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Reservoir Characteristics of the Smackover Formation at the Little Cedar Creek Field, Conecuh County, Alabama
The Little Cedar Creek Field appears to be the largest Smackover field discovered in the northern U.S. Gulf Coast in the last three decades (Fig. 1). By June 2006, the field demonstrated an oil column of at least 850 feet, and extended along strike over eight miles in length (Fig. 1). The goal of this short communication is to present characteristics of the reservoir lithofacies in this field.
Seven lithofacies were recognized (Heydari and Baria, 2005). From base to top they include the following: (1) laminated peloid wackestone (S-1); (2) bioturbated, peloid packestone (S-2); (3) microbial bindstone (S-3); (4) laminated peloid wackestone-packstone (S-4); (5) bioturbated peloid packstone (S-5); (6) peloid-ooid grainstone (S-6); and (7) wackestone, shale, and siltstone (S-7) (Fig. 2).
Characteristics of these lithofacies indicate deposition in a ramp environment (Fig. 3). The stacking pattern of these lithofacies suggests that the Smackover Formation in this field consists of one shoaling upward cycle (Fig. 2). The deposition of the cycle began with the laminated peloid wackestone lithofacies which overlays the conglomerate of the Norphlet Formation with a very sharp contact, indicating that a rapid transgression submerged dunes and alluvial fans of the Norphlet Formation (Fig. 2). A long-term progradation resulted in the deposition of the observed stacking pattern (Fig. 3). The deposition of this cycle was terminated by the grainstones of the beach (lithofacies S-6) and siliciclastics of tidal flat environments (lithofacies S-7).
Of the seven lithofacies of the Smackover Formation, two form excellent reservoirs. These are the microbial bindstone (S-3) and the peloid-ooid grainstone lithofacies (S-6) (Fig. 2). The two reservoirs are separated from one another by a non-porous and non-permeable horizon (Figs. 2 and 4).
The microbial lithofacies of the Smackover Formation has been the center of intense study due to its excellent porosity and permeability characteristics (Kopaska-Merkel, 1994; Kopaska-Merkel and Schmid, 1999; Parcell, 2002; Mancini et al., 2004). The microbial bindstone reservoir at the Little Cedar Creek Field is 10 to 30 feet thick (Fig. 2). It consists of pellets, peloids, ostracods, and forams which were binded and cemented by a combination of microbial and abiotic processes resulting in the formation of reefal character (Fig. 5). The porosity of microbial lithofacies at the Little Cedar Creek Field ranges from 2 to 30%. Dominant porosity types are framework and intergranular (Figs. 2 and 5). The microbial reef reservoir is overlain by a bioturbated peloid pack-stone lithofacies with little porosity and permeability which forms the seal (Fig. 2). The dominant process which resoluted in the preservation of porosity was marine diagenesis including microbial binding and cementation.
The upper reservoir consists predominantly of a cross-laminated peloid and ooid grainstone and ranges in thickness from 10 to 30 feet (Fig. 2). This reservoir grades upward into wackestones and then into green and red shales. Its dominant porosity types are intergranular, modic, vuggy, and intercrystalline (Fig. 6). Processes which resulted in the reservoir formation were meteoric in nature causing dissolution of originally aragonitic grains.
Unlike virtually all other Smackover fields in the Eastern Gulf, Little Cedar Creek does not possess a Buckner Anhydrite top seal immediately above the reservoir. Although these stratal geometries and facies are not uncommon throughout the Smackover trend of southwestern Alabama, Little Cedar Creek Field is unique in that both of its reservoirs are composed predominantly of limestone, not dolomite, as is the case in most Smackover fields in this region.
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