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

ABSTRACT: Macroscopic Reservoir Heterogeneity in Upper Jurassic Smackover Formation Oil Plays, Southwest Alabama

Upper Jurassic Smackover Formation oil fields in southwest Alabama are grouped into five oil plays and four subplays. These plays differ in carbonate reservoir characteristics and in the nature of macroscopic heterogeneity as a result of depositional, diagenetic, and halokinetic processes. The plays and subplays can be classified into three groups (low, moderate, and high) that differ in the scale, type, and range of macroscopic reservoir heterogeneity.

Low macroscopic heterogeneity values characterize reservoirs in the Pickens, Gilbertown, and West Bend fault systems subplay of the regional peripheral fault trend play and the Mississippi interior salt basin play; low values probably are also associated with the Choctaw ridge complex subplay of the basement ridge play. Low heterogeneity results from the restriction of reservoirs to relatively homogeneous intervals of areally extensive porous grainstones with associated macroscopically homogeneous moldic and secondary intraparticle pore systems. Reservoirs in the Conecuh and Pensacola-Decatur ridge complexes subplay of the basement ridge play and the Pollard and Foshee fault systems subplay of the regional peripheral fault trend play exhibit high macroscopic heterogeneity values which can be attributed to the diversity of reservoir rock types and to the patchy distribution of porosity in intercrystalline and complex pore systems. Macroscopic heterogeneity values in the Mobile graben fault system play range from low to high. This range in heterogeneity values is due to variable reservoir rock types and pore system characteristics within the play. This variability results from the general north-south orientation of the Mobile graben fault system, which is approximately perpendicular to depositional and diagenetic trends. Moderate macroscopic reservoir heterogeneity values in the Wiggins arch complex play result from the diversity of reservoir strata present and the variable nature of the intercrystalline pore systems.

Delineation of the variability and distribution of macroscopic heterogeneity in Smackover reservoirs will assist in the exploration for hydrocarbons in these carbonate strata. In addition, the recovery of oil from the reservoirs can be improved by understanding the relationship of Smackover reservoir heterogeneity to rock types, diagenesis, and pore system characteristics.

This research was partially funded by the United States Department of Energy under contract No. DE-FG22-89BC14425.

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