MINK, ROBERT M., BERRY H. TEW, STEVEN D. MANN and ERNEST A. MANCINI, Geological Survey of Alabama, Tuscaloosa, AL

ABSTRACT: Characterization and Geologic Framework of the Jurassic (Oxfordian) Smackover Formation in the Alabama State Coastal Waters Area

The Jurassic (Oxfordian) Smackover Formation is an important and prolific hydrocarbon producing geologic unit in the onshore Gulf of Mexico area, including onshore Alabama. To date, however, no Smackover strata containing commercial accumulations of hydrocarbons have been encountered in

Alabama state coastal waters area (ACW). A regional study of the geologic framework of the Smackover Formation was undertaken in order to characterize the unit in the ACW, compare strata in the ACW with strata in the productive onshore area, and to assess the future hydrocarbon potential of the Smackover in the offshore Eastern Gulf of Mexico.

The Smackover Formation in the study area accumulated in a highly modified carbonate ramp setting. Variations in the general basinward slope of the ramp are associated with pre-Smackover topographic features. In the onshore area north of the Wiggins arch complex (a major east-west trending structural feature), an inner ramp developed in the area of the Mississippi interior salt basin and the Manila and Conecuh embayments. Smackover rocks that were deposited in the inner ramp setting have been extensively studied in association with the exploration for and development of the significant hydrocarbon reserves occurring in these strata. South of the Wiggins arch complex, in extreme southern onshore Alabama and in the ACW, an outer ramp developed. Four lithofacies associations are recogniz d in the outer ramp setting. They are the lower, middle, and upper outer-ramp and the coastal dolostone lithofacies. The lower outer-ramp lithofacies represents initial carbonate deposition associated with the Smackover transgression and, in general, consists of low energy, finely laminated strata. The middle outer-ramp lithofacies is characterized by low energy, organic rich rocks that represent progressive upward-deepening associated with continued relative sea level rise. The upper outer-ramp lithofacies represents progradation of high energy deposits in association with regression and overall relative sea level fall. The coastal dolostone lithofacies includes dolomitized upper outer-ramp and, possibly, middle outer-ramp lithofacies. This study indicates that the coastal dolostone lit ofacies, where present, accounts for most of the reservoir-grade porosity in the outer ramp setting south of the Wiggins arch complex; the lower, middle, and upper outer ramp lithofacies are, to a large degree, tight. Intercrystalline porosity is volumetrically 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 of hydrocarbons in the outer-ramp lithofacies of the Smackover. In addition, prolific hydrocarbon source rocks occur 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 bee encountered in the ACW.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.