Late Cretaceous–to–present-day mixed carbonate–clastic deposition along the Nicaraguan platform, western Caribbean Sea, has evolved from a tectonically controlled, rifted upper Eocene shallow–to–deep-marine carbonate–siliciclastic shelf to an upper Miocene–to–present-day tectonically stable shallow-marine carbonate platform and passive margin. By integrating subsurface data of 287 two-dimensional seismic lines and 27 wells, we interpret the Cenozoic stratigraphic sequence as 3 cycles of transgression and regression beginning with an upper Eocene rhodolitic–algal carbonate shelf that interfingered with marginal siliciclastic sediments derived from exposed areas of Central America bordering the margin to the west. During the middle Eocene, a carbonate platform was established with both rimmed reefs and isolated patch reefs. A late Eocene forced regression produced widespread erosion and subaerial exposure across much of the platform and was recorded by a regional unconformity. The Oligocene–upper Miocene sedimentary record includes a southeastward prograding delta of the proto-Coco river, which drained the emergent area of what is now northern Nicaragua. The late Miocene–to–present-day period marks a period of strong subsidence with the development of small pinnacle reefs. We describe favorable petroleum system elements of the Nicaraguan platform that include (1) Eocene fossiliferous limestone source rocks documented as thermally mature in vintage exploration wells and seen as active gas chimneys emanating from inferred carbonate reservoirs; (2) upper–to–middle Eocene reservoirs in patch and pinnacle reefs, middle Eocene calcareous slumps, and Oligocene fluvial-deltaic facies documented in wells; and (3) regional seal intervals that consist of both regional unconformities and Eocene–Oligocene intraformational shale.
The Nicaragua Rise is a broad shallow-water Upper Cretaceous–Cenozoic platform that extends across the northern Caribbean Sea from Central America to Jamaica (Figure 1). The Nicaragua Rise is bounded on the north by the Cayman Trough and on the southeast by the Hess Escarpment (Figure 1). The shallow-water Nicaraguan carbonate platform ranges in depth from 2 to 61 m (7 to 200 ft) below sea level and comprises an area of approximately 54,700 km2 (∼24,300 mi2) or 10.2% of the Nicaragua Rise. The Nicaraguan platform consists of a 1.5–7-km (4921–16,404-ft)-thick succession of Cenozoic shallow–to–deep-marine carbonate with some fluvial influences (Muñoz et al., 1997).
Figure 1. A regional map of the Nicaragua Rise showing the (1) location of study area, (2) regional faults and tectonic features, (3) locations of wells used in the study, (4) location of two-dimensional seismic data used in the study, and (5) oil and gas shows in exploration wells modified from Carvajal-Arenas et al. (2015). Blue lines indicate seismic line locations as previously interpreted by Carvajal-Arenas et al. (2015).
Petroleum exploration of the Nicaraguan offshore of the Nicaragua Rise started in the 1940s with Gulf Oil drilling of the Gulf Oil 1 Twara and Gulf Oil 1 Punta Gorda wells near the Nicaraguan coastline (Figure 2). By the end of the 1970s, major American companies, including Shell, Chevron, Union Oil, Texaco, and ExxonMobil (known as Mobil Exploration at the time, but its Esso subsidiary also had a significant concession of the Nicaraguan platform) had drilled 27 offshore wells in water depths ranging from 15 to 51 m (50 to 169 ft) that mainly targeted four-way closures within reefal and anticlinal traps. By the beginning of the 1980s, Occidental Petroleum had drilled the Miskito wells that targeted reefal and anticlinal four-way closures. Occidental Petroleum 2 Miskito was drilled on a positive seismic reflection anomaly, which was revealed to be a Miocene dike intruding an Eocene sedimentary section (Carvajal-Arenas, 2017).
Figure 2. Location map of study area showing major faults, structural highs, major Cenozoic depocenters, present-day carbonate banks, and other prominent bathymetric features. Oil and gas shows from exploration wells are modified from Carvajal-Arenas et al. (2015).
The majority of the 28 wildcat exploration wells drilled on the western Nicaragua Rise have reported widespread hydrocarbon shows and, in some cases, recovered volumes of noncommercial oil and gas (Figure 2; Table 1). Limited production wells include the Unocal 1 Tuapi well, which flowed at a rate of 119 bbl light oil per day in 1978 (42°–45°API) (Carvajal-Arenas et al., 2015), and the Waterford Oil 1 Touche well, which had a 1957 blowout with a maximum pressure of 3800 psi (26.2 MPa) (Hoylman and Chilingar, 1965; Muñoz et al., 1997) (Table 1). The Waterford Oil 1 Touche well blowout indicated the existence of an active petroleum system that has remained untested (Muñoz et al., 1997; Carvajal-Arenas et al., 2015).
Table 1. Hydrocarbon Occurrences on the Nicaraguan Platform Identified from 11 Wells and Outcrop Data, with Source-Rock Data
(Depth Interval, Total Organic Carbon, Vitrinite Reflectance, Biostratigraphy, and Formation Name) Modified from Carvajal-Arenas et al.
From the 1980s to the present, there has been a three-decade-long hiatus of exploration on the Nicaraguan platform with the exception of Noble Energy’s deep-water well in 2013 (Noble Energy 1 Paraiso Sur well) drilled in 370 m (1214 ft) of water to test a reef target in the Nicaraguan platform (Bunge et al., 2016). This well encountered excellent quality reservoir associated with Eocene heterozoan reefs but encountered no significant hydrocarbon accumulation (Bunge et al., 2016). The reasons for failure of the Noble Energy 1 Paraiso Sur well according to Bunge et al. (2016) included one or more of the following: (1) failure of containment or the lack of charge from the source rock, (2) late charge timing, (3) poor conduit for migration, and (4) breached seal.
To better understand the hydrocarbon frontier area of the western Nicaragua Rise, we used all available exploratory information, which includes an extensive compilation of well logs, lithological descriptions, and two-dimensional (2-D) seismic data dating back approximately half a century to construct a Cenozoic sequence stratigraphic framework for the Nicaraguan platform in Nicaraguan waters. This subsurface information was integrated with plate tectonic reconstructions by Rogers et al. (2007), Escalona and Norton (2015), and Sanchez et al. (2015, 2019) to understand how the regional counterclockwise rotation of the Chortis block and western Caribbean plate affected mixed carbonate–clastic sedimentation, paleogeography, and petroleum systems of the Nicaraguan platform since the Late Cretaceous. We reevaluated and improved the geologic history of the Nicaraguan carbonate platform, which recorded various influences, including climate, global sea level, variations in clastic sediment supply, oceanic circulation, and regional topographic changes. The objective of this paper is to use the principles of sequence stratigraphy and the plate tectonic framework to better understand the source, reservoir, and seal rock elements of the inferred petroleum system of the Nicaraguan platform.
TECTONIC AND GEOLOGIC BACKGROUND
The western Nicaragua Rise is composed of several crustal provinces that include (1) the Chortis block underlain by continental crust beneath the shallower upper Nicaragua Rise in the Honduran offshore area (Venable, 1994; Sanchez et al., 2015); (2) the accreted oceanic arc system known as the Siuna terrane (Venable, 1994; Flores et al., 2007; Rogers et al., 2007; Lewis et al., 2011), or, alternatively, the Mesquito composite oceanic terrane (MCOT) (Baumgartner et al., 2008) and the “Great Arc of the Caribbean” (Ott, 2016); and (3) transitional to oceanic pre-Cenozoic thinned continental or oceanic plateau crust underlying the lower Nicaragua Rise in the offshore area of Nicaragua (Pindell and Kennan, 2009; Carvajal-Arenas and Mann, 2018) (Figure 3A).
Figure 3. (A) Regional map of the western Nicaragua Rise showing crustal provinces in the study area and (B) table summarizing locations and rock types described in outcrops and wells in Nicaragua that include metamorphic rocks (true basement), exhumed intrusive igneous rocks, and interbedded volcanic and sedimentary rocks commonly associated with the imaging limit of the seismic reflection lines.
The Nicaraguan platform is located along the projected offshore trend of the Siuna terrane (Venable, 1994), which is a subarea of the MCOT (Baumgartner et al., 2008). All these terranes are composed by a series of accreted terranes of Mesozoic intraoceanic island arc or accretionary prism affinity of Pacific origin with diverse blocks of igneous and sedimentary origin overprinted by various metamorphic events (Baumgartner et al., 2008; Flores et al., 2015). In the locality of Siuna in northern onshore Nicaragua, a part of the Siuna terrane–MCOT outcrops known as the Siuna serpentinite mélange (SSM), which corresponds to a typical Early Cretaceous subduction zone mélange of an older Jurassic oceanic arc (Flores et al., 2007) (Figure 3B). The SSM is composed by a matrix of serpentinite and blocks of metavolcanic rocks, metasediment, metabasite, and metasomatite (Flores et al., 2015). The Siuna terrane is unconformably or tectonically overlain by a Cretaceous (Albian to Maastrichtian?) volcano-sedimentary succession similar to the overlap succession of the Chortis block in Honduras (Flores et al., 2007; Rogers et al., 2007) and observed offshore using marine geophysical data on the northern Nicaragua Rise (Sanchez et al., 2015) (Figure 4A). To the south of the Siuna terrane–MCOT in onshore Nicaragua, close to the border with Costa Rica, the El Castillo mélange has been defined by Baumgartner et al. (2008) as a radiolarite block that is tectonically embedded in serpentinite with the oldest fossil recovered so far to be of the latest Triassic (Figure 3A).
Figure 4. (A) Generalized lithostratigraphic chart of the northern and southern Nicaraguan platform summarizing its petroleum systems, major tectonic events, and eustatic sea-level curve modified from Haq et al. (1987), Muñoz et al. (1997), Carvajal-Arenas et al. (2015), and Flores et al. (2015). Locations of the regional seismic lines and wells on which the stratigraphic columns of the northern and southern sections were based are shown in Figures 4 and 5, respectively. (B) Tectonic reconstructions of the Caribbean region from the Late Cretaceous to present day modified from Escalona and Norton (2015) and Sanchez et al. (2015) showing major events in the study area: (1) translation of the CLIP by left-lateral displacement of the HEFZ and collision of the Chortis block (Chortis) against the Siuna terrane (Siuna) with subsequent counterclockwise rotation of the larger, sutured area, (2) intraplate extension and formation of the Eocene Verolania rift, and (3) eastward migration of extension and formation of the Miocene–to–present-day San Andres rift as a pull-apart basin between left-lateral strike-slip faults. EOC = Eocene; J = Jurassic; J3 = Late Jurassic; K1 = Late Cretaceous; K2 = Early Cretaceous; MIO = Miocene; OLI = Oligocene; PAL = Paleocene; PLIO = Pliocene; Yucatan = Yucatan Basin.
