traces (Ramos et al., 2006; Gibert et al., 2011). Some scientists have realized that, although trilobite tracks typical of the Cruziana ichnofacies are commonly regarded as indicators of open-marine offshore to nearshore settings, their presence in heterolithic facies can no longer be taken as an absolute indicator of deposition in subtidal settings in the early Paleozoic, and indeed, they may have been notably more common within intertidal deposits than currently envisioned (Mángano et al., 2014). The nonactualistic sedimentary model presented in this study incorporates this observation so that the Cruziana ichnofacies is also considered a common characteristic element of shallow tidal flat settings (Figure 10).

CONCLUSIONS

Where encountered in the subsurface of the northern Murzuq, the Hawaz Formation is represented by a clastic succession mainly comprising fine- to locally medium–grained quartzarenites and subarkosic arenites, with subordinate sublithic arenites, up to 210 m (690 ft) thick. Fifteen major lithofacies, comprising sandstones and heterolithics, have been recognized and grouped into seven correlatable facies associations. These include the following: (1) tidal flat (HWFA1); (2) subtidal complex (HWFA2); (3) abandoned subtidal complex (HWFA3); (4) middle to lower shoreface (HWFA4); (5) burrowed shelfal and lower shoreface (HWFA5); (6) burrowed inner shelf (HWFA6); and (7) shelfal storm sheets (HWFA7), all deposited within the framework of an intertidal to subtidal setting.

A clear relationship exists between facies and reservoir quality for the Hawaz Formation. The best reservoir quality sandstones are those comprising facies association HWFA2 (subtidal complex) with an average porosity of 11% and horizontal permeability of 125 md and general absence of thick mud drapes and interlayered claystones.

The depositional model for the Hawaz Formation cannot be compared with an actualistic sedimentary analog because of the major differences stemming from the following: (1) the absence of fauna and especially flora in subaerial environments, which directly determines coastal dynamics; (2) the difference in relative sea level and its control on erosion in shallow marine settings together with the low-gradient depositional setting, which promoted very wide facies belts compared with most present-day moderate- to high-gradient depositional systems; (3) the difference in tidal ranges reflecting the progressive change in the distance between Earth and the moon, and finally; (4) the characteristic ichnofacies observed in the Hawaz are not present in modern environments.

The Hawaz Formation can be divided into three main depositional sequences (DS1–DS3), each with characteristic systems tracts bounded by key surfaces: maximum regressive surface, maximum flooding surface, and unconformable shoreline ravinement surface.

Based upon this systems tracts architecture, a genetic zonation composed of five zones has been proposed (HWZ1 to HWZ5). This new stratigraphic zonation should serve as a useful tool to improve the management in oil production from the Hawaz Formation. The Hawaz Formation extends laterally hundreds of kilometers away from the study area, forming an excellent regional reservoir across the Murzuq and southern Ghadames (Berkine) Basins and, to a lesser extent, as the laterally equivalent unit III in the Illizi Basin. The facies schemes, depositional model, and zonation framework proposed here should also be applicable to existing or potential Hawaz reservoirs elsewhere within this larger region.

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ACKNOWLEDGMENTS

Special thanks are due to the Libyan National Oil Corporation, notably Bashir Garea and Khaeri Tawengi, and partners Total, Equinor, and OMV both for the technical support in this project and permission to publish the results. Special thanks are also due to Edward Jarvis of CGG Robertson for discussions concerning the key core descriptions. Thanks are also due to colleagues Manuel Ron, Javier Buitrago, Mikel Erquiaga, Lamin Amh Abushaala, and Mourad Bellik for introduction to the project and their continuous help throughout the study. Special thanks to Eduard Remacha and Francisco Pángaro for constructive discussions in the field and in the office. Support from the Ministerio de Economía y Competitividad (Project Sedimentary Sediment Routing Systems: Stratigraphic Analysis and Models CGL2014-55900-P) and Generalitat de Catalunya (2014SGR467) is gratefully acknowledged. Thanks to reviewers David Boote and Jonathan Redfern and also to the editors of the AAPG Bulletin for their constructive comments that have improved the content of this paper.

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