Abstract: Area-Balanced Structural Model of a Fractured Chalk Reservoir: Toward Revitalizing Gilbertown Field, Choctaw County, Alabama

PASHIN, JACK C.
Geological Survey of Alabama, Tuscaloosa, AL;
RICHARD H. GROSHONG, and JIN GUOHAI
University of Alabama, Tuscaloosa, AL

Gilbertown Field is the oldest oil field in Alabama and has produced more than 14 million barrels of oil from fractured chalk of the Cretaceous Selma Group and from sandstone of the Eutaw Formation since 1944. The field is at the southern margin of the Gilbertown graben, which is part of the peripheral fault trend of the Gulf Coast basin. Oil production from the field has declined to economically marginal levels, and production from the Selma Group ceased in 1996. To identify opportunities to revitalize production from the Selma Group, we are developing area-balanced models of the Gilbertown graben.

Area-balanced restorations indicate that the Gilbertown graben began forming as a half graben during the Late Jurassic and evolved into a full graben during the Early Cretaceous. Requisite strain values are locally higher than 20 % in Jurassic strata and decrease to near zero in Cretaceous strata, indicating that small-scale deformation in the reservoir intervals is localized within the fault zones. Faultseal diagrams and completion records establish that a critical seal is developed where clay shale in the hanