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Pore Facies of Smackover Carbonate Reservoirs in Southwest Alabama

KOPASKA-MERKEL, DAVID C., and STEVEN D. MANN, Geological Survey of Alabama, Tuscaloosa, AL

Pore facies are rock units that are defined by certain proportions of pore types, contain specific pore-throat-size distributions, and exhibit certain consequent fluid-flow properties. Pore facies may contain only one pore type but more typically are characterized by a combination of several pore types. Data used in this study consist of point counts of thin sections that are plotted on ternary diagrams whose apices are pore types (ternary pore plots), commercial porosity and permeability data from core analyses, qualitative thin-section descriptions, and qualitative core descriptions.

Two partially intergrading pore facies are recognized in the Smackover of southwestern Alabama. These pore facies are defined based on the two most common pore types in the Smackover-particle-moldic (including secondary intraparticle) and intercrystalline pores-which together account for more than 85% of total porosity in the Smackover. Rocks assigned to the moldic pore facies exhibit similar depositional fabrics and experienced similar diagenetic processes. For example, this pore facies is most commonly composed of peloid and ooid grainstone (some dolomitized), modified by early cementation and particle dissolution. By contrast, in the intercrystalline pore facies, destruction of primary fabrics by nonfabric-selective dolomitization is almost the sole determinant of pore-system chara teristics. The moldic pore facies dominates to the northwest (Choctaw, western Clarke, Washington Counties), and the intercrystalline pore facies is dominant to the south and east (Mobile, Monroe, Baldwin, Escambia, and Conecuh Counties). Pore facies distributions overlap in some areas (e.g., western Monroe County), indicating that multiple pore facies occur in many Smackover fields.

The two pore facies exhibit substantially different petrophysical characteristics. The mean slope of regression lines of porosity on ln (natural log) permeability for the intercrystalline pore facies is 0.47, with a range of 0.19 to 0.90. The mean slope for the moldic pore facies is 0.22 with a range of 0.18 to 0.27. The higher slopes for the intercrystalline moldic pore facies mean that permeability values may be more precisely predicted from porosity data in this pore facies. The mean maximum permeability for the intercrystalline pore facies is 130 md; the corresponding value is 91 md for the moldic pore facies. High-permeability fluid conduits are more common in the intercrystalline pore facies than in the moldic pore facies. In addition, all high-permeability examples of the moldi pore facies

contain substantial amounts of interparticle porosity and are found near the Smackover subcrop. Porosity values are commonly higher in the moldic pore facies, which has a range of mean porosity of 10.2 to 28.0% compared to 9.6 to 20.5% for the intercrystalline pore facies. Greater hydrocarbon volumes can be stored in reservoirs dominated by the moldic pore facies, but connectivity is better in the intercrystalline pore facies.

Mixtures between the two pore facies are moderately common. Mixed pore systems commonly resemble the intercrystalline pore facies in the slope of the regression line of porosity on ln permeability. In strata with mixed pore systems, isolated particle-moldic pores are commonly connected by networks of intercrystalline pores which control transport of fluids out of particle-moldic pores.

 

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)