ABSTRACT

Post-salt Upper Jurassic siliciclastic and carbonate rocks in the East Texas/North Louisiana/Mississippi region record the early 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 from south-central Arkansas to central Texas, and were influenced by initial salt movement. Subsequent Haynesville-age Gilmer carbonate shoals in Texas rimmed rift-related high blocks, as well as highs formed by early salt movement. Siliciclastic sedimentation progressively dominated east of Shreveport, and submarine fans developed in association with a lowstand episode. During early Cotton Valley time, a major southeastward advance of the ancestral Red River delta formed the Taylor and Bossier sand series; while to the east, lower Terryville shoreline sands began to prograde southward in front of an expanding lagoon. During late Cotton Valley 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 a subsequent major sea-level fall, and was then covered by Winn carbonates deposited during the subsequent sea-level rise.

High-potential exploration plays remain in Upper Jurassic strata. Smackover and Buckner/Gilmer shoals have produced over 10 trillion cubic feet of gas (TCFG) and 750 million barrels of oil (MMBO) in East Texas and North Louisiana, but large segments of the complex atolls along the Sabine Uplift have not been defined. Complex shoals may rim old rift-related highs such as the Sabine and Wiggins highs. The Gilmer ("Cotton Valley Lime") pinnacle reefs of the Robertson-Leon trend may develop (in one form or another) over a broad swath of east-central Texas into westernmost Louisiana. Haynesville-age submarine fans may extend eastward into Mississippi. Updip Cotton Valley is an established 10-TCF tight-gas sandstone play in Texas and Louisiana, but its basinward extensions have high untapped potential. Significant high-quality Upper Cotton Valley 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 lowstand wedge should occur in east-central Texas and in southwestern Mississippi.