Regional porosity evolution in the Smackover Formation of Alabama

Abstract

The Upper Jurassic (Oxfordian) Smackover Formation is the most prolific oil producer in Alabama. Smackover strata in Alabama were deposited on carbonate ramps under conditions of stable relative sea level. Reservoir rocks consist primarily of nonskeletal (primarily oolitic and pelletal) dolograinstone, with lesser amounts of nonskeletal dolopackstone, microbial doloboundstone, and nondolomitized nonskeletal grainstone and packstone. Porous and permeable strata are classified on the basis of the relative proportions of genetic pore types as belonging to one of three pore facies: moklic, intercrystalline, and intermediate (mixtures of moklic and intercystalline pores at various scales). Equivalent vitrinite reflectance (Ro) data, commercial core-plug porosity data, conventional core description, and point counting of petrographic 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 Smackover reservoirs of Alabama, as they are in most sedimentary rocks. However, the overall correlation between porosity and Ro is not strong, indicating that other factors significantly affected porosity evolution. Factors of rock type (limestone vs. dolomite), depositional environment, and depositional fabric did not strongly influence regional porosity evolution. By contrast, mode of dolomitization as expressed by pore-facies classification had a direct effect on regional porosity evolution. This results from regional geographic trends in mode of dolomitization. Subdivision of the data set by pore facies increases correlation coefficients of porosity-Ro as a function of geographic variation in pore-facies assignment. Intermediate pore systems are more porous than moldic pore systems at a given value of Ro. Reservoirs with intercrystalline pore systems were too few to analyze quantitatively, but these reservoirs plot approximately within the field defined by intermediate reservoirs on a porosity-Ro plot, and resemble intermediate reservoirs petrophysically. Smackover reservoir rocks on the north flank of the eastern (Alabama) end of the Wiggins arch, a major paleohigh that was partially exposed throughout deposition of the Smackover, are unusually porous for the Smackover of Alabama. Smackover reservoirs in this area are unique in other ways as well: they are more completely dolomitized, contain more hydrocarbons than would a similar number of Smackover fields elsewhere in Alabama, are dominated by intercrystalline pore systems (uncommon elsewhere in the Alabama Smackover), and exhibit values of mean porosity, mean permeability, standard deviation of permeability, Dykstra-Parsons coefficient, and microscopic reservoir heterogeneity (variable defined by Kopaska-Merkel and Mann, 1992) that deviate from regional trends.