Joshua Rosenfeld’s article in the April EXPLORER issue’s Historical Highlights provides an interesting compilation of observations selected to support an unproven hypothesis: that the large influx of Paleogene Wilcox sandstones in the Gulf of Mexico can be linked to a major evaporative drawdown of the basin, loosely aligned with and possibly triggering the Paleocene-Eocene thermal event.
Today this Wilcox deepwater play has the largest ultimate technically recoverable resources of any reservoir in U.S. Federal waters at 22.6 BBOE (from 26 discoveries, according to 2014 data from the Bureau of Ocean Energy Management). It extends into Mexico, as documented with recent discoveries at Trion and other Pemex wells. Industry exploration, led by Chevron, Shell and other companies, and evolving scientific thinking on the source terranes, fluvial to deepwater depositional systems, chronostratigraphy, geochemistry, and paleoclimate indicate that a catastrophic drawdown of the basin water level did not occur. Rather, consideration of natural processes of source to sink sediment routing and basin depositional history can fully explain the remarkable distribution, quality and continuity of this unique sandstone-prone interval in the deep basin.
But let us consider first the key aspects of this play as revealed in numerous industry wells and scientific studies. We offer our observations based on decades of oil industry experience working the Wilcox play, more than 55 peer-reviewed academic papers and one book on Gulf of Mexico sedimentary basin.
Evolution of Exploration and Scientific Thinking
Since the first discovery in 1928, Wilcox reservoirs have been a target of onshore Texas deep gas wells, drilled in places like Duval Co., home of the Seven Sisters Field and Zapata Co. where Shell found the Fandango field. The largest – Sheridan field in Colorado County – was discovered in 1940 and has produced 2.4 trillion cubic feet. Few of us working these onshore plays considered the possibility that the Wilcox extended offshore and into areas below and beyond the salt canopy of Alaminos Canyon, Walker Ridge and other deepwater protraction blocks where oil discoveries emerged following drilling of the Wilcox play opener well, BAHA No. 2. This play expanded to the east, both updip and downdip where it now covers more than 35,000 square kilometers (figure 1).
However, our understanding of deepwater plays was also evolving in tandem with exploration. Long-run out submarine fans were studied in outcrop, modeled in laboratories and mapped in the subsurface. We now routinely find the limits of many Gulf of Mexico deepwater plays have been extended much further basinward (for example, Upper to Lower Miocene and Oligocene), not just in the Wilcox (figure 2). This is paralleled by the dramatic improvement in seismic imaging and illumination that allowed industry to identify deepwater traps previously hidden below the salt canopy.
A corporate leader in this deepwater Wilcox play is Chevron, who discovered the giant Jack Field in 2004. Larry Zarra, a stratigrapher who has worked for Chevron for nearly 30 years, has long considered the factors that contributed to deposition of the deepwater Wilcox. In the past two decades he has conducted detailed interpretations of more than 13,000 feet of deepwater Wilcox conventional cores from the Wilcox 1, 2, 3 and 4 sequences, variously representing the inboard, outboard, and west trends (figure 1), plus a few nearshore Louisiana wells. As described in AAPG Search and Discovery No. 51616, Zarra’s 2019 interpretation points to Wilcox deepwater reservoirs as deposited in a lower slope to basin floor setting. The rock record examined contains no evidence supporting the hypothesis of a basin drawdown; there are no indications of shallow water facies, no evaporites and no signs of catastrophic events. This is a consensus view among all operators active in the deepwater play. The deepwater Wilcox is almost entirely composed of lower slope to basin floor turbidites and associated depositional systems. Mass transport deposits (plus or minus 10 meters) occur, but are extremely rare, and unconfined fan systems are limited to the Wilcox 1 interval in the outboard trend. The nature of turbidite flows is that they proceed down depositional gradient until the gradient decreases. Distance is irrelevant in sedimentary gravity flows as long as there is a slight gradient (equal or greater than 0.25 degrees). In a high input system such as the Wilcox, it is not surprising to find high net sand lithofacies far out on an extended slope.
Zarra devised a definitive chronostratigraphic framework for the deepwater Wilcox, first published in 2007, and recently updated and revised with Chevron collaborators in 2019 on AAPG Search and Discovery (No. 51616). The chronostratigraphy chart illustrates that the Wilcox reservoir interval is comprised of nine third-order sequences that span 10.4 million years, from 61.5 to 51.1 ma (see Appendix 1 chart of Zarra et al. 2019). This is hardly a short duration, catastrophic process as was suggested by the Rosenfeld article. Zarra notes further that events occurring within this time interval cannot automatically be considered contemporaneous, especially when discrete events cited by drawdown hypothesis proponents are temporally uncalibrated or are separated by millions of years. For example, the Wilcox 4 sequence that Rosenfeld and others discussed as the thick sandstone interval seen in the BAHA No. 2 and Trident No. 1 wells was deposited between 61.5 and 60.5 ma. The canyons that Rosenfeld proposed as conduits for this deposit were formed much later. Cut and early fill of the onshore Texas Yoakum Canyon is dated to 56.7 ma (Zarra, unpublished data) and the onshore Mexico Chicontepec Canyon cut and early fill is dated to 54 ma, well into the lower Eocene (see Vasquez, Cossey, et al. paper in 2014). The PETM signals a very short duration global event precisely dated worldwide at 55.96 to 55.84 ma. While Rosenfeld considers that this event may be correlated to deposition of the massive Wilcox 4 sand, these events are separated by about 5 ma. Zarra has described the P/E Boundary from the Anchor 3 well (figure 3), possibly the only cored PETM interval in the deepwater northern GoM, as a deepwater laminated shale with thin-bedded structured siltstones and with no evidence of evaporates or mineralization related to unusual water chemistry (Zarra et al. 2019 AAPG Search and Discovery No. 51616).