The Siuna terrane is equated by most previous scientists to the Great Arc of the Caribbean because of its island-arc affinity and similar lithologic and geochemical compositions (Venable, 1994; Lewis et al., 2011; Sanchez et al., 2015; Ott, 2016). In this study, we consider that the Upper Jurassic–Lower Cretaceous Siuna terrane–MCOT is the metamorphic and igneous basement of island arc and oceanic plateau affinity underlying the Cenozoic sedimentary section of the Nicaraguan platform in which Upper Cretaceous–Paleocene volcanism is associated to the Great Arc of the Caribbean (Rogers et al., 2007; Lewis et al., 2011; Ott, 2016) (Figures 3, 4A).
The complex Late Cretaceous–to–present-day stratigraphic evolution of the Nicaraguan platform was influenced by several tectonic events.
1. Late Cretaceous–to–Paleocene arc-continent collision of the Siuna terrane–MCOT with the Chortis block that juxtaposed the Great Arc of the Caribbean, the Caribbean large igneous province (CLIP), and the Chortis block, resulting in the formation of the Colon fold-thrust belt in Honduras (Rogers et al., 2007), inversion and basin formation along the Nicaraguan platform (Sanchez et al., 2015), and development of a mature Cretaceous volcanic arc (Flores et al., 2006). The collisional event also resulted in Late Cretaceous–Paleocene volcanism in the region and contributed to counterclockwise rotation of the Chortis block (Avila et al., 1984; Mann, 2007; Rogers et al., 2007; Sanchez et al., 2015) (Figure 4B).
2. From Paleocene to early Eocene, a pattern of west-to-eastward–younging north-south rifts containing intermediate to basic volcanic rocks marks the tectonic transition from collision and assembly of the terranes of the western Caribbean plate to eastward strike-slip motion, formation of the Cayman Trough pull-apart basin, and widespread east-west intraplate extension across the northwestern Caribbean plate (Figure 4B). The oldest (Paleocene) rifts are located in Jamaica and associated with bimodal alkaline and calcalkaline volcanism (Mann and Burke, 1990; Ott, 2016), whereas the younger (early Eocene) rifts are located in the southern Nicaraguan platform (e.g., the Verolania rift) and are associated with Paleogene bimodal volcanism (Figure 3B).
3. A middle Eocene–to–late Miocene thermally driven sag phase followed the early Eocene rifting event on the Nicaragua Rise. This thermal sag occurred above rifts in the Jamaica area (Mann and Burke, 1990; Ott, 2016) and above the subsided fold-thrust belt on the upper Nicaragua Rise off the coast of Honduras (Sanchez et al., 2015).
4. A late Miocene–to–present-day phase of rifting and rift-shoulder uplift caused by east-west extension along the San Andres rift in the lower Nicaragua Rise (Figure 4B). The recently active San Andres rift formed in the Miocene as a left-stepping pull-apart basin in the left-lateral Pedro Bank fault zone (Carvajal-Arenas and Mann, 2018) (Figure 2). Miocene–to–present-day opening of the San Andres rift was accompanied by an east-to-west migration of widespread Miocene–to–present-day basaltic alkaline volcanism that includes basaltic volcanos on Providencia Island on the lower Nicaragua Rise (Concha and Macía, 1993, 1995) and Corn Island on the southern end of the Nica ridge (Ryan and Zapata, 2003) (Figure 2). A final pulse of Pliocene extension triggered a younger phase of alkali volcanism at Pearl Lagoon and Cukra Hill in onshore Nicaragua (Wadge and Wooden, 1982; Carvajal-Arenas and Mann, 2018).
Review of Stratigraphic Data from the Nicaraguan Platform and Adjacent Land Areas
Sedimentation along the Nicaraguan platform started in the Early Cretaceous with deposition of deep- to shallow-water shelf limestone and interbedded volcanic rocks of the Aptian–Albian Atima Formation, now exposed onshore in northwestern Nicaragua (Venable, 1994; Rogers et al., 2007; Baumgartner et al., 2008; Flores et al., 2015) (Figure 4A). The Aptian–Albian section rests unconformably on the oceanic Siuna terrane–MCOT and is composed of planktonic foraminifera-bearing hemipelagic shale and distal volcaniclastic turbidites (Flores et al., 2007, 2015; Rogers et al., 2007; Baumgartner et al., 2008). In offshore Honduras, a 1300-m (4300-ft)-thick Aptian–Albian carbonate section was penetrated by the Caribe 1 well (Emmet et al., 2011; Sanchez et al., 2015), similar to the section described by Flores et al. (2015) in northern onshore Nicaragua.
The uppermost section of the Atima Formation corresponds to shallow-water limestone with a rich assemblage of oysters, solitary corals, and sponges in a dark, organic-rich micritic matrix, suggesting low-energy eutrophic conditions (Flores et al., 2006). The Atima Formation is overlain by the shaly carbonate equivalent of the Albian–Cenomanian Krausirpe Formation in Honduras (Mills and Hugh, 1974; Rogers et al., 2007; Flores et al., 2015; Sanchez et al., 2015) that is also called the Gaure Limestone by Emmet et al. (2011) (Figure 4A). According to Erlich et al. (2003), unproven Cretaceous source rocks of the Nicaragua Rise may have been deposited in deep-water areas of the Caribbean plate during the Turonian–Santonian.
The Late Cretaceous Period on the Nicaragua Rise was characterized by the occurrence of high primary productivity typical of tropical areas like the Caribbean. High production of organic matter and its transport to the deep ocean areas resulted in low marine oxygen levels and the deposition of high-quality Caribbean source rocks in the form of dark, laminated organic-rich shale interbedded with pelagic limestone and marlstone (Erlich et al., 2003).
Upper Cretaceous platform limestone has been described from deformed outcrops in the Siuna terrane–MCOT near the town of Siuna (Venable, 1994; Muñoz et al., 1997; Rogers et al., 2007; Flores et al., 2015) where a 10–200-m (30–660-ft)-thick succession of shallow water and fossiliferous limestone rests unconformably on top of the Lower Cretaceous section (Flores et al., 2007) (Figure 4A). Venable (1994) also studied several outcrops near the town of Siuna that include Upper Cretaceous black shale, limestone, and conglomerate deposited in a shallow-marine environment, equivalent to the Kraursipe Formation as defined by Rogers et al. (2007) and formerly defined by Gordon (1993) as the Cantarranas Formation of the Yojoa Group in Honduras (Figure 4A).
The same Upper Cretaceous rudist-bearing limestone has been observed in the Chortis block in Honduras (Rogers et al., 2007) and has been logged in offshore wells Ocean Drilling Program (ODP) 999B and Deep-Sea Drilling Project 1001A in the Colombia Basin (Carvajal-Arenas et al., 2015). In the Nicaraguan platform, the ExxonMobil 1 Nica well drilled 10 m (33 ft) of backreef and forereef facies of presumed Late Cretaceous age (Muñoz et al., 1997; Carvajal-Arenas et al., 2015). According to the Nicaraguan Ministry of Energy and Mines (MEM) (2010) a sample of chert from the Siuna terrane was analyzed by the National Petrographic Service in Houston, Texas, and estimated that the sapropelic content of the kerogen was 77%, with a rich source-rock potential.
The post-Cenomanian succession of Central America and the western Nicaragua Rise includes well-rounded, well-sorted, and imbricated pebble conglomerate of the fluvial redbeds of the Valle de Angeles Formation (Rogers et al., 2007; Baumgartner et al., 2008; Flores et al., 2015). The MEM (Nicaraguan Ministry of Energy and Mines, 2010) reported locally preserved, clastic Upper Cretaceous sedimentary rocks related to the subsequent erosion of the Colon Mountains fold-thrust belt of northwestern Nicaragua and eastern Honduras (Rogers et al., 2007). In the Colon Mountains, Upper Cretaceous–Paleocene extrusive volcanic rocks of the Wampu Formation and breccia of the Tabacon Formation overlie potential source rock intervals of the Atima and Krausirpe Formations (Rogers et al., 2007) (Figure 4A).
The entire Cretaceous succession is intruded by diorite to granodiorite dikes dated to be circa 60 Ma (Ar–Ar in biotite in Venable, 1994) (Figures 3B, 4). Lewis et al. (2011) conducted petrographic and trace-elements analyses on the granitoids penetrated by wells Chevron 1 Toro Cay and Occidental Petroleum 1 Miskito (Figure 3) for which the authors found no evidence of an intrusive contact between these granitoids and the overlaying sedimentary Eocene sections therefore assigning these stocks to the Upper Cretaceous–Paleocene. Other Upper Cretaceous volcanic successions were drilled by Noble Energy 1 Paraiso Sur well in the southeastern Nicaraguan platform (Bunge et al., 2016).
Paleocene (?) dense calcareous shale with pyrite sedimentary rocks known as the Touche Formation has been only reported on the Nicaraguan platform from the Waterford Oil 1 Touche well, which was drilled into a proposed Paleogene rift (Arden, 1975; Flores et al., 2007; Emmet et al., 2011) (Figure 4A). Some authors include the uppermost Paleocene at the base of the Punta Gorda Formation (Muñoz et al., 1997). However, scarce microfaunal preservation in this interval yields little support for the Paleocene–Eocene transition (Martinez Tiffer et al., 1991). We consider the Punta Gorda Formation to be Eocene and the Touche Formation, drilled only by Waterford Oil 1 Touche well, to be the only documented Paleocene present in the study area as determined by Beicip-Franlab (1993, personal communication) (Figure 4A).
Other areas in which Paleocene sedimentary rocks are known from the western Caribbean include the inverted Wagwater rift of Jamaica (Mann and Burke, 1990), the lower Nicaragua Rise, and depocenters drilled in the Colombia Basin at ODP sites 999B and 1000 (Carvajal-Arenas et al., 2015). Over most of the Nicaraguan platform and the Honduran platform, the Paleocene stratigraphic interval is either absent or sparsely preserved unless deposited in rift settings as in the Wagwater rift in Jamaica (Arden, 1975; Emmet et al., 2011; Sanchez et al., 2015).
