KOPASKA-MERKEL, DAVID G., Geological Survey of Alabama, Tuscaloosa, AL; and JAMES W. SCHMOKER, U.S. Geological Survey, Denver, CO

ABSTRACT: Controls on Porosity Evolution in Carbonate Reservoirs of the Smackover Formation (Upper Jurassic) Alabama

Two factors that influenced porosity evolution in the Alabama Smackover are thermal exposure and mode of dolomitization. Equivalent vitrinite reflectance (R<o>) and core-plug porosity (phi) for 32 Smackover fields in Alabama exhibit the commonly observed trend of overall reduction of porosity with increasing thermal exposure. However, Smackover reservoir strata in Alabama are anomalously porous for their level of thermal maturity (R<o> = 0.64-1.62). Also, the correlation between R<o> and "phi" is unusually weak. Core-plug porosity and R<o> data from Smackover fields exhibit a weak inverse power-function relationship; r{2} = 0.21-0.24. Consideration of factors such as rock type, depositional environment, and geographic location weakens the correlation between "p i" and R<o>. By contrast, incorporation of variation in mode of dolomitization (mimetic vs. nonmimetic) increases r{2} between "phi" and R<o>. Smackover reservoir rocks are assigned to three distinct pore facies controlled by mode of dolomitization. The moldic pore facies is the product of early mimetic dolomitization and widespread dissolution of aragonitic allochems. The intercrystalline pore facies owes its pore-system topology primarily to nonmimetic dolomitization. Intermediate pore systems incorporate both moldic and intercrystalline pore systems. Intercrystalline reservoirs were too few to analyze. Moldic and intermediate pore systems exhibit inverse power-function relationships between R<o> and "phi" with r{2} values from 0.24 to 0.36 and 0.27 to 0.64, respectiv ly. The slopes of regression lines are similar for these two facies, but intermediate pore systems are substantially more porous at a given value of R<o>.

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