Oil Plays in Smackover Reservoirs of the Eastern Gulf Coastal Plain

MANCINI, ERNEST A., Geological Survey of Alabama and University of Alabama, Tuscaloosa, AL, and R. M. MINK, B. H. TEW, D. C. KOPASKA-MERKEL, and S. D. MANN, Geological Survey of Alabama, Tuscaloosa, AL

Five Smackover (Upper Jurassic, Oxfordian) oil plays can be delineated in the eastern Gulf Coastal Plain. These include the basement ridge play, the regional peripheral fault trend play, the Mississippi interior salt basin play, the Mobile graben fault system play, and the Wiggins arch complex play. Plays are recognized by basinal position, relationships to regional structural features, and characteristic petroleum traps. Within two plays, subplays can be distinguished based on oil gravities and reservoir characteristics. Reservoirs are distinguished primarily by depositional setting and diagenetic overprint.

The basement ridge play is updip of the regional peripheral fault trend where the Jurassic Louann Salt is thin or absent; structures in this trend formed on pre-Jurassic basement rocks. The basement ridge play is characterized by structural and combination traps. Reservoirs in the Choctaw ridge complex subplay are partially to completely dolomitized peritidal oolitic, peloidal, and oncoidal grainstone. Reservoirs produce heavy oils with gravities of 15 to 25 degrees API at depths ranging from 10,200 to 10,900 ft. Reservoirs of the Conecuh and Pensacola-Decatur ridge complexes subplay are subtidal to supratidal oolitic, oncoidal, intraclastic, and peloidal dolograinstone and dolopackstone, fenestral dolostone, quartz sandstone, and algal doloboundstone. Reservoirs in this subplay produ e intermediate to light oils with gravities of 30 to 55 degrees API. Reservoir depth ranges from 12,600 to 15,500 ft in this subplay.

The regional peripheral fault trend play is basinward of the updip limit of Louann Salt and is typified by salt-related structural features. These structural features occur in association with the Gilbertown, West Bend, Pollard, and Foshee fault systems and are generally parallel to the basin margin. The regional peripheral fault trend play is exemplified by salt-related structural and combination traps. Reservoirs of the Gilbertown and West Bend fault systems subplay are peritidal, nondolomitic to completely dolomitized oolitic, oncoidal, and peloidal grainstone and produce intermediate oils with gravities of 35 to 45 degrees API at depths ranging from 11,000 to 12,500 ft. Reservoirs of the Pollard and Foshee fault systems subplay are subtidal to supratidal partially to completely do omitized peloidal grainstone to packstone. Production is light oil of 50 degrees API gravity at depths of 15,000 to 15,700 ft.

The Mississippi interior salt basin play is downdip from the Gilbertown fault system and is characterized by structural and combination traps associated with salt tectonism in this basin. Reservoirs are peritidal nondolomitic to completely dolomitized oolitic and peloidal grainstone and packstone that produce intermediate to light oils of 40 to 50 degrees API gravities at depths of 13,100 to 13,600 ft.

The Mobile graben fault system play is located along the eastern limit of the Mississippi interior salt basin and is typified by salt-induced structural and combination traps and Smackover peritidal peloidal and oolitic dolograinstone and crystalline dolostone reservoirs at depths ranging from 12,300 to 17,100 ft. Production from these reservoirs is crude oil with intermediate gravities of 40 to 45 degrees API.

The Wiggins arch complex play is in a downdip basinal position and is exemplified by structural and combination petroleum traps associated with stratigraphic thinning and salt flow. The traps occur along the flanks of pre-Mesozoic paleohighs associated with this complex. Subtidal and supratidal peloidal and oncoidal dolograinstone and dolopackstone and crystalline dolostone reservoirs at depths of 16,200 to 18,400 ft produce light oils of 45 to 60 degrees API gravities in this play.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)