While the bulk of deepwater oil and gas discoveries in the northern Gulf of Mexico offshore areas lie within Cenozoic sandstones, Mesozoic siliciclastic reservoirs have been somewhat overlooked due to the depth and age of the reservoir section. That began to change with emergence of the Norphlet deepwater play, kicked off by discoveries in Mississippi Canyon and Desoto Canyon protraction blocks in 2010-13 by Shell and partners. The Norphlet reservoirs are aeolian (dryland) reservoirs located in modern abyssal water depths. However, the exploration potential of the younger Cretaceous section remains unknown, with just a handful of meaningful tests in deepwater areas.
That might be changing as BP and its partner Hess are drilling a potential play opening well in Mississippi Canyon protraction block 518. Prior to this well, dry holes at Baha II (AC 557-1), presently uneconomic gas discoveries at Davy Jones II (South Marsh Island 234-14) and Highlander (McMoran Jeannette Minerals No. 1), stranded oil in low permeability reservoirs in Tiber (KC 102-1), and rare salt-rafted hints of Cretaceous sandstones elsewhere are all that 20 years of pursuing the Cretaceous deepwater play had yielded until now.
Undrilled Salt Expulsion Rollovers
However, some of the largest salt structures in the Gulf of Mexico basin remained undrilled in the Mississippi Canyon protraction block. Called “salt expulsion rollovers,” these asymmetric structures often form due to a process involving sediment loading, salt evacuation and eventual collapse of salt flanking strata onto a basal, usually flat weld (where strata of different ages are juxtaposed). The final result are large structural closures, often 1,000 or more feet in scale. Sometimes these features are misinterpreted as sedimentary clinoforms, but the scale of these clearly exceeds the 600-foot scale of continental shelf margins.
The oil and gas industry has long hoped to test these immense Mesozoic structures, but uncertainties surround the reservoir section, given the lack of nearby penetrations. Enthusiasm for drilling has been dampened by successive downturns in oil prices and concerns over the unknown pressure regime within structures. However, there continues to be an obvious attraction of large prospects in an oil-rich super basin with a well- established production infrastructure that lowers the threshold for economic success, even in our current low-price environment.
Regional work by industry and by academic research consortiums has helped to reduce risk on reservoir presence. Building on the prior published work of Brian Horn and others, our GBDS research project reviewed updip wells in two sand-prone fairways, the Keathley Canyon and Mississippi Canyon axes. The Keathley Canyon axis wells include the aforementioned Baha II, Tiber, Davy Jones II, and Highlander wells, so this avenue is a better-defined pathway into the deepwater of Alaminos Canyon and Keathley Canyon protraction blocks. By contrast, the Mississippi Canyon axis has no equivalent deepwater well penetrations. However, a series of onshore wells and penetrations in state waters provide some indications of sandstone content that signal a potential pathway into the deepwater.
We examined one particular well, Arco Biloxi Marshlands P-2 that cored a series of highly porous and permeable sandstones with oil staining. While earlier work by Kurtus Woolf of UT-Austin interpreted these as shallow marine tidal deposits, the somewhat equivocal sedimentary structures could alternatively be considered deposits of sediment gravity flows. Our paleogeographic reconstruction also suggests slope depth waters.
We published our work in 2016 in Interpretation, an AAPG-Society of Exploration Geophysicists joint journal. We also showed the core description and maps in our book, “The Gulf of Mexico Sedimentary Basin: Depositional Evolution and Practical Applications,” published by Cambridge University Press in 2019.
Testing in Mississippi Canyon
The oil and gas industry is one of the few venues where scientific hypotheses are often directly evaluated by the drill bit. In this case, BP and its partners decided to test one of these large structures in Mississippi Canyon block 518, spudding a well, informally named Galapagos Deep in May of this year. BP produces from the nearby Galapagos field from Cenozoic reservoirs. This Mesozoic prospect and others like it are based in large part on mapping of wide azimuth seismic data, expensive but essential datasets to properly image and map structure and stratigraphy in these complex salt structures. Besides reservoir risk, uncertainties regarding timing of source rock maturation and migration timing relative to tectonic evolution of the salt structure are also being investigated. Seismic velocity analysis also plays a role in lithology prediction. Proprietary technologies such as these are critical in reducing exploration risk and uncertainty and results often may not disclosed for years following drilling.
One part of the seismic side of the story, however, was published in seminal paper by Andrew Harding and co-authors with Noble Energy in December of 2016 at the Gulf Coast Section of the Society of Sedimentary Geology Conference, which I helped to organize. Using WAZ datasets with special processing, Harding and his co-authors defined a series of Cretaceous primary basins, termed the Western, Central, and Eastern Cretaceous basins (WCB, CCB and ECB). Basin boundaries are salt walls and backstops where a sand transport fairway hits a dead end. Thus, the expected primary pathways for siliciclastics are mapped with considerable detail.
BP and its current partner Hess (and earlier partner Noble Energy) mapped the area as a foundation for this first test of these Mesozoic expulsion rollovers. However, other companies such as BHP, Chevron, Occidental, LLOG, and Murphy have over the years have picked up leases over these Mesozoic prospects, likely for Cenozoic targets but also with an eye toward offset or play extension wells if the Galapagos Deep well is successful. Thus, Galapagos Deep is a potential play opener, with considerable follow-up potential if the well is indeed successful. In any case, an important but expensive piece of the regional Cretaceous subsurface depositional history will have been obtained.
References are available upon request to the author, John Snedden, .