The Microfaunal Indicators
Marcie Phillips of the University of Texas-Austin, who previously worked for Noble Energy on the Wilcox as a biostratigrapher, agrees with Zarra on the longevity of Wilcox deposition based on microfossil ranges. She also notes that the foraminifers observed in deepwater wells in the northeastern GoM are predominantly, if not entirely, planktonic, indicating a pelagic deepwater depositional environment. Benthic forams, the majority of which are shallow shelf-dwellers, are rarely observed in these wells because their numbers decrease exponentially with depth. In the deepwater wells in the northern GoM, through the Paleocene, including the PETM, agglutinated forams are dominant and abundant indicating a turbid, high-energy submarine environment. Most significant are the abundant radiolarians are noted in nearly every deepwater well across the GoM. Radiolarians are exclusively planktonic and marine and can only tolerate normal levels of salinity (35-40 ppm), which plainly refutes the Rosenfeld hypothesis of a shallow, hypersaline GoM. Thus, hypersaline submarine conditions through the PETM cannot be supported by micropaleontological evidence. The observable microfossil assemblages are diverse and abundant and are predominated by species requiring marine pelagic waters, normal marine salinity and sufficient nutrients – conditions that would most certainly do not exist in an extreme environment such as a shallow, hypersaline water body.
The Geochemical and Paleoclimate Record
Robert Cunningham is petroleum systems consultant to the GBDS project who has published several GoM source rock papers including one the Tithonian-centered source rocks. “Offshore GoM petroleum interpreted to be sourced from the Paleocene-Eocene and distal source rock extracts do not show evidence of hypersalinity or salinity stratification and stagnation that would be expected under drawdown conditions” he said. Further, he notes that significant isolation and drawdown (by up to 2,000 meters, according to Rosenfeld and co-authors in several publications) during the warm Paleogene should have led to density stratification, deoxygenation and organic enrichment (especially at the PETM) through fluvial plumes and evaporative enhancements similar to pre-Messinian sapropels. Excluding more restricted neritic or bathyal settings such as the Chicxulub impact crater, this is not seen in multiple wells across the deepwater northern GoM.
We agree with Rosenfeld et al that Paleogene climate change, particularly the thermal maximum at the Paleocene/Eocene boundary, was important in Gulf of Mexico basin history, but disagree that it resulted in a major sea level drawdown. Detailed analysis of the mineral and chemical alteration indices in Paleogene sandstones and mudstones by Angela Hessler was published in 2017. Her work indicates that Paleogene climate changes enhanced source terrane weathering and forced other hydrologic changes in the hinterland producing a greater siliciclastic yield and more quartz-rich sandstones, a pattern persisting for several millions of years. Observations and BQART modeling of sediment supply published by Jinyu Zhang of UT-Austin and coauthors in 2016 showed that during the rapid temperature spikes of the PETM and other hyperthermals (at least 17 from 57.5-52.5 ma) sediment supply increased by 20-100 percent. The increase in nutrient supply during the PETM and subsequent hyperthermals clearly affected radiolarian abundances as well, as noted by Marcie Phillips.
Source to Sink
William Galloway, a senior scientist and emeritus faculty member at UT-Austin, has long worked on the Wilcox as part of the Gulf Basin Depositional Synthesis research project. In 2011, he extended his mapping of the Wilcox northward to the Rocky Mountain source terranes and defined the drainage catchments that supplied sediment to the basin during the peak of the Laramide orogeny. His peer-reviewed papers demonstrate the massive influx of siliciclastics during Wilcox time can clearly be linked to hinterland tectonic events. Subsequent papers by others (Mike Blum, Glenn Sharman and others) have confirmed this source to sink sediment routing using advanced provenance techniques such as detrital zircon geochronology. In 2018, we published a paper refining these drainage catchment maps (figure 4), but also demonstrating that the globally-based empirical scaling relationships between drainage basin size and submarine fan run out length and width are valid for the Paleogene of the northern Gulf of Mexico. In other words, big (and long) rivers often produce big submarine fans, and this is true for the Paleogene Wilcox.