Lower Eocene Stratigraphy
The lower Eocene basal Punta Gorda Formation consists of massive limestone interbedded with calcareous shale and is locally marly to the northern part of the study area (Carvajal-Arenas et al., 2015) (Figure 4A). In the southern part of the Nicaraguan platform, limestone of the Punta Gorda Formation interfingers with localized basaltic and andesitic flows, breccia, tuff, and lapilli with minor amounts of sandstone as described from the Texaco 1 Centeno well (Muñoz et al., 1997) (Figure 3B). To the southeast, the Noble Energy 1 Paraiso Sur well encountered clean limestone and marlstone deposited on top of eroded Cretaceous volcanic rocks of the East Nica and Nica-Tinkham ridges (Bunge et al., 2016). Based on the allochem assemblages and degree of preservation of the skeletal remains encountered in Noble Energy 1 Paraiso Sur well, Bunge et al. (2016) determined that deposition occurred in a transition from inner ramp, backshoal, shoal, and foreshoal setting to a middle ramp and foreshoal to backshoal setting.
Widespread gas shows reported within the basal Punta Gorda Formation are associated with beds of fractured lignite and carbonaceous beds as recorded in the Unocal 1 Coco Marina and Unocal 1 Huani wells (Table 1). Other hydrocarbon shows within this same interval at the base of the Punta Gorda Formation include (1) dead oil logged in the Chevron 1 Prinzapolka well, (2) traces of free oil in the Shell 3 Perlas well, (3) oil shows in the Shell 1 Perlas well, (4) minor gas shows in Shell 2 Perlas well, and (5) traces of free black asphalt in the ExxonMobil 1 Tyra well (CanOcean Resources, 1980, personal communication) (Table 1).
Porosity ranges from fair to good in the Punta Gorda Formation as documented at the Shell 1 Perlas well with 10%–12% matrix and 5% vuggy porosity (Table 1) and at the Chevron 1 Nasa Cay well limestone with 15% porosity confined to calcilutite intervals (Carvajal-Arenas et al., 2015; Carvajal-Arenas, 2017). The porosity from the Noble Energy 1 Paraiso Sur well log averaged approximately 19% (Bunge et al., 2016).
Middle–to–Upper (?) Eocene Stratigraphy
The middle Eocene Punta Gorda Formation mainly consists of fossiliferous limestone and micrite with good porosity and thicknesses ranging from 760 to 1800 m (2500 to 6000 ft) (CanOcean Resources, 1980, personal communication). Porous, reefal bioclastic limestone is reported at the top of this formation (Martinez Tiffer et al., 1991). Gas shows from this formation occur in many wells including the Occidental Petroleum Miskito, Unocal 1 Coco Marina, Unocal Ledacura, Unocal 1 Tuapi, Unocal 1 Martinez Reef, Chevron 1 Toro Cay, and Gulf Oil 1 Punta Gorda (Table 1). A gas kick with hydrocarbon residue was reported from the Chevron 1 Nasa Cay well (Carvajal-Arenas et al., 2015) (Table 1). Excellent reservoirs, characterized by an approximately 35% vuggy porosity in limestone has been reported in Unocal 1 Diriangen well (Carvajal-Arenas et al., 2015). This interval is considered both a source and reservoir rock (Muñoz et al., 1997; Bernardo and Bartolini, 2015; Carvajal-Arenas et al., 2015).
Over large areas of the Nicaraguan platform, the upper Eocene sedimentary section is absent with the exception of the area of the Chevron 1 Prinzapolka and Chevron 1 Escondido wells (CanOcean Resources, 1980, personal communication) in which no oil shows and porosity values have been reported for the Eocene interval. According to Carvajal-Arenas et al. (2015), the presence of this upper Eocene unconformity across the Nicaraguan platform can be considered as a high-quality seal based on its angular truncation of the underlying section as documented in the Shell 3 Perlas well.
Oligocene–to–Upper (?) Miocene Stratigraphy
The Oligocene interval for the northern part of Nicaraguan platform consists of a fluvial to deltaic succession of lithic sandstone Mosquitia Group (Figure 4A). The basal Mosquitia Group consists of reddish gray to light brown shale and fine-grained sandstone with thin layers of conglomerate and pebbly sandstone of the Kamanon Formation. The Kamanon Formation is overlain by marly limestone to the south (Figure 4A). This deltaic wedge is up to approximately 360 m (∼1200 ft) thick as documented in the onshore Gulf Oil 1 Twara well (Muñoz et al., 1997).
Overlying the Kamanon Formation is a thick succession of unfossiliferous sandstone and conglomerate with a red, shaly matrix known as the Barren Red Beds Formation (Figure 4A). The youngest unit of the Mosquitia Group corresponds to a series of angular, lithic gravels and coarse-grained sandstone of the Bragman’s Bluff Formation with thickness up to 328 m (1000 ft) (CanOcean Resources, 1980, personal communication). In the upper Oligocene–Miocene deltaic succession, sandstone bodies are 10–15 m (30–50 ft) thick and interbedded within the argillaceous section with an average porosity of 25% (Carvajal-Arenas et al., 2015). The Oligocene–Miocene successions are normally lean in organic matter with no potential source rocks in this area (Bernardo and Bartolini, 2015; Carvajal-Arenas et al., 2015). To the south, reefal limestones are up to 1200 m (4000 ft) thick with fair to good primary porosity of approximately 10%–20% as reported in Shell 1 Atlantico, Shell 1 Perlas, Shell 2 Perlas, Shell 3 Perlas, ExxonMobil 1 Nica, and Texaco 1 Centeno wells (Carvajal-Arenas et al., 2015).
Upper Miocene–to–Present-Day Stratigraphy
The youngest unit of the Nicaraguan platform corresponds to the shallow-marine Martinez Reef Formation (Figure 4A). This unit includes fine- to coarse-grained limestone with bivalves, gastropods, shell fragments, algae, corals, and abundant foraminifera. In the northwestern area, interbedded continental shale contains unconsolidated and poorly sorted quartz–to–lithic-rich sandstone with some minor lignite beds of the Coco River known from several wells (i.e., Unocal 1 Ledacura, Chevron 1 Nasa Cay, Chevron 1 Zelaya, and Unocal 1 Coco Marina [CanOcean Resources, 1980, personal communication]). The presence and development of Pleistocene bioherms have been observed in several nearby wells (i.e., ExxonMobil 1 Nica, ExxonMobil 1 Tyra, and Shell 1 Perlas). Oil and gas shows are present in Unocal 1 Huani and Unocal 2 Tuapi, respectively (Carvajal-Arenas et al., 2015) (Table 1).
DATA SET USED FOR THIS STUDY
The subsurface data used for this study include 2-D seismic lines and data from 27 vintage wells in the Nicaraguan platform drilled between 1944 and 1978. Both seismic and well data were kindly provided by the Nicaraguan MEM (Figure 1). Seismic data include 287 lines (2-D) acquired during different seismic campaigns from the 1970s to the 1980s and reprocessed by the Nicaraguan MEM in 2004. The dominant frequency of the seismic data is approximately 20 Hz with an average seismic velocity of 1600 m/s and an average vertical resolution of approximately 20 m (∼70 ft).
Well data provided by the Nicaraguan MEM include (1) lithology reports done by CanOcean Resources in 1980, which include biostratigraphic data and subsequent formation age estimation; (2) regional well correlations done by Beicip-Franlab (1993, personal communication) based on core observations; and (3) a geochemical study, including RockEval data that were conducted by Japex Geoscience Institute, Inc. (JGI Inc.) in 2003. A compilation of well data are shown in Table 1, which includes (1) oil and gas shows reported by CanOcean, (2) geochemical data from JGI Inc. for source-rock intervals, and (3) formation lithology and biostratigraphic age data as reported by CanOcean.
All of the wells have digital wire-line log data (Carvajal-Arenas et al., 2015), but only digital logs from the Occidental Petroleum 1 Miskito and Occidental Petroleum 2 Miskito wells were made available to us for this study. The seismic well ties and seismic interpretation were previously done by Carvajal-Arenas et al. (2015) and were incorporated into this study. No access to core data was granted by the Nicaraguan MEM, biomarkers analysis was not done, and some basic well information from the Waterford Oil 1 Touche and ExxonMobil 1 Tinkham wells was missing from the lithologic reports made available to us by the Nicaraguan MEM.
METHODOLOGY USED FOR THIS STUDY
In this study, a detailed integration of 2-D seismic data with well data and lithology was carried out to determine the environment of deposition for the following time intervals of the Nicaragua Rise: Late Cretaceous–Paleocene (?), early Eocene, middle Eocene, late Eocene, Oligocene, early-to-middle Miocene, and late Miocene to present day. Burial plots were constructed using PetroMod for four different wells to understand the history of regional basin uplift and subsidence on the Nicaraguan platform of the Nicaragua Rise, the relationship to events previously studied on the upper Nicaragua Rise in the Honduras area (Sanchez et al., 2015), and to infer the timing of tectonic events.
Figure 5. West-to-east (A) uninterpreted and (B) interpreted dip seismic transect crossing the northern part of the Nicaraguan platform in the dip direction and showing Mesozoic (?) structures, Eocene synrift carbonate rocks of the Punta Gorda Formation; Oligocene–Miocene mixed clastic and carbonate rocks of the Mosquitia Group, and upper Miocene–to–present-day carbonate rocks of the Martinez Reef Formation. The inset map shows the location of line and wells shown. MEM = Nicaraguan Ministry of Energy and Mines; Neg = negative; Pos = positive.
The conventional sequence stratigraphy model of Posamentier and Vail (1988) was applied to this dominantly shallow-water carbonate platform setting to determine the eustatic sea-level changes versus local, relative sea-level changes within the stratigraphic evolution of the area. In tropical, carbonate settings, sediment accumulation is also constrained by in situ production of the “carbonate factory” in which carbonate facies and their fabrics respond to changing sea-level positions (Pomar, 2001). Carbonate sequence stratigraphy applied here to the Nicaraguan platform (Pomar and Ward, 1999; Pomar, 2001) does not follow all of the tenets of clastic sequence stratigraphy that is based on sediment hydrodynamics and physical accommodation as described by Van Wagoner et al. (1990), Posamentier and Allen (1999), Catuneanu et al. (2009, 2011), and Neal and Abreu (2009). For evaluating tectonic controls on the area, we use previous plate tectonic models for the area that include regional plate models by Escalona and Norton (2015) and a modification of this model by Sanchez et al. (2015).
