Mesozoic carbonate petroleum systems in the northeastern Gulf of Mexico area

Mancini, E. A., Marcello Badali, T. M. Puckett, and W. C. Parcell, Center for Sedimentary Basin Studies and Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama

Abstract

Three Mesozoic petroleum systems are known from the eastern Gulf Coastal Plain onshore area of Mississippi, Alabama and Florida. These include the Upper Jurassic Norphlet/Smackover/Haynesville /Cotton Valley (Oxfordian - Tithonian); the Lower Cretaceous Hosston/Sligo, James/Rodessa, Moorings- port/Paluxy, Andrew/Dantzler (Aptian-Albian); and the Upper Cretaceous Tuscaloosa/Eutaw/Selma (Cenomanian - Maastrichtian) systems. Total production from these three petroleum systems exceeds 11.5 TCF natural gas and 2.8 BBO.

Based on studies of the offshore continental shelfal area of the northeastern Gulf of Mexico, only the Upper Jurassic and Lower Cretaceous deposits of Mesozoic strata are found to be hydrocarbon productive in this area. Also, based on regional studies of the onshore and offshore shelfal areas, only Upper Jurassic and Lower Cretaceous carbonates are projected as viable Mesozoic carbonate petroleum systems in the continental slope areas of the northeastern Gulf of Mexico.

The primary petroleum source rocks for these two Mesozoic petroleum systems are believed to be Oxfordian to Kimmeridgian carbonates and Aptian to Albian carbonates and shales. The Tithonian shales and carbonates are effective source rocks in Mexico and therefore may also have potential in the outer continental shelf (OCS). The Upper Jurassic source beds are postulated to be gas and condensate prone in the OCS area of the northeastern Gulf of Mexico, while the Lower Cretaceous source beds are believed to be oil prone. The natural gas and condensate produced from the James Limestone in the OCS region is speculated to be sourced from Upper Jurassic (Smackover) carbonates, and the oil produced from the Washita carbonates in the OCS area is believed to be sourced from post James /Rodessa /Ferry Lake carbonates.

The principal carbonate petroleum reservoirs in the continental slope areas of the northeastern Gulf of Mexico are postulated to be Upper Jurassic deep water microbial boundstones developed on the outer portion of a distally steepened carbonate ramp. Deep water microbial buildups have been observed from outcrop analogs in western Europe. Some of these microbial buildups exceed 30 meters in thickness. In addition to these potential Upper Jurassic reservoirs, Lower Cretaceous lowstand carbonate wedge deposits comprised of reef debris rudstones and shelf-derived grainstones are believed to have high reservoir potential in this area. Such deeper water deposits have been observed on seismic reflection profiles located seaward of the Lower Cretaceous rimmed shelf margin. Diagenetic studies of the Washita carbonate reservoirs in the OCS Main Pass Blocks 253-254 Field indicate that these potential reservoirs would have been subjected to favorable diagenetic processes of dolomitization, dissolution, and karstification associated with repeated exposure of the Cretaceous shelf margin during post-Cenomanian time.

Although the presence of natural gas and condensate in the James Limestone and oil in Washita strata in the OCS region is believed to be attributed to petroleum seal rock distribution, understanding the distribution of regional petroleum seal rocks in the OCS area is problematic. The Lower Cretaceous (Rodessa /Ferry Lake) anhydrites above the James Limestone should serve as effective regional seals, and their presence is speculated as one of the reasons natural gas and condensate sourced from Upper Jurassic (Smackover) carbonates is trapped in the James Limestone, while Washita reservoirs in the OCS region are productive of oil. These stratigraphic relationships suggest that the oil in Washita reservoirs in the Main Pass Field is sourced from Albian source beds overlying the James/Rodessa/Ferry Lake interval. However, the onshore Upper Jurassic regional seal, the Buckner anhydrites, are projected to be absent in this OCS region, therefore, hydrocarbons generated by Upper Jurassic source beds have no regional barriers to flow in pre-James /Rodessa/Ferry Lake strata. Thus, potential Knowles, Sligo, and James reservoirs are postulated to contain natural gas and condensate sourced from Upper Jurassic carbonates in the slope areas of the northeastern Gulf of Mexico and post-James /Rodessa /Ferry Lake reservoirs are speculated to contain oil sourced from Albian carbonates in this region. Tithonian (Cotton Valley) shales have the potential of serving locally as a confining seal for hydrocarbon accumulations, but these shales are believed to lack the regional lateral continuity, and/or uniformity of thickness and/or lithology to act as effective regional seal rocks.

Seismic reflection profiles indicate the presence of salt-related structures and paleotopographic basement features in the northeastern Gulf of Mexico. Petroleum traps in this region, therefore, are postulated to be combination traps involving favorable stratigraphic relationships and salt tectonics. The timing of hydrocarbon generation, expulsion, migration and entrapment appears favorable for petroleum accumulation.

Therefore, the continental slope areas of the northeastern Gulf of Mexico appear to have high potential for successful hydrocarbon exploration and development.