Canyons, Escarpments and Karst
William Galloway also was one of the first researchers to investigate the origin of the Lavaca-Yoakum Canyon system of central Texas, where drilling and logging revealed a pinchout of Wilcox sandstones against a mud-filled canyon. Careful well log correlation, paired with 1980s vintage 2-D seismic and industry cores suggested that the canyon formed in a subaqueous, submarine environment. Years later, one of our students, Colin White (now a doctoral candidate at Stanford), used reprocessed ION seismic data to further study the Lavaca and Yoakum canyons (figure 5). He determined that these canyons must have formed due to retrogradational mass failure in a deep slope setting, based upon numerous slump structures in core and other mass transport processes. A key supporting point is the released biostratigraphic study of a well immediately adjacent to the canyon, the Lola Fuhrken-1 well (figure 6). Rashel Rosen, a well-known Gulf Coast biostratigrapher, interpreted the Wilcox section as having formed in water depths from upper slope to outer shelf. Biostratigraphically-calibrated sedimentation rates are rather high in the Wilcox interval and decrease upward, a pattern noted by Rosenfeld in his Historical Highlights article. However, the upward decreasing sedimentation rate can be linked to changes in drainage basin configuration, and hinterland paleoclimatic events that resulted in lower discharge. It should also be noted that the Lola Fuhrken well is located outside of the canyon complex and therefore the canyon itself must have been in even deeper water depths given the 3,000-foot scale of the Yoakum canyon (figure 5).
Paul Weimer, a past president of AAPG and 2020 Sidney Powers medalist, has published numerous papers on the GoM basin and concurs with the view that the Paleogene drawdown hypothesis is not based on our current understanding of submarine erosional processes and karst formation. “Discussion of erosional features in the southeastern Gulf of Mexico within the Cretaceous strata of the lower carbonate slope does not fit with current understanding of submarine erosion,” he said. Deepwater currents have been documented as causing significant erosion in many carbonate margins of the world. In fact, several independent researchers have documented that the adjacent southeastern portion of Florida Escarpment has retreated several kilometers since the late Miocene. Dredged samples of Aptian (Lower Cretaceous) through lower Tertiary strata have been recovered along the 2,000 meters of the near vertical submarine escarpment. The interpreted cause of the erosion is a combination of continuous deep-marine current erosion and ongoing mass wasting. No significant falls in sea level have ever been documented as the cause of this erosion. Thus, it can be deduced that deepwater currents are sufficient to cause the erosion, as cited by Rosenfeld.
In addition, suggested timing of two important events around the Gulf of Mexico Basin is misleading. The Poza Rica reservoirs have been determined by Xavier Janson and Robert Loucks of the Bureau of Economic Geology to be base-of-slope gravity flows deposited during the Aptian and Albian (late Early Cretaceous). The key diagenetic event for preserving reservoir porosity was the early subsurface dolomitization of the carbonate debrites and turbidites. There was no freshwater diagenesis in the Poza Rica reservoirs. Further, karstification at the Golden Lane Atoll, as documented by Andrew Horbury and others at Pemex, began during the Cenomanian (early Late Cretaceous). The age of the sealing strata at these fields vary from Turonian though Oligocene, indicating that the atoll was partially exposed between 2 to 50 my. Exposure thus began approximately 30 my prior the proposed Paleocene drawdown event.
The Ocean Gateways
The primary driver for the Paleogene drawdown invoked by Rosenfeld and others is interpreted closure of the Florida straits with hard docking of Cuba against the North American plate. However, tectonic reconstructions of the basin at this time suggest otherwise. Ian Norton, who is a research scientist for the PLATES project at UT-Austin, has done restorations for many areas across the globe over his 40-year career at ExxonMobil and UT. His model for the position of Cuba (figure 7) shows that the trench and ocean basin to the north of Cuba remained open during this time interval including the PETM. Convergence continued through the early Eocene until approximately 48 Ma (top UW) when advance of the Cuban plate system ceased. This reconstruction for the Cuban fold-thrust belt follows the “Pacific Origin” model for formation of the Caribbean, based on earlier work of James Pindell, Kevin Burke and John Dewey with refinements in timing as summarized in Pindell and Lorcan Kennan (2009) and incorporation of geologic data from Cuban researchers such as Manuel Iturralde-Vincent.
The existence of a second connection to the Atlantic Ocean is well-documented in the geologic literature, in contrast to the informal source quoted in Rosenfeld’s article. Nine peer-reviewed papers provide objective information on the genesis and character of the Suwannee strait that existed across central Florida from the Cretaceous until Middle Miocene.
With strong evidence collected from multiple geoscience disciplines, we provide an alternative view to that suggested by Rosenfeld and others to explain the Paleogene sandstone influx that formed one of the most important reservoir intervals in the northern GoM, while at the same time respecting their freedom to consider contrasting views and data. One can be entertained by iconoclasts espousing unique views, but we know from experience that a solid foundation of scientific investigation must support expensive exploration campaigns, particularly those with such a broad, regional scale as the deepwater Wilcox.
(Note: A list of key peer-reviewed papers supporting this article is available upon email request to the senior author at [email protected].)