GENERAL OBSERVATIONS OF BASEMENT STRUCTURE AND STRATIGRAPHIC UNITS OF THE NICARAGUAN PLATFORM
Regional 2-D seismic profiles across the northern (Figure 5) and southern Nicaraguan platform (Figure 6) reveal that the top of the “basement” corresponds to the imaging limit of the seismic reflection data and is overlain by the Cenozoic stratigraphic units. As previously mentioned, the sub-Cenozoic successions indicated below the seismic imagining limit have not been drilled in the Nicaraguan platform and are inferred mainly from deformed, onshore outcrops in Nicaragua and Honduras (Muñoz et al., 1997; Rogers et al., 2007; Flores et al., 2015; Sanchez et al., 2015) (Figures 5, 6). Carvajal-Arenas et al. (2015) observed seismic reflections below the surface where seismic imaging is poor and interpreted these basement reflectors as representing stratigraphic layering of an inferred Cretaceous section that underlies large areas of the Nicaraguan platform (top of green interval in Figures 5 and 6). Moreover, Bunge et al. (2016) interpreted the top of Jurassic as the top of a metamorphic and igneous basement near the East Nica and Nica ridges with a preserved Cretaceous (Turonian) section underneath Upper Cretaceous volcanic rocks in the southeastern area of Nicaraguan platform. Therefore, we infer that below the limit of seismic reflection imaging observed in our data set, there is a combination of deformed Cretaceous sedimentary and volcanic successions that possibly overlies the older Siuna terrane–MCOT that crops out on land in Central America.
Figure 6. West-to-east (A) uninterpreted and (B) interpreted dip seismic transect crossing the southern part of the Nicaraguan platform showing Paleocene (?) structural highs bounding the Eocene Verolania rift, Eocene mixed clastic and carbonate rocks of the Punta Gorda Formation, Oligocene–Miocene carbonate rocks of the Mosquitia Group, and upper Miocene–to–present-day carbonate rocks of the Martinez Reef Formation. Inset map shows location of the seismic line and location of wells along the seismic line. MEM = Nicaraguan Ministry of Energy and Mines; Neg = negative; Pos = positive; TWT = two-way traveltime.
The lower Eocene section pinches out and onlaps against prominent ridges as observed in the Diriangen Basin, Tuapi Basin, and Verolania rift, which indicates that early Eocene deposition was controlled by previously inverted pre-Eocene ridges (Muñoz et al., 1997; Emmet et al., 2011; Bunge et al., 2016) (Figures 2, 5, 6). Examples of these ridges include the (1) Misquito plateau, (2) Vounta high, (3) Tuapi and Prinzapolka ridges, (4) Nica-Tinkham ridges, and (5) El Bluff high. The middle Eocene section represents the transition from a rift to sag-basin stage and is unconformably overlain by postrift Oligocene–Miocene successions (Sanchez et al., 2015) (Figures 5, 6). High-angle normal faulting is widespread across the platform with higher-density fault deformation in the northern area of the platform related to basin subsidence (Figure 5). The relatively undeformed upper Miocene to present-day postrift succession unconformably overlies the more deformed, older section (Figures 5, 6).
Burial History Plots of Wells from the Nicaraguan Platform
Burial plots record the complex tectonic history of the Nicaraguan platform from its Late Cretaceous collision and accretion against the Great Arc of the Caribbean to development of Paleogene, east-west–oriented rifts. After this collision, the Caribbean plate ceases its counterclockwise rotation and begins to translate eastward along the Cayman Trough strike-slip boundary by the late Eocene (Mann and Burke, 1990; Gordon and Muehlberger, 1994; Mann et al., 1995; Mann, 2007; Pindell and Kennan, 2009; Sanchez et al., 2015). Burial history plots were calculated for Occidental Petroleum 1 Miskito and Unocal 1 Ledacura wells in the northern area and the Shell 1 Atlantico and Shell 1 Perlas wells in the southern area. The different stages of the platform evolution are indicated on the burial plots of Figure 7 and include the following tectonic stages.
1. The Late Cretaceous (?) to Paleocene: Subsidence during the Late Cretaceous is associated to localized east-west intra-arc extension with associated volcanism developed in the Nicaragua Rise caused by crustal fragmentation with deep, narrow rifts (Arden, 1975; Mann and Burke, 1984) (stage 1, Figure 7A). The Paleocene includes an uplift and basin exhumation event related to the continued accretion and subduction of the thick plateau (CLIP) crust underneath the Nicaragua Rise (stage 2, Figure 7) (Pindell and Kennan, 2009; Lewis et al., 2011). Therefore, collision and accretion of the Siuna terrane–MCOT against the continental Chortis block (Venable, 1994; Rogers et al., 2007; Sanchez et al., 2015, 2019) is responsible for the Late Cretaceous–early Paleocene (?) inversion of the ridges formed along the Nicaraguan platform caused by transpressional deformation that includes major strike-slip faulting (Infinity Energy Resources, 2013; Sanchez et al., 2019).
2. We propose that these pre-Eocene ridges (e.g., Nica-Tinkham ridges) correspond to inverted, compressional features formed during the Paleocene, which are (1) composed of deformed Cretaceous interbedded sedimentary and volcanic rocks, (2) capped by Upper Cretaceous volcanic rocks (Bunge et al., 2016), and (3) locally intruded by Upper Cretaceous–Paleocene granitoids (Flores et al., 2007; Rogers et al., 2007; Lewis et al., 2011) prior to their inversion. The Paleocene exhumation event has also been described in Tela Basin and Patuca Basin in Honduras (Emmet et al., 2011; Sanchez et al., 2015, 2019) and had resulted in poor preservation of the Paleocene section throughout the Nicaraguan platform and Honduras borderlands (Arden, 1975).
3. The early Eocene–middle Eocene: The Eocene was characterized by continuous basinal subsidence associated with synrift to postcollisional relaxation in a sag-basin stage recorded as stage 2 (Sanchez et al., 2015) (Figure 7). As established by Muñoz et al. (1997), extensional stresses resulted in strike-slip faulting and rifting that migrated from west to east during the Eocene. This same Paleogene rift migration has been described by Mann and Burke (1990), Sanchez et al. (2015), and Ott (2016) for Jamaica and the northern Nicaragua Rise during the transitional period from collision to strike-slip faulting.
4. The late Eocene–Oligocene: A pulse of uplift characterizes the late Eocene–Oligocene transition. By late Eocene, the majority of the Nicaraguan platform was exposed coinciding with a low eustatic sea level caused by formation of Antarctic polar ice caps (Iturralde-Vinent and MacPhee, 1999). Moreover, the opening of the Cayman Trough to the north of the Honduras borderlands also triggered tilting and uplift of the platform as shown in stage 3 in Figure 7 (Rosencrantz et al., 1988; Leroy et al., 2000; Sanchez et al., 2015).
5. The Miocene to present day: Increased subsidence during this period is attributed to the early Miocene reactivation of the left-lateral Pedro Bank strike-slip fault zone and linked to the opening of the San Andres pull-apart basin (Carvajal-Arenas et al., 2015; Ott, 2016; Carvajal-Arenas and Mann, 2018). Strike-slip faulting may have also affected the subsidence of the Verolania rift along the Prinzapolka and Tuapi ridges (Muñoz et al., 1997) and is shown as stage 4 in Figure 7.
Figure 7. Well burial histories for the northern and southern Nicaraguan platform: (A) Miskito 2 well (modified from Carvajal-Arenas et al., 2015), (B) Ledacura 1 (CanOcean Resources, 1980, personal communication), (C) Atlantico 1 (CanOcean Resources, 1980, personal communication), and (D) Perlas 1 (modified from Carvajal-Arenas et al., 2015). Different basin phases include (1) steady subsidence and development of carbonate platform from 50 (?) to 38 Ma; (2) 38–23 Ma uplift, erosion, and preservation of a major regional unconformity with a hiatus from circa 38 to 33 Ma; (3) 23–18 Ma increase in subsidence, drowning of carbonate platforms, and reactivation of Pedro Bank left-lateral strike-slip fault zone and San Andres pull-apart basin; (4) continuous 18–5 Ma subsidence with maximum transgression during the middle to late Miocene; and (5) Pliocene (Plio.) to Pleistocene (Pl.) maximum pulse of east-west extension across the San Andres pull-apart with increased subsidence, low sea level, and brief basin uplift and erosion (Carvajal-Arenas and Mann, 2018). Cret. = Cretaceous; L. = Late; Q. = Quaternary.
6. By the middle Miocene, widespread extension occurs across the lower Nicaragua Rise with the initial formation of the San Andres rift as recorded by stage 5 steady subsidence in the Shell 1 Perlas and Shell 1 Atlantico wells of the southern Nicaraguan platform (Figure 7C, D). By the middle to late Miocene, uplift and exposure of the San Andres Island occurs as the eastern shoulder of the San Andres rift with eruption of middle Miocene volcanic rocks on Providencia Island (Kerr, 1978; Wadge and Wooden, 1982; Concha and Macía, 1993, 1995; Carvajal-Arenas and Mann, 2018). In the northern Nicaraguan platform, the maximum pulse of extension (described by Carvajal-Arenas and Mann, 2018) began in the Pliocene and is recorded by a sharp increase in subsidence shown as stage 5 in the Unocal 1 Ledacura and Occidental Petroleum 2 Miskito wells (Figure 7A, B).
Sequence Stratigraphic Interpretation of Subsurface Data from the Nicaraguan Platform
To effectively display the cycles of regression–transgression recognized in this study, a stratigraphic datum was defined by the best-preserved transgressive surface (transgressive surface 2, called here “datum TS2”), which represents a landward shift of the system coinciding with an early Miocene episode of high eustasy as defined by Haq et al. (1987) and shown on the sea-level curve in Figure 4A. Maximum flooding surfaces (MFSs) are chosen to tie well data to the same level because most of the area is under flooding conditions, and a widespread condensed section was deposited and preserved (Posamentier and Allen, 1999). Because the MFS is not well preserved throughout the area, the TS2 was chosen instead as the datum (datum TS2 in Figures 8–10).
Figure 8. West-to-east well correlation in the dip direction across the northern part of the Nicaraguan platform showing well lithology, depositional environments, and sequence stratigraphic interpretation, including system tracts and major bounding surfaces. The sequence stratigraphic interpretation shows three cycles of regression–transgression within the lower Eocene–to–present-day section within a mixed carbonate and clastic environment with a less well expressed LST1 within the lower Eocene section. Locations of wells are shown on the inset map. X areas indicate no data available.
