Deep Mesozoic Gas Play in the Central and Eastern Gulf Coast Plain

Summarized and graphic reprinted with permission from AAPG. Full article titled “Mesozoic (Upper Jurassic-Lower Cretaceous) Deep Gas Reservoir Play, Central and Eastern Gulf Coastal Plain” was published in AAPG Bulletin, March 2008, v. 92, no. 3, pp. 283-308.

Evolution of the onshore interior salt basins and subbasins is related to the origin of the Gulf of Mexico. The Gulf of Mexico is a passive continental margin characterized by extensional rift tectonics and wrench faulting. Major negative features in the U.S. Gulf coastal plain include the North Louisiana and Mississippi Interior salt basins and Manila and Conecuh subbasins. Deposition was associated with rifted margin tectonics and was a result of basement cooling and subsidence that produced accommodation space for sediment accumulation. The greatest accommodation space was generated during the Late Jurassic to the Early Cretaceous.

Eleven transgressive-regressive stratigraphic sequences have been recognized within the onshore interior salt basins and subbasins for the central and eastern Gulf coastal plain. These sequences form a framework for the facies interpretation and reservoir identification in this study. Organic geochemical analyses indicate that the Upper Jurassic organic-rich laminated lime mudstone of the lower to middle part of the Smackover Formation serves as the regional source of most hydrocarbons. Burial history and thermal maturation profiles indicate that gas expulsion in the North Louisiana and Mississippi Interior salt basins began first in the southern part of these basins during the Early Cretaceous to Tertiary. Hydrocarbon flow pathway modeling indicates that the main source of secondary, nonassociated thermogenic gas produced from reservoirs in the North Louisiana salt basin is generated from the Smackover lime mudstone beds in the southern part of the basin.

Flow pathway modeling also indicates the importance of vertical as well as lateral migration in this petroleum system. The tectonic history of the basin resulted in normal and wrench faulting associated with fractures that facilitated vertical migration and stratigraphic and erosion surfaces that enhanced lateral migration. Late vertical migration through overlying strata probably resulted in mixing of thermogenic gas with gas that originated from the conversion of oil to gas in deeply-buried Upper Jurassic and Lower Cretaceous reservoirs. The chief differences between the North Louisiana and Mississippi Interior salt basin geohistories

Sequence-stratigraphic framework for the central and eastern Gulf coastal plain illustrating the transgressive and regressive (T-R) sequences recognized and the major petroleum seal rocks identified (modified from Mancini et al., 2006b, c).

In the North Louisiana salt basin, the estimate of secondary, nonassociated thermogenic gas in the Upper Jurassic Smackover from source rocks below 12,000 ft is 4,800 Tcf. Using a gas expulsion, migration, and trapping efficiency of 2-3%, the authors estimate that 96-144 Tcf of gas is potentially available. With cumulative production of about 29 Tcf, this leaves 67-115 Tcf of gas remaining in place. Using a 65% gas recovery factor, 44-75 Tcf of gas is potentially recoverable. Reservoir facies with gas potential in central Louisiana are interpreted to be preserved at depths greater than 12,000 ft. Potential gas reservoir facies are projected to occur in the central/southern Mississippi area at depths greater than 16,500 ft.

Workshop Topics
To Look Forward To
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• 4/6-8 U.S. Oil & Gas Technology Summit (U.S. Oil Expo Group) - Natchez, MS.

•  5/TBD Little Cedar Creek Field Case Study (Mississippi Geological Society) - Jackson, MS.

• 6/4 Enhanced Oil Recovery and CO2 EOR - Shreveport, LA.

• 7/23 Microbial Reservoir Play, Central and Eastern Gulf (Shreveport Geological Society) - Shreveport, LA.

• 8/TBD Sequence Stratigraphy and Its Application to Petroleum Exploration in Onshore Mesozoic Salt Basins, Gulf Coastal Plain - New Orleans, LA.