Sequence Stratigraphic Framework of the Upper Jurassic Smackover and Related Units, Western Gulf of Mexico
MOORE, CLYDE H., and YEHEZKEEL DRUCKMAN, Lousiana State University, Baton Rouge, LA
Post-Norphlet Upper Jurassic subsurface rocks of the western Gulf may be divided into four sequences encompassing some 12 million years (Lower Zuni A-4). The transgressive systems tract of the lower sequence (Smackover) consists of laminated muddy carbonate rocks, and is the source for upper Jurassic hydrocarbons. The highstand systems tract of the Smackover consists of blanket ooid grainstones deposited on a platform. The Smackover sequence is terminated by a relative sea level fall, with subaerial exposure and development of secondary porosity in the platform interior.
The Buckner "B" begins with a siliciclastic lowstand fan ("C" sand). Highstand deposits of the "B" consist of subaqueous lagoonal evaporites and an ooid grainstone prograding barrier system. Reflux of Buckner brines into Smackover grainstones resulted in regional dolomitization.
The Buckner "A" is similar to the "B" except that the lagoon is dominated by siliciclastics. The Gray Sands of northern Louisiana may represent a lowstand fan initiating the "A" sequence.
The last sequence (Gilmer) is dominated by siliciclastics in Arkansas and Louisiana and limestones in east Texas. Its highstand systems tract in Texas is ooid dominated and is similar to the Buckner "B" and "A." The regionally extensive Bossier Shale represents the transgressive systems tract of the next major sequence (Cotton Valley).
Jurassic hydrocarbon production is controlled by sequence architecture: porosity of Smackover highstand deposits are dominantly secondary, developed by exposure during sea level fall and by dolomitization during the subsequent Buckner "B" highstand, while Buckner and Gilmer highstand porosity is primary; Smackover traps are structural because of the blanket nature of the highstand system, while Buckner traps are stratigraphic developed in progradational highstand wedges. Siliciclastic lowstand fans are key units in deciphering the sequence stratigraphy of the Upper Jurassic of the western Gulf.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)