REGIONAL POROSITY EVOLUTION IN THE SMACKOVER FORMATION OF ALABAMA

The Upper Jurassic (Oxfordian) Smackover Formation is the most prolifi c oil producer in Alabama. Smackover strata in Alabama were deposited on carbonate ramps under conditions of stable relative sea level. Rese rvoir rocks consist primarily of nonskeletal (primarily oolitic and pe lletal) dolograinstone, with lesser amounts of nonskeletal dolopacksto ne, microbial doloboundstone, and nondolomitized nonskeletal grainston e and packstone. Porous and permeable strata are classified on the bas is of the relative proportions of genetic pore types as belonging to o ne of three pore facies: moldic, intercrystalline, and intermediate (m ixtures of moldic and intercrystalline pores at various scales). Equiv alent vitrinite reflectance (R(o)) data, commercial core-plug porosity data, conventional core description, and point counting of petrograph ic thin sections were used to evaluate the relationships among thermal exposure, pore types, and porosity evolution in Smackover reservoirs in Alabama. Porosity and thermal exposure are inversely related in Sma ckover reservoirs of Alabama, as they are in most sedimentary rocks. H owever, the overall correlation between porosity and R(o) is not stron g, indicating that other factors significantly affected porosity evolu tion. Factors of rock type (limestone vs. dolomite), depositional envi ronment, and depositional fabric did not strongly influence regional p orosity evolution. By contrast, mode of dolomitization as expressed by pore-facies classification had a direct effect on regional porosity e volution. This results from regional geographic trends in mode of dolo mitization. Subdivision of the data set by pore facies increases corre lation coefficients of porosity-R(o) as a function of geographic varia tion in pore-facies assignment. Intermediate pore systems are more por ous than moldic pore systems at a given value of R(o). Reservoirs with intercrystalline pore systems were too few to analyze quantitatively, but these reservoirs plot approximately within the field defined by i ntermediate reservoirs on a porosity-R(o) plot, and resemble intermedi ate reservoirs petrophysically. Smackover reservoir rocks on the north flank of the eastern (Alabama) end of the Wiggins arch, a major paleo high that was partially exposed throughout deposition of the Smackover , are unusually porous for the Smackover of Alabama. Smackover reservo irs in this area are unique in other ways as well: they are more compl etely dolomitized, contain more hydrocarbons than would a similar numb er of Smackover fields elsewhere in Alabama, are dominated by intercry stalline pore systems (uncommon elsewhere in the Alabama Smackover), a nd exhibit values of mean porosity, mean permeability, standard deviat ion of permeability, Dykstra-Parsons coefficient, and microscopic rese rvoir heterogeneity (variable defined by Kopaska-Merkel and Mann, 1992 ) that deviate from regional trends.