A MICROBIAL SMACKOVER FORMATION AND THE DUAL RESERVOIR – SEAL SYSTEM AT THE LITTLE CEDAR CREEK FIELD IN CONECUH COUNTY OF ALABAMA

The Smackover Formation is only 80 – 100 feet (24 – 30 m) thick and consists of 7 distinct lithofacies at the Little Cedar Creek Field (LCCF) in Conecuh County of Alabama. From the base to the top, the following lithofacies are recognized: (1) a laminated peloid wackestone (mid-ramp) which overlies the red conglomerates (alluvial fan) of the Norphlet Formation with a sharp contact, (2) a bioturbated, peloid packstone (mid-ramp), (3) a microbial bindstone (inner ramp), (4) a laminated peloid wackestone – packstone (inner ramp), (5) a bioturbated peloid packstone (lagoonal), and (6) a peloid-ooid grainstone (beach). The sequence of lithofacies and their respective depositional environments indicate a shoaling upward cycle that formed by southward progradation following the rapid transgression of the Smackover sea. Virtually every lithofacies of the Smackover Formation exhibits microbial features, making the entire thickness of the formation microbial in origin at this location.

The microbial bindstone and ooid grainstone lithofacies are highly porous and permeable forming two distinct reservoirs at the LCCF. The microbial bindstone reservoir (10 – 30 feet) consists primarily of pellets and peloids binded by microbially and abiotically precipitated cements. Framework and intergranular pores generate porosities of 2 – 25% and permeabilities as high as 1.5 Darcies. The microbial reef reservoir is overlain by the non-porous and non-permeable bioturbated peloid packstone lithofacies (5 – 20 ft) forming the seal over this reservoir. The cause of the preservation of porosity in the microbial bindstone was marine cementation preventing extensive burial compaction.

The ooid grainstone reservoir (10 – 30 feet) is cross-laminated and has intergranular, moldic, vuggy, and intercrystalline porosity types. The abundance of microbially coated grains and composite particles suggest a low-energy beach where microbial activities were an integral part of the environment. The ooid grainstone reservoir grades upward into nonporous and nonpermeable wackestone and packstone facies, and eventually to green and red sandstone and shale layers. The reason for the preservation of porosity in the ooid grainstone reservoir was early meteoric diagensis which produced moldic and intercrystalline pore spaces.