On the Nicaraguan platform, transgressive surfaces and MFSs are missing especially in the landward direction (e.g., transgressive surface 1 [TS1], datum TS2, maximum flooding surface 1 [MFS1], and maximum flooding surface 2 [MFS2] shown on Figures 8–10). As defined by Pomar and Ward (1999), these cycles are commonly truncated and incomplete in shallow-marine carbonate settings and do not correspond to parasequences. Furthermore, the limited access to well log and core data does not allow the identification of higher-resolution parasequences and sets of parasequences. For this reason, we identify only second-order cycles in this study using the criteria of previous studies (Kerans and Tinker, 1997; Brachert et al., 2003; Catuneanu et al., 2009; Neal and Abreu, 2009).
Figure 9. West-to-east well correlation in the dip direction across the southern Nicaraguan part of the platform showing well lithology, depositional environment, and sequence stratigraphy interpretation, including system tracts and major bounding surfaces. The sequence stratigraphic interpretation shows three cycles of regression–transgression. Locations of wells are shown on the inset map. X areas indicate no data available.
Sequence boundary 1 (SB1 in Figures 8–10), equivalent to the top of the seismic imaging limit in Figures 5 and 6, corresponds to the lower contact of the basal lower Eocene Punta Gorda Formation. The SB1 is a diachronous surface that separates nonconformable lower Eocene shallow-marine carbonate successions from localized continental (fluvial?) environments of the poorly imaged, underlying Upper Cretaceous (?) shallow-marine successions (?). This surface was observed in Occidental Petroleum 1 Miskito, Occidental Petroleum 2 Miskito, Gulf Oil 1 Twara, Chevron 1 Toro Cay, and Gulf Oil 1 Punta Gorda wells and is inferred to be present across the entire platform (Figures 8–10) but is best seen in the seismic illustrations in Figures 10 and 11.
Figure 10. Well correlation in the south-north strike direction across the northern and southern Nicaraguan platform showing well lithology, depositional environment, and sequence stratigraphic interpretation, including system tracts and major bounding surfaces. The sequence correlation shows three cycles of regression–transgression with variable system tract preservation within the lower Eocene–to–present-day section of mixed carbonate and clastic deposition. Locations of wells used for sequence stratigraphic interpretation are shown on the inset map. X areas indicate no data available.
The SB1 represent a hiatus of circa 10 m.y., approximately the duration of the Paleocene epoch (Figure 4A) as reported by Bunge et al. (2016) in Noble Energy 1 Paraiso Sur well. The SB1 is overlain by a section of continental sand and shale that is mixed with restricted–to–shallow-marine marlstone of the lowstand system tract 1 (LST1 in Figures 8–10) that was deposited within a short period of low sea level (Haq et al., 1987).
Figure 11. (A) Seismic line AA′ crossing the northern part of the Nicaraguan platform showing main bounding surfaces and system tracts; (B) sequence stratigraphic interpretation showing lapout terminations (black arrows) and shoreline trajectory (purple dotted arrows) within the Cenozoic section. The LST3 was not observed in this area because of the lack of seismic coverage. (C) Inset map showing the location of Chevron 1 Nasa Cay well to seismic line AA′. Neg = negative; Pos = positive; TWT = two-way traveltime.
During the LST1, algae reefs developed mostly restricted to the northern part of the platform. These lower Eocene reefs are documented by the Unocal 1 Martinez Reef, Unocal 1 Tuapi and Unocal 1 Huani, and Unocal 1 Ledacura wells (Figures 8 and 10, respectively) (Brachert et al., 2003). In the northern area of the platform, well data (CanOcean Resources, 1980, personal communication) show the occurrence of fine-grained limestone (mudstone–wackestone), calcareous shale, claystone, chert, and silty limestone. To the south, different sedimentary conditions prevailed with deposition of fine-grained limestone (wackestone to grainstone), volcanic tuff, dolomite, and dolomitized tuff, shale, siltstone, sandy limestone, and chert with subordinated siliciclastic sedimentation near the Texaco 1 Centeno well (Muñoz et al., 1997) (Figures 9, 10). Bentonite, a mineral formed because of alteration of volcanic rocks, dolomite, and lignite, is also present in the southernmost area of the platform (Carvajal-Arenas, 2017).
The TS1 represents a shift from a restricted shallow-marine environment to a deeper-marine environment with the development of pinnacle reefs documented by the Unocal 1 Martinez Reef, Unocal 1 Tuapi, and Unocal 1 Huani wells to the north and the Shell Perlas wells to the south (Figure 8). The TS1 also corresponds to outer-shelf facies with decreased continental input to the south (Shell 1 Atlantico well in Figures 9 and 10) and marks the transition from the early to the middle Eocene. The LST1, bound at the top by the TS1, is better preserved in wells drilled landward, whereas in wells located toward the basin, this interval is not clearly identified (Figure 9). To the north, the TS1 is characterized by a basal, catch-up, and isolated carbonate reef deposited during a period of increasing sea level and high accommodation (Figure 7). To the south, pelagic foraminifera were reported in the middle Eocene sections of Shell 1 Rama and Shell 1 Atlantico wells (Table 1), indicating outer-shelf–to–basinal deposition caused by enhanced accommodation in a sag-basin setting and with overall deepening of the Verolania rift.
The MFS1 and MFS2 (Figures 8–10) correspond to the base of a fossiliferous limestone layer, except in sections of the northern area of the platform where MFS2 is assumed to occur within a band of continental sedimentation as documented in several wells (Gulf Oil 1 Twara, Chevron 1 Toro Cay, Chevron 1 Nasa Cay, Chevron 1 Zelaya, and Unocal 1 Ledacura) (Figure 10). Coquina layers commonly cap the catch-up pinnacle reefs drilled by Unocal 1 Tuapi, Unocal 1 Huani, and Unocal 1 Martinez Reef, Unocal 1 Ledacur, and Shell Perlas wells (Figures 8, 10). The highstand system tract 1 (HST1) is characterized by a fossil-rich limestone unit deposited along a keep-up carbonate platform with variable preservation, particularly in wells drilled over paleohighs where the HST1 has been eroded by sequence boundary 2 (SB2) as observed in several wells (ExxonMobil 1 Tinkham, Chevron 1 Escondido, ExxonMobil 1 Tyra, Gulf Oil 1 Twara, and Chevron 1 Toro Cay wells in Figures 9 and 10, respectively).
The SB2 (Figures 8–10) is a diachronous, erosional unconformity seen throughout the Nicaraguan platform. The SB2 has been described by Carvajal-Arenas et al. (2015) and Sanchez et al. (2015) in offshore Nicaragua, Jamaica, and Honduras, respectively, and likely formed as a result of a global sea-level retreat (Iturralde-Vinent and MacPhee, 1999). Most of the upper Eocene section is absent because of erosion and partial exposure of the Nicaraguan platform recorded by the SB2 directly overlaid by fluvial-deltaic successions of the Oligocene section of the Mosquitia Group to the north. In offshore Honduras, the entire Oligocene section is missing as known from the Unocal 1 Coco Marina and Main Cape-1 wells (Sanchez et al., 2015).
In approximately half of the wells drilled in the study area, this Oligocene fluvial-deltaic succession includes the Kamanon Formation at the base of the Mosquitia Group (Figure 4A). These clastic units were deposited in a northwest-to-southeast direction and mostly inhibited carbonate development, particularly in the northern Nicaraguan platform. The SB2 also corresponds to a type 1 sequence boundary, marking a forced regression with fluvial incision and localized recrystallization and dolomitization of the underlying Punta Gorda Formation (e.g., ExxonMobil 1 Tyra well at approximately 1800-m [∼5900-ft] depth in Figure 10). To the south, sedimentary facies change laterally from shallow to restricted marginal marine carbonate characterized by sandy limestone, calcareous sandstone, and shaly limestone of the base of the Mosquitia Group in proximity to the Shell Perlas wells (Figure 10).
The TS2, also defined as a surface datum (datum TS2 in Figures 8–10), corresponds to the base of the lower Miocene interval of the Mosquitia Group (Figure 4A). To the north, this surface represents the onset of a deltaic backstepping and is inferred to be within floodplain-type aggradational continental shale in wells Gulf Oil 1 Twara, Gulf Oil 1 Punta Gorda, and Chevron 1 Toro Cay (Figure 8). Unfossiliferous redbeds, sandstone, and conglomerate with a red shaly matrix from the Barren Red Beds Formation are also part of this deltaic wedge and are present in several wells (Gulf Oil 1 Twara, Chevron 1 Toro Cay, Unocal 1 Martinez Reef, Chevron 1 Nasa Cay, Unocal 1 Coco Marina, Unocal 1 Ledacura, and Chevron 1 Zelaya wells in Figures 8 and 10). These redbeds grade into shallow-marine prodelta shale and siltstone (Unocal 1 Tuapi and Unocal 1 Huani wells in Figure 8) (Beicip-Franlab, 1993, personal communication). To the south, datum TS2 is marked by a basal outer-shelf interbedded limestone and calcareous shale interval (Texaco 1 Centeno, ExxonMobil 1 Nica, ExxonMobil 1 Tinkham, and Shell 1 Atlantico wells) (Muñoz et al., 1997). These units mark a basin deepening and the beginning of the transgressive system tract 2 (TST2 in Figure 9).
Datum TST2 is tied to the top by the MFS2 and is characterized by fossil-rich layers of biocalcarenite with fine-grained sandstone, dolomite, and sandy limestone containing larger foraminifera (e.g., Astrocyclina in Shell 1 Perlas well shown in Table 1) in a shallow to locally restricted to marginal marine environment similar to that of MFS1 (datum TS2 and MFS2 in Figures 8–10). Continental shale is present further south in the Chevron 1 Escondido and Chevron 1 Prinzapolka wells (Figure 9) along with tidal flat deposits that include carbonate buildups, bird’s-eye structures, stromatolites, algal boundstone, and continental carbonate as seen in the Shell 1 Perlas wells (CanOcean Resources, 1980, personal communication; Beicip-Franlab, 1993, personal communication).
By the late Miocene, an overall shift from a clastic to carbonate setting characterizes the highstand system tract 2 (HST2) with dominant deposition of dense limestone with thin sandstone intervals (Figures 8–10). Restricted continental conditions prevailed in the area close to Gulf Oil 1 Punta Gorda, Gulf Oil 1 Twara, Chevron 1 Toro Cay, and Unocal 1 Ledacura wells with deposition of pebbly, clay-bound gravel, unconsolidated lithic- to quartz-rich sandstone, and conglomerate and minor lignite beds (Figure 8) (CanOcean Resources, 1980, personal communication). To the east, fossiliferous limestone with dolomitic and carbonaceous intervals and thin beds of lignite and traces of pyrite were reported in the area of the Occidental Petroleum Miskito wells (Carvajal-Arenas et al., 2015) (Figure 8).
The youngest bounding surface corresponds to the transgressive surface of erosion (TSE) observed at the top of the HST2. The TSE is observed at the base of the upper Miocene to present-day Martinez Reef Formation and is characterized by a unit that includes biocalcarenites with corals, crinoids, gastropods, foraminifera, and bryozoans with layers of coarse-grained quartz (CanOcean Resources, 1980, personal communication). Its occurrence is related to brief pulses of sea-level rise that occurred during the late Miocene to the present day as shown in the sea-level curve in Figure 4A. Locally, this surface corresponds to a hiatus with erosion and no deposition. Preservation of a transgressive lag was observed as a layer of conglomeratic sandstone in the ExxonMobil 1 Nica well at a depth of approximately 400 m (∼1300 ft) (Figure 10). This surface was not clearly observed in some wells in the southern part of the platform (Figure 9) where some of the upper Miocene succession is missing and is expected to have a corresponding basinward lowstand system 3 (LST3?) (TSE in Unocal 1 Huani, Unocal 1 Tuapi, and Occidental Petroleum 1 Miskito wells in Figure 8 and Texaco 1 Centeno, ExxonMobil 1 Nica, Chevron 1 Nasa Cay, and Unocal 1 Ledacura in Figure 10).
Seismic Data Supporting Sequence Stratigraphic Interpretations
Identification of system tracts, major flooding surfaces (e.g., transgressive surfaces, MFSs, and TSEs), and sequence boundaries was carried out using 2-D seismic lines tied to well data (Mitchum, 1977; Catuneanu et al., 2009; Neal and Abreu, 2009). Most of the major surfaces that bound systems tracts are recognized from seismic sections, with their correlative conformities observed in the furthermost eastern flank of the Nicaraguan platform. On seismic data, SB1 and SB2 are observed throughout the study area as strong, continuous high-amplitude reflections with toplap-truncated reflections of the underlying units (SB1 and SB2 in Figures 11 and 12). The expression of these and other stratigraphic surfaces in chronostratigraphic displays (Wheeler diagrams) will be discussed below.
Figure 12. (A) Regional seismic line AA′ showing sequence stratigraphy, system tracts, and main bounding surfaces of the southern part of the Nicaraguan platform; (B) zoom of seismic line AA′ crossing the Nica ridge area showing detailed interpretation of system tracts across and along the Nica high; and (C) seismic line and interpretation of BB′ showing the Cenozoic section of the Verolania rift basin with lapout terminations (black arrows) and shoreline trajectory (purple dotted lines). The LST3 is only observed on seismic data and has not been identified in wells. (D) Inset map showing area of seismic line and surrounding wells. Neg = negative; Pos = positive; TWT = two-way traveltime.
Below the imaging limit, the seismic image is characterized by low- to high-amplitude, wavy, locally chaotic, and discontinuous–to–laterally continuous reflections. In comparison, the lower Eocene succession that corresponds to the LST1 also shows onlapping of low- to moderate-amplitude, mostly laterally continuous reflections, and locally in a wedge-shaped pattern typical of rift successions and typically onlapping against preexisting structures (cf. SB1 in Figures 11 and 12). Because Cretaceous units outcrop in onshore Nicaragua and assuming these units and their deeper-water equivalents are present along the platform, then SB1 separates these older Cretaceous sequences from the overlying Eocene carbonate rocks with a hiatus of circa 8–10 m.y., which is equivalent to most of the missing Paleocene.
The MFSs are not directly observed in the seismic sections. Instead, these surfaces are inferred along offlapping terminations that separate sequence boundaries and transgressive surfaces (MFS in Figures 11 and 12). Therefore, the transgressive and highstand system tracts are grouped as packages of retrograding–aggrading–prograding, moderate- to high-amplitude, smooth, laterally continuous offlapping reflections (transgressive system tract 1 [TST1]–HST1, TST2–HST2 and TST–HST3? in Figure 12). In the northern area of the platform, the MFS has been interpreted as a strong reflection separating the datum TS2 and the TSE with westward sediment thinning below where it is interpreted to be a correlative conformity (Figure 11).
The LST2 is characterized by a package of prograding, dull-to-strong, high-amplitude, laterally semicontinuous–to–continuous, wavy, offlapping reflections in the southern area of the platform (Figure 12). To the north, these reflections are dominantly strong, wavy, and laterally discontinuous with a concave base interpreted as a fluvial-deltaic succession of the Oligocene basal Mosquitia Group. A third lowstand system tract (LST3?) is observed in the southern Nicaraguan platform as prograding–aggrading, strong, laterally continuous reflections (Figure 12). This system tract is associated with a brief late Miocene platform exposure as previously described and has not been drilled by any well.
On top of the LSTS?, a TSE is observed as a relatively flat dull reflection with landward onlapping above, basinal offlapping, and toplap-truncated reflections below (Figure 12B, C). Above the TSE, retrograding packages of strong, high-amplitude, laterally continuous reflectors were observed in the northern area (Figure 11), with locally wavy reflections in the southern area caused by present-day reef development corresponding with the upper Miocene–to–present-day Martinez Reef Formation (Figure 12A, B).
Application of Carbonate Sequence Stratigraphy for Interpreting Three Types of Lower-to-Middle Eocene Reef Development on the Nicaraguan Platform
The Eocene interval on the Nicaraguan platform marks a favorable period for the accommodation and extensive growth of carbonate reefs (Burgess et al., 2013). Based on seismic interpretations tied to well logs, we propose three types of reef development for the study area: (1) type 1: lower Eocene isolated patch reefs, (2) type 2: middle Eocene pinnacle reefs, and (3) type 3: middle Eocene elongated patch reef system (Figure 13).
Figure 13. (A) Seismic sections showing examples of proposed type 1 lower Eocene isolated patch reefs (green shaded areas) in the northern area of the Nicaraguan platform, (B) seismic sections showing examples of type 2 middle Eocene pinnacle reef (blue shaded areas) in the central area of the Nicaraguan platform, (C) seismic sections showing examples of type 3 lower Eocene elongated patch reef systems in the southern area of the Nicaraguan platform, and (D) location maps for seismic lines, wells, structural highs, and Eocene reefs shown above on the seismic lines in (A), (B), and (C). Neg = negative; Pos = positive.
In the northern area, type 1 lower Eocene patch reefs are recognized as small domal features with smooth morphology (Figure 13A, B). This domal morphology indicates an algal-prone composition and relatively low accommodation rates as seen in several wells (i.e., Unocal 1 Martinez Reef, Unocal 1 Tuapi, and Unocal 1 Huani [Figure 13A, B]).
By the middle Eocene, the combination of eustatic and tectonically triggered transgression and enhanced accommodation during a phase of basin relaxation (stage 2 in Figure 7 and 13C) favored the transition from type 1 lower Eocene patch reefs to type 2 pinnacle reefs in the northern platform. This transition marked the initiation of TST1 and widespread carbonate platform reefal catch-up successions observed in several wells (i.e., Unocal 1 Martinez Reef, Unocal 1 Huani, and Unocal 1 Tuapi in Figure 8 and Unocal 1 Ledacura in Figure 10).
These type 2 pinnacle reefs are easily identified in the seismic section because of their conical shape formed during aggradation, upward thickening, and depositional wings at the flanks of the reef (Burgess et al., 2013). This shape indicates that carbonate production could keep up with progressively greater rates of relative sea-level rise (Figure 13A, B). These cycles of upward thickening are interpreted as aggradational stacking, corresponding to the TST1 (Figure 11). An upward thinning at the top of these pinnacle reefs would indicate slowing rates of sea-level rise and a decrease in the keep-up potential of the carbonate pinnacle reefs (Figure 13A, B). The progradational stacking pattern (HST1) with the preservation of a condensed section of coquina layers at the bottom of this sequence marks the MFS1 (MFS1 and HST1 in Figures 8 and 11, respectively).
In the southern Verolania rift, the East Nica and Nica-Tinkham ridges provided the substrate for an elongated, northeast-trending patch reef system by the early Eocene (Bunge et al., 2016) (type 3 in Figure 13C, D). Similar to the lower Eocene type 1 reefs formed in the northern area of the platform (Figure 13A, B), the type 3 reef system of the southern area developed under conditions of relatively low accommodation because of reefal growth on preexisting bathymetric highs. These conditions also allowed the growth of longer-lived, laterally extensive keep-up reef systems (Figure 13C, D). Carbonate slumps related to these reef systems are observed in seismic data as fan-shaped features with high-amplitude, laterally continuous reflections (Figure 13B, C) and was described in well data by Beicip-Franlab (1993, personal communication). These slumps have lower porosities (∼5%–10%) in the Shell 1 Atlantico well (Carvajal-Arenas, 2017) compared to porosities reported in other carbonate intervals that may result from cementation associated to diagenetic fluids leached from the interbedded volcanic minerals (Figures 3B, 9>).
Because carbonate sediment production occurs largely in situ in contrast to siliciclastic sedimentary rocks that area transported, moderate rates of relative sea-level rise may permit a tropical carbonate factory to produce sedimentary volumes at much higher rates than siliciclastic systems. The transgressive systems tracts in the Nicaraguan platform range in thickness from 500 to 1500 m (1600 to 4900 ft) (as seen in Figures 8–10) compared to conventional sequence stratigraphic clastic models in which transgressive system tracts are typically considerably thinner (Posamentier and Allen, 1999; Catuneanu et al., 2009, 2011).
DISCUSSION OF MAIN RESULTS
A summary of our main results based on interpretation of the 2-D seismic lines tied to well data is graphically illustrated by (1) the allostratigraphy of the Cenozoic sequences (Figure 14), (2) the paleogeographic evolution of the Nicaraguan platform in map view (Figure 15), and (3) a representation of the paleogeography in oblique view (Figure 16). We summarize the main elements and supporting seismic and well data for the stratigraphy and tectonic controls for the western Nicaragua Rise from the Late Cretaceous to the present day in the discussion below.
Figure 14. Chart showing proposed allostratigraphic interpretation for the Eocene–to–present-day section of the Nicaraguan platform showing (A) Wheeler diagram and eustatic sea-level curve (Haq et al., 1987) and labeled system tracts with bounding surfaces. Cross-sectional interpretations for the sequence stratigraphy are shown for the following periods: (B) early Eocene, (C) middle Eocene, (D) late Eocene to early Oligocene, (E) late Oligocene, (F) early to middle Miocene, and (G) late Miocene to present day. EOC = Eocene; J = Jurassic; J3 = Late Jurassic; K = Early and Late Cretaceous; MFS = maximum flooding surface; MIO = Miocene; OLI = Oligocene; PAL = Paleocene; PLIO = Pliocene; Q = Quaternary; SB = sequence boundary; TS = transgressive surface; TSE = transgressive surface of erosion.
