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Review of Late Jurassic Depositional Systems and Potential Hydrocarbon Plays of the Northern Gulf of Mexico Basin

T. E. Ewing
Venus Exploration, Inc., San Antonio TX

Post-salt Upper Jurassic siliciclastic and carbonate rocks in the East Texas - North Louisiana - Mississippi region record the post-rift evolution of the interior zone rift basins of the northern Gulf of Mexico Basin. Four major river systems were fixed in space but variable in supply rate; they carried sand and mud in from the northwest (ancestral Red River), the north (ancestral Ouachita River), the northeast (ancestral Mississippi River), and the east (ancestral Alabama River). Shoal-water carbonate deposits formed when siliciclastic inputs were low and shallow-water, high-energy conditions prevailed – usually in association with basement highs, salt swells and abandoned deltaic shelf margins.

Smackover shoal-water carbonates formed within an extensive carbonate ramp and were influenced by initial salt movement. Haynesville / Gilmer carbonate shoals in Texas rimmed rift-related high blocks, as well as highs formed by early salt movement. East of Shreveport, siliciclastic sedimentation dominated, and submarine fans developed in association with a lowstand episode. In Lower Cotton Valley (LCV) time, a major southeastward advance of the Texas delta formed the Taylor and Bossier sand series; to the east, lower Terryville shoreline sands began to prograde southward in front of an expanding lagoon. In Upper Cotton Valley (UCV) time, continued clastic sedimentation formed wave-dominated deltas in Texas and upper Terryville strike-fed sandstones in north Louisiana. At the close of Cotton Valley deposition in the earliest Cretaceous, regional transgression allowed Knowles carbonates to ’colonize’ the abandoned shelf edge and form a thick shelf-edge reef complex on its seaward slope. The significant ’Calvin’ lowstand wedge of sandstones was deposited offshore of this edge in Louisiana during the subsequent major sea-level fall, and was then covered byWinn carbonates deposited during the subsequent sea-level rise.

High-potential exploration plays remain in the Upper Jurassic sequences. Smackover and Haynesville / Gilmer shoals have produced prolifically in East Texas, but large segments of the complex atolls along the Sabine Uplift have not been defined. “Cotton Valley Lime” pinnacle reefs of the Robertson-Leon trend may develop in a broad swath of east-central Texas into westernmost Louisiana. Haynesvilleage submarine fans may extend eastward into Mississippi. Updip LCV and UCV are a mature tight-gas sandstone play in Texas and Louisiana, but their downdip extensions have high untapped potential, particularly the downdip LCV sandstones of the “Bossier” sandstone play of the Freestone-Robertson area and the Poole and other sandstones in Louisiana. Significant high-quality UCV production similar to the North Louisiana “blanket sands” may still be found in central Mississippi. The earliest Cretaceous (Knowles/Calvin/Winn) margin is poorly tested. Calvin-like sandstones of the lowstandwedge should occur in east-central Texas and in southwestern Mississippi, downdip of significant fluvial axes.

The major challenge for exploration in the Jurassic trends is prediction of adequate reservoir quality to support commercial completions. Higher porosity and permeability may be associated either with early migration of hydrocarbons or with relative isolation from diagenetic ground-water systems. Both processes suggest more favorable reservoir conditions in the deep-water extensions of deltaic systems (Bossier, Poole, Gray) relative to the deltaic and strandplain cores.

AAPG Search and Discovery Article #90901©2001 GCAGS, Annual Meeting, Shreveport, Louisiana