TEW, BERRY H., Geological Survey of Alabama and University of Alabama, Tuscaloosa, AL, ROBERTY M. MINK and STEVEN D. MANN, Geological Survey of Alabama, Tuscaloosa, AL, and ERNEST A. MANCINI, Geological Survey of Alabama and University of Alabama, Tuscaloosa, AL
ABSTRACT: Geologic Framework of the Jurassic (Oxfordian) Smackover Formation in the Alabama Coastal Waters Area
The Jurassic (Oxfordian) Smackover Formation is a prolific hydrocarbon-producing geologic unit in the onshore Gulf of Mexico area, including southwest Alabama. However, no Smackover strata containing commercial accumulations of oil or gas have thus far been discovered in the Alabama state coastal waters area (ACW). This study of the regional geologic framework of the Smackover Formation was done to characterize the unit in the ACW and to compare strata in the ACW with productive Smackover intervals in the onshore area. In the study area, the Smackover Formation was deposited on a highly modified carbonate ramp. Variations in the general basinward slope of the ramp are associated with pre-Smackover topographic features. In the onshore Alabama, north of the Wiggins arch complex, an inne ramp developed in the area of the Mississippi interior salt basin and the Manila and Conecuh embayments. South of the Wiggins arch complex, in extreme southern onshore Alabama, and in the ACW, an outer ramp formed that was characterized by a much thicker Smackover section. In the outer ramp setting, four lithofacies associations are recognized: lower, middle, and upper outer ramp lithofacies (ORL), and the coastal dolostone lithofacies. Initial carbonate deposition associated with incipient Smackover transgression is represented by the lower ORL, which, generally, consists of finely laminated strata that accumulated in a tidal-flat setting. The middle ORL is characterized by organic-rich mudstone; these strata represent progressive upward-deepening associated with continued relative sea level rise. The upper ORL records basinward progradation of high-energy deposits in association with regression and overall upward shallowing. The coastal dolostone lithofacies includes dolomitic upper and, possibly, middle ORL. Data indicate that the coastal dolostone lithofacies, which is present immediately south of the Wiggins arch complex and pinches out basinward, accounts for most of the reservoir-grade porosity in the outer ramp setting; the lower, middle, and upper ORL, for the most part, are nonporous. Volumetrically, intercrystalline porosity is the most important pore type in the coastal dolostone lithofacies. Numerous data in the ACW area indicate that halokinesis has created structural conditions favorable for accumulation and entrapment of oil and gas in the outer ramp lit ofacies of the Smackover. Further, prolific hydrocarbon source rocks are present in the ACW, as evidenced by the significant natural gas accumulations in the Norphlet Formation. To date, however, reservoir quality rocks of the coastal dolostone lithofacies coincident with favorable structural conditions have not been encountered in the ACW.
AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.