Late Cretaceous–to–Paleocene Interval
Recently emplaced, Upper Cretaceous igneous intrusions and volcanic rocks were folded, uplifted, subaerially eroded, and covered by Upper Cretaceous–to–Paleocene (?) basal conglomerate along an unconformity surface of fractured and weathered igneous-volcanic rocks as reported in Occidental Petroleum 1 Miskito well (Lewis et al., 2011; Sanchez et al., 2015, 2019). The Hess Escarpment fault zone was a tectonically active strike-slip fault separating the Siuna terrane–MCOT and the CLIP (Baumgartner et al., 2008; Carvajal-Arenas et al., 2015). The Hess Escarpment fault zone was a positive area with localized folding during the Upper Cretaceous (Bowland, 1993; Baumgartner et al., 2008) with shallow-marine deposition (Figure 15A). Lower-to-middle Cretaceous folded and thrusted shallow-marine limestones are widely exposed in onshore Honduras and northern Nicaragua and are likely equivalent to similarly deformed carbonate facies found at the basal part of the Nicaraguan platform onshore (Venable, 1994; Rogers et al., 2007) and imaged on deeply penetrating seismic lines offshore in Honduras (Sanchez et al., 2015, 2019). The onshore Upper Cretaceous continental Valle de Angeles Formation, a thick redbed unit containing angular clasts of Lower-to-middle Cretaceous marine carbonate units, is interpreted as the clastic wedge associated with this Late Cretaceous folding, thrusting, and uplift event (Figure 15A) (Arden, 1975; Rogers et al., 2007; Emmet et al., 2011). By the Paleocene, most of present-day areas of Nicaragua and Honduras were uplifted and subaerially exposed.
Early Eocene Interval
Lower-to-Upper Cretaceous sedimentary and crystalline rocks that once were exposed during the Paleocene are largely covered during the early Eocene by mixed layers of siliciclastic and carbonate lower Eocene sedimentary rocks across the Nicaraguan platform (Figure 15B). The hiatus between the seismic imaging limit and the basal sedimentary unit corresponds to SB1 and initiated the LST1 (Figure 14A, B) that was deposited in a shallow-marine setting with restricted deep-marine deposition in the Verolania rift (Figures 15B, 16B).
Figure 15. Paleogeographic reconstruction of the Nicaraguan platform showing depositional environments and changes in shoreline locations that accompanied Paleogene–to–present-day counterclockwise block rotation of the Chortis block. White arrows indicate the amount of counterclockwise block rotation based on paleogeographic reconstructions by Sanchez et al. (2015). (A) Late Cretaceous–Paleocene (?) collision of the Chortis block against the Siuna terrane, uplift and deformation of fold-thrust belt in Colon Mountains, and progressive counterclockwise rotation of structural highs; (B) early Eocene formation of tectonically controlled mixed-carbonate platform accompanied by east-west rifting of the Verolania rift; (C) development of a middle Eocene sag basin overlying the older Verolania rift with platform drowning; (D) late Eocene eustatic lowstand accompanied by extensive erosion; (E) formation of Oligocene deltaic wedge; (F) early Miocene–middle Miocene regional intraplate extension with initial formation of the San Andres rift and rift-related volcanism; and (G) late Miocene–to–present-day volcanism and formation of a stable carbonate platform. Locations of wells used to constrain paleogeography are shown on the inset map. MCOT = Mesquito composite oceanic terrane.
We propose that the pre-Eocene (Paleocene deformed?) ridges like the Nica-Tinkham–West Tinkham ridges, Prinzapolka ridge, Wounta high, and El Bluff high remained positive areas that controlled Eocene clastic and carbonate sedimentation on the Nicaraguan platform. Fragmented and discontinuous red algae-rich carbonate platforms formed along the study area with reefs on top of these ridges (Figures 15B, 16B).
Continental siliciclastic successions related to the erosion of the ridges are found in offshore Honduran wells (Sanchez et al., 2015) and on the southernmost area of the Nicaraguan platform close to Texaco 1 Centeno well (Figures 15B, 16B). The Mosquito plateau and Gulf Oil 1 Twara, Chevron 1 Toro Cay, and Gulf Oil 1 Punta Gorda wells are areas that remained exposed by the early Eocene (Figures 15B, 16B). We relate a major change in the location and orientation of the Eocene shoreline from northeast to north-south to the continuous counterclockwise rotation of the Chortis block and Siuna terrane–MCOT (Avila et al., 1984; Mann and Burke, 1984; Gordon, 1993; Mann et al., 1995; Mann, 2007; Rogers et al., 2007; Carvajal-Arenas et al., 2015; Sanchez et al., 2015).
Figure 16. Schematic three-dimensional paleogeographic reconstruction of the Nicaraguan platform shown in Figure 14, including depositional environments based on wells and seismic facies, changes in shoreline locations, and volcanic features. (A) Late Cretaceous–Paleocene (?); (B) early Eocene; (C) middle Eocene; (D) late Eocene–early Oligocene; (E) late Oligocene; (F) early to middle Miocene; and (G) late Miocene to present day. Locations of wells that were used to constrain the paleogeographic interpretations are shown on the inset map.
In the Verolania rift on the southern part of the Nicaraguan platform, different conditions prevailed with deposition of interbedded limestone and interbedded submarine lava flows that are present in the Shell 1 Atlantico well (Beicip-Franlab, 1993, personal communication) and basaltic and andesitic flows, breccia, tuff, lapilli, and dykes in Shell 1 Perlas, Shell 2 Perlas, Shell 3 Perlas, Texaco 1 Centeno, Chevron 1 Escondido, Chevron 1 Prinzapolka, ExxonMobil 1 Tinkham, and ExxonMobil 1 Nica wells (Figure 3B). This widespread Eocene volcanism indicates the possibility of an eastward-dipping subducting slab from the Central American subduction zone causing intraplate extension along the Nica-Tinkham ridges and the area of the Verolania rift (Muñoz et al., 1997; Brandes and Winsemann, 2018; Carvajal-Arenas and Mann, 2018) (Figure 16B).
Middle Eocene Interval
A decrease in the extent of regional volcanism as shown in Figures 9 and 10 accompanies the final stages of intraplate rifting and initiation of the subsequent sag phase (Figure 16C). The early-to-middle Eocene represents a period of high sea level and climate warming that favored the development of carbonate platforms that may have been controlled by widespread release of methane from ocean-floor sediments (Iturralde-Vinent and MacPhee, 1999).
The middle Eocene landward shoreline shift accompanied the fossiliferous shallow-marine inner-to-outer carbonate platform with lesser amounts of algal preservation (Figures 15B, 16B). Toward the west, continental shale containing coal and plant remains are reported from the Gulf Oil 1 Punta Gorda and Gulf Oil 1 Twara wells (CanOcean Resources, 1980, personal communication). In the southern and most distal area of the platform, gravity-driven deposits are present along the flanks of the Nica-Tinkham ridges (CanOcean Resources, 1980, personal communication; Beicip-Franlab, 1993, personal communication; Carvajal-Arenas et al., 2015) (Figures 15B, 16B).
By the end of the middle Eocene transgressive event, the deposition of fossiliferous limestone marks the preservation of a condensed section interpreted by us as an MFS1 and the initiation of the HST1 (MSF1 in Figure 14A, C). The complete filling of the Verolania rift yielded to a continuous and stable carbonate platform that is rich in fossil remains (Figures 15C, 16C).
Late Eocene Interval
The late Eocene interval on the Nicaragua Rise was marked by a strong fall in eustatic sea level that has been proposed to represent the initiation of the icehouse climate, the rapid expansion of the Antarctic ice sheets, and a global fall of sea level (Haq et al., 1987; Iturralde-Vinent and MacPhee, 1999). After the late Eocene, the tropical climate cooled and became drier, and changes in ocean circulation and cooling in Antarctica brought an end to the long greenhouse climate that had existed since the Mesozoic (Iturralde-Vinent and MacPhee, 1999). Thus, a major erosional event took place throughout the Nicaragua Rise corresponding to the SB1 and the onset of the LST2 (Figure 14A, D).
A major basinward shoreline shift is also seen on seismic data (Figures 11, 12). Because carbonate is much more prone to dissolution and erosion than siliciclastic sedimentary rocks, it was not unusual for much of the Nicaraguan platform to become subaerially exposed as illustrated in Figures 15D and 16D.
By the Oligocene, sediment supply increased because of the erosion of recently uplifted land areas in Nicaragua, and a deltaic system developed along the northwestern area of the Nicaraguan platform (Figures 15E, 16E). We propose that this fluvial-deltaic succession corresponds to an ancestral river of Oligocene age, which we call the proto-Coco river after the nearby modern Coco river of northeastern Nicaragua. We propose that the proto-Coco river formed during a time of low eustatic sea level within the LST2 (Figures 14E, 15E, 16E). To the south, an open carbonate platform developed in shallow marine to inner neritic conditions with localized patch reef formation favored by the distal location of the deltaic wedge (Figures 15E, 16E). Mutti et al. (2005) determined that mass carbonate production started to decrease circa 27 Ma in the Nicaragua Rise as a precursor of the global event known as the middle–late Miocene carbonate crash (Mutti et al., 2005). By the late Oligocene, the shoreline orientation remains in a constant north-south orientation and is consistent with the accompanying eastward translation (without rotation) of the Caribbean plate by this time (Figure 15E).
Early Miocene–to–Middle Miocene Interval
During the early Miocene, the continuous shallow-water carbonate shelf of the Nicaragua Rise extended from the Honduras–Nicaraguan mainland to the modern island of Jamaica and disintegrated into isolated banks and shelves dissected by a series of intervening seaways in the northeastern Nicaragua Rise like the Rosalind Channel, Diriangen Channel, and Bawihka Channel (Iturralde-Vinent and MacPhee, 1999; Mutti et al., 2005) (Figure 2).
