Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, Texas 78712–1722
Cretaceous Carbonate Sequences and Reservoir Systems of the Gulf of Mexico Basin
GCAGS Education/Leadership Forum 2.0: Foundational Talks for the Gulf of Mexico II (GRBCC, Assembly Area A)
Tuesday, September 22, 2015, 2:00 pm
Cretaceous reservoirs dominate world hydrocarbon resources with the combined effect of rich source intervals (commonly combined with Tithonian source) and prolific reservoir facies. Reservoirs in the Middle East, Venezuela, and the Gulf of Mexico (GOM) add to this picture. The Greenhouse setting of the Cretaceous was marked by warm climates, minimal eustatic fluctuations and platform exposure, and sluggish oceanic circulation, which favor extensive source bed formation and widespread reservoir development. The GOM Lower Cretaceous alone provides 25% of the cumulative oil production and 15% of the natural gas for this prolific basin. Remarkable similarities exist in sequence evolution and productive facies of Middle Eastern and GOM carbonates, but major differences in diagenesis (mostly cementation) have caused these reservoirs to pale relative to their Middle Eastern counterparts.
GOM Cretaceous passive margin strata comprise 8 super-sequences ranging from earliest Cretaceous through Campanian. They are named here for their platform elements: (1) Calvin, (2) Knowles, (3) Sligo/Cupido, (4) James/Cow Creek, (5) Rodessa/Glen Rose/Ferry Lake/Sunniland/El Abra, (6) Edwards/Georgetown/Stuart City/El Abra, (7) Buda, and (8) Eagle Ford/Austin/Tokio/Eutaw. Little data are available for the Calvin, but the remaining sequences illustrate a full spectrum of reservoir styles ranging from deepwater basinal debris fans and chalks to midshelf patch reefs and microporous tidal flats.
Barremian-Aptian Sligo-Cupido carbonates form the first circumGulf rimmed platform. Production from this trend is dominated by gas in the northern GOM. Transgression linked to oceanic anoxic event (OAE) 1a causes shutdown of Sligo carbonates and deposition of Pearsall shales. Recovery and stacking of Albian Glen Rose and Edwards/“Comanche” platforms sees a return of the first isolated patchreef formation from Arizona to Florida. Later, a well-organized reef-rimmed phase formed (which is analogous to the Sligo margin) that extended from the Campeche, around the GOM and to the Bahama-Florida megabank and beyond. Hydrocarbon production from this interval is dominated by large fields in eastern and southern Mexico (Tuxpan and Campeche), with mostly gas production coming from Albian carbonates in the U.S. GOM. The final interval of carbonate production in the Cretaceous accumulates during OAE2, with drowning of the platforms and deposition of the hemipelagicpelagic Eagle Ford/Austin strata. Production from these intervals represents the current frontier of unconventional drilling. One of the last major frontiers in the GOM will be in the northern GOM forereef slope and basin of the AptianAlbian platforms. Issues with source, trap, and reservoir quality will have to be overcome to realize the potential in this setting, but similar carbonates are prolific in the Poza Rica trend of eastcentral Mexico.