ABSTRACT: A Subsurface Study of the Denkman Sandstone Member, Norphlet Formation, Hatters Pond Field, Mobile County, Alabama

R. Scott Higginbotham, Leonard M. Young, Elvon G. Anderson, Lawrence R. Baria

Hatters Pond field is in east-central Mobile County in southwestern Alabama. The field was discovered in 1974 by Getty Oil Company and produces from both the Norphlet and Smackover formations. The structural trap involves salt movement along the west side of the Mobile Fault System that resulted in a faulted salt anticline. The Norphlet Formation of southwestern Alabama consists of red to gray siltstone and pinkish to gray sandstone with conglomerate layers. Three facies have been distinguished within the Norphlet Formation: a lower shale, a red siltstone sequence, and an upper quartzose unit. The thickness of the formation ranges from a feather edge to more than 800 ft (234.8 m) in southwestern Alabama.

The Upper Jurassic Denkman Sandstone Member of the Norphlet Formation at Hatters Pond field is a medium- to fine-grained, well-sorted arkosic sandstone between the underlying Norphlet redbed lithofacies and the carbonates of the overlying Smackover Formation. Here, the Denkman Member can be subdivided into a massive upper unit and a low- to high-angle cross-stratified lower unit.

The sandstones are quartz-rich with a high percentage of feldspars. The majority of the feldspar grains observed are potassium feldspar. Microcline is usually less altered when compared with other types of feldspar grains. The major types of feldspar replacement include illitization, hematitization, dolomitization, chloritization, calcitization, vacuolization, and anhydritization. Carbonate replacement of feldspars is very abundant, mostly by ferroan dolomite. Rock fragments are not abundant in the Denkman Member, although there is good evidence of a metamorphic/volcanic source area. The sandstones are cemented by dolomite, calcite, anhydrite, and quartz and feldspar overgrowths. The lower Denkman unit is slightly more porous than the upper Denkman unit. The pore-lining authigenic cla , illite, greatly reduces permeability and porosity in these sandstones.

Primary source rocks for Denkman sediments at Hatters Pond field were derived from the Paleozoic Appalachian structural front to the north-northeast. Other positive features of the region, including the Conecuh Ridge, Pensacola Arch, Wiggins Arch, and Stockton Ridge, not only served as secondary source areas for Denkman sediments at Hatters Pond field, but probably aided in channeling clastics between these highs to the areas of deposition. The uplifted horst blocks of the Mobile Graben also may have served as a sediment contributor.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990