As shown by stage 4 in Figure 7, a major pulse of increased subsidence affects the platform with retreat of the deltaic wedge at the mouth of the proto-Coco river and the initiation of the TST2 (Figure 14F). Mutti et al. (2005) described foundering of the Nicaragua Rise caused by opening of seaways like the Pedro Channel (Cunningham, 1998) that produced changes in ocean chemistry and limited neritic carbonate production along the northeastern Nicaragua Rise. Well data from CanOcean Resources (1980, personal communication) and Beicip-Franlab (1993, personal communication) show that continued preservation of calcareous microfossils from the southern Nicaraguan platform indicates that this event did not have a significant impact on Miocene carbonate productivity in this area (Figures 15F, 16F). Volcanic activity was widespread during this period across the Nicaragua Rise with active Miocene volcanoes on Providencia and Corn Islands (Ryan and Zapata, 2003; Carvajal-Arenas and Mann, 2018).
Late Miocene–to–Present-Day Interval
A pulse of rapid subsidence occurs at the end of the late Miocene as a result of reactivation of the Pedro Bank fault zone and slab rollback and trench retreat in Central America (Brandes and Winsemann, 2018) causing extension of the San Andres rift in the lower Nicaragua Rise (Figure 14G) (Carvajal-Arenas et al., 2015; Carvajal-Arenas and Mann, 2018). The San Andres rift drowns to its current depth of 3000–3500 m (9800–11,500 ft) after coalescing of a series of embayments similar to the present-day submergence of estuaries at Laguna de Perlas along the southern coast of Nicaragua (Figures 15G, 16G). We propose that the Miskito Cays in the northern area of the platform formed as remnant barrier islands of the proto-Coco river delta. The Miskito Archipelago is capped and surrounded by present-day patch coral reefs, estuaries, mangroves, seagrass beds, and islets (Ryan and Zapata, 2003). This rapid subsidence triggers the formation of present-day pinnacle reefs as the carbonate system tries to keep pace with the surrounding, rapidly subsiding basin (Carvajal-Arenas et al., 2015) (Figures 15G, 16G). To the southeast, the recently formed Providencia Island, San Andres Island, and Corn Island become favorable sites for the formation of Miocene–Quaternary patch reefs, mangrove, and lagoon deposits in San Andres Island and modern fringing reefs around Providencia Island (Kerr, 1978; Geister, 1992; Vargas-Cuervo, 2004) (Figures 15G, 16G). To the west, Pleistocene Pearl Lagoon and Cukra Hill volcanoes formed by the Pliocene.
Application of This Study for Future Petroleum Exploration of the Nicaraguan Platform
We have integrated seismic and well data within a sequence stratigraphic framework to make inferences on potential and known petroleum system elements of the Nicaraguan platform. Source-rock intervals are identified in the lower and middle Eocene (Figure 17; Table 1) with further potential of deeper source rock within the proposed offshore extension of the Cretaceous succession outcropping in onshore Nicaragua (shaly facies of the Atima and Kraursipe Formations) (Figure 4).
Figure 17. Schematic representation of the different Cenozoic petroleum system elements in the Nicaraguan platform based on data set and modified from Bunge et al. (2016). Also shown are system tracts, major bounding surfaces, and general lithology. Fm. = Formation; GAC = Great Arc of the Caribbean.
The general lack of lateral continuity caused by rapid lateral facies changes is an adverse effect on the quality, distribution, and total organic carbon content of Eocene source rocks. Moreover, Eocene source rocks were deposited in restricted to shallow-marine conditions with continental influx from exposed areas of Central America, as summarized in the paleogeographic maps in Figures 15 and 16. Continental sedimentary influx leads to the preservation of mixed kerogen type II and III and an oil- and gas-prone basin with an important gas component as proposed by Carvajal-Arenas et al. (2015). The most continuous, organic-rich intervals are observed within the northeast-southwest–trending depocenters like the Verolania rift, Tuapi Basin, and Diriangen Basin and are also associated to transgressive surfaces and fossiliferous layers of condensed sections (i.e., MFSs) (Figure 17).
In addition, higher thermal maturity of Eocene source rocks may only exist in similar areas of higher sedimentation rates that occurred during transgressive to highstand episodes and with subsequent, thicker overburden and deeper burial. According to Beicip-Franlab (1993, personal communication) and Carvajal-Arenas et al. (2015), the lower Eocene source rock in the Shell 1 Perlas, Shell 2 Perlas, and Shell 3 Perlas wells is within the oil generation window, indicating that an overburden of approximately 4 km (∼13,000 ft), as shown in burial plots in Figure 7 is sufficient for hydrocarbon generation. The presence of dead oil in ExxonMobil 1 Nica and Chevron 1 Prinzapolka wells, black asphalt in the ExxonMobil 1 Tyra well, and gilsonite in the Occidental Petroleum 2 Miskito well may also be explained by overcooking of source-rock intervals resulting from heat released from surrounding volcanic material. More distal areas in the abyssal plains of the western part of the study area are commonly immature as a result of thin overburden (Bernardo and Bartolini, 2015; Carvajal-Arenas et al., 2015). The same unfavorable hydrocarbon situation of thin overburden is present in the westernmost flank and areas in the north of the present-day Nicaraguan platform where the sedimentary section is too thin for hydrocarbon generation accumulation along with little preservation of a continuous sealing horizon as observed in several wells in this area (e.g., Texaco 1 Centeno, Gulf Oil 1 Twara, Gulf Oil 1 Punta Gorda, and Chevron 1 Toro Cay wells) and the Unocal 1 Tuapi well (Figure 17).
Eocene reservoirs in the study area include dolomitized intervals, weathered and fractured carbonate intervals, basal volcanic units, basal conglomerate, and untested calcareous turbidites. The best reef reservoirs known from well data are associated with Eocene transgressive system tracts. Eocene reef reservoirs overlying the top of ridges may lack sufficient lithostatic pressure to contain hydrocarbons.
Other potential reservoirs include Oligocene–Miocene deltaic successions associated with the proto-Coco river system overlying SB2 (Figure 17). Carvajal-Arenas et al. (2015) proposed that the lack of seal rock is the highest risk factor for this study area. We consider that the most promising continuous sealing facies are marine shale deposited along transgressive surfaces (Figure 17).
Reasons for failure in the exploration history documented in vintage wells from the 1960s and 1970s from the Nicaraguan platform that are summarized in Figure 1 and Table 1 are attributed to (1) rudimentary subsurface seismic quality and mapping from this era; (2) lack of data from critical, offshore areas, and lost data from critical areas; and (3) using incorrect geologic models for petroleum system evaluation.
CONCLUSIONS AND IMPLICATIONS FOR FUTURE HYDROCARBON EXPLORATION
• The Late Cretaceous–to–present-day development of the Nicaraguan platform on the western Nicaragua Rise records tectonically controlled deposition with strong influence by eustatic sea-level variations and Late Cretaceous–Eocene west-to-east counterclockwise rotation of the Chortis block and Siuna terrane–MCOT (Figures 15, 16).
• Two stages of intraplate rifting and associated volcanism affected the Nicaraguan platform: (1) Eocene rifting and formation of the Verolania rift, and (2) Miocene formation of the San Andres rift (Figure 7).
• The Nicaraguan platform records three cycles of transgression–regression during the Cenozoic with the development of a discontinuous (Eocene) to a continuous (late Miocene) carbonate platform with episodes of fluvial influence (Figure 13).
• Source rocks include lower Eocene fossiliferous lagoonal limestone and middle Eocene deep-marine calcareous shale (Figures 8–10). The best Eocene source-rock facies are associated with coquina layers preserved during MFSs and condensed sections associated with major transgressive surfaces. The most laterally continuous source-rock intervals are within depocenters like the Verolania rift in the south and the Diriangen and Tuapi Basins in the north (Figure 17).
• Reservoir rocks include three types of Eocene algae-rich reefs: (1) type 1 comprises lower Eocene rimmed reefs formed during lowstand conditions along elongated preexisting ridges, (2) type 2 comprises middle Eocene pinnacle reefs formed during highstand conditions and enhanced basin accommodation, and (3) type 3 comprises lower Eocene patch reefs formed during lowstand conditions along the platform edge (Figure 13).
• Other favorable reservoir intervals include dolomitized, weathered, and fractured carbonate formed during subaerial exposure in the late Eocene–Oligocene transition as a consequence of low eustatic sea level (Figure 17). Oligocene–to–lower Miocene thick fluvial-deltaic sandstone packages show fair to good porosity (∼20%), and well sorting deposited during low sea levels represent an excellent secondary reservoir for early Eocene source rocks (Figure 17; Table 1).
• Untested but potential reservoirs and traps include lower-to-middle Eocene gravity-driven slumps associated with platform and reef collapsed along the edges of platforms (e.g., East Nica and Nica-Tinkham ridges). These slumps may potentially seal the lower Eocene source-rock interval of the Punta Gorda Formation (Figure 17).
• The upper Eocene–Oligocene unconformity represents the most continuous and extensive seal across the Nicaraguan platform with underlying units truncating against its base as an angular contact. Additionally, in the northern part of the platform, transgressive shale associated to the datum TS2 overlies this upper Eocene–Oligocene unconformity. A similar scenario is observed with shale deposited on top of the middle Miocene–upper Miocene TSE capping sandstone reservoirs of the Oligocene–Miocene Mosquitia Group (Figure 17).
• Other localized sealing intervals include intraformational calcareous shale within the Eocene Punta Gorda Formation (Figures 8–10, 17; Table 1).
• Observed traps include four-way closures from structures anticlines, reefal highs, sediment pinch-outs against highs, unconformities, and slump deposits (Figure 17).
• Major risk factors proposed for the Nicaraguan platform include (1) poor seal continuity and capacity, (2) lack of overburden, (3) lack of source-rock continuity, and (4) insufficient maturation, source-rock generation, and charge (Figure 17).
• Cretaceous units have been defined in onshore Nicaragua, which may have continuous offshore continuations beneath the Cenozoic Nicaraguan carbonate platform. The predicted depth of these onshore Cretaceous successions is expected at 5000 m (16,400 ft) beneath the Nicaraguan platform (Figures 5, 6).
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The authors thank R. Bacca and V. Artiles from the Nicaraguan Ministry of Energy and Mines who kindly provided the seismic and well data used for this study and provided us permission to publish these data in this paper. We would also like to thank Pete Emmet, Lisa Rebora, Rick Roberson, and the anonymous reviewer for their valuable comments during the review process. We also thank Peter Baumgartner and Claudia Baumgartner-Mora for sharing their knowledge on Nicaragua’s geology. We give special thanks to Rasheed Ajala for his technical assistance. We thank the industry sponsors of the Conjugate Basin, Tectonics and Hydrocarbons consortium for their continued support.