AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Carbon Isotope Stratigraphy of the Cretaceous Gulf of Mexico: Global Utility and Regional Implications
(1) Department of Geological Sciences, The University of Texas at Austin, Austin, TX.
(2) Bureau of Economic Geology, The University of Texas at Austin, Austin, TX.
Carbon isotope chemostratigraphy is a powerful tool that allows correlation of globally significant environmental perturbations such as Cretaceous Oceanic Anoxic Events (OAEs). Using cores of the Cretaceous Comanche Shelf in south Texas, this study establishes a continuous carbon isotope profile spanning the lower Hauterivian to lower Coniacian (~135 Ma to ~88 Ma). The profile consists of isotope curves from major oil and gas producing carbonate intervals in the Gulf of Mexico and allows correlation to reservoirs in the Middle East, as well as carbonate successions in Asian, Pacific, and Tethyan regions. The South Texas Carbon Isotope Profile (STCIP) is constructed using 920 samples from 9 cores of platform limestone and shale. Cores are dispersed through numerous depositional environments including peritidal, reefal, and deep subtidal. Sample material was obtained from powdered carbonate matrix, including mud, peloids, and fine skeletal debris. Dolomitic samples were avoided. An array of well logs aided correlation of cores and isotope profiles. Results and applications of the STCIP are global and regional in nature. We show that each shale unit in the Comanche Shelf is equivalent to globally recognized OAEs including the: Hammett Shale (OAE 1-A), Bexar Shale (OAE 1-B), Del Rio Clay (OAE 1-D), and Eagle Ford Shale (OAE 2). These OAEs are likely the source rock of hydrocarbons in the Cretaceous Gulf of Mexico and equivalent global reservoirs. An exquisite match in STCIP curve character to published data demonstrates the Pearsall Formation in south Texas is the temporal equivalent of the Shu’aiba Formation in Oman. At a finer scale in each respective formation, the Hammett Shale is equivalent to the Hawar Shale (OAE 1-A), oil producing grainstones of the James Member are equivalent to oil producing platform carbonates of the Shu’aiba, and the upper Bexar Shale is equivalent to shales of the Bab Member (OAE 1-B). At the regional scale, STCIP data of the Comanche Shelf illustrate the diachronous nature of lithofacies and formation contacts between the prolific natural gas producing Eagle Ford Shale and overlying Austin Chalk. The profiles confirm that a significant portion of distal thickening in Eagle Ford black shale lithology is due to a facies change from Austin Chalk limestone to black shale. Knowledge of this partial time equivalence between the Eagle Ford and Austin Chalk formations should constrain regional correlations of gas producing intervals.