A Microbial Smackover Formation and the Dual Reservoir–Seal System at the Little Cedar Creek Field in Conecuh County of Alabama
Ezat Heydari1 and Lawrence Baria2
1Department of Physics, Atmospheric Sciences, and General Science, P.O. Box 17660, Jackson, Mississippi 39217; [email protected]
2Jura-Search Inc., P.O. Box 997, Jackson, Mississippi 39205; [email protected]
The Smackover Formation is only 80–100 feet (24–30 m) thick and consists of 7 distinct lithofacies at the Little Cedar Creek Field 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). These nearly pure carbonate lithofacies are overlain by red and green shale, sandstone, and conglomerate (lithofacies 7) interpreted as Smackover siliciclastics tidal flat deposits. 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. Such a situation was probably caused by harsh environmental conditions imposed by the geometry of the embayment and the ramp, low energy conditions, and poor seawater circulation.
The microbial bindstone and ooid grainstone lithofacies are highly porous and permeable forming two distinct reservoirs at the Little Cedar Creek Field. 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 diagenesis which produced moldic and intercrystalline pore spaces.
Unlike virtually all other Smackover fields in the Eastern Gulf, the Little Cedar Creek Field does not possess a Buckner anhydrite top seal. Furthermore, the Little Cedar Creek Field is also unique because both of its reservoirs are composed of limestone, not dolomite, as is the case in most Smackover fields in this region. At the time of this investigation the stratigraphic limits of the field remain undefined, and development drilling continues.
AAPG Search and Discovery Article #90080©2005 GCAGS 55th Annual Convention, New Orleans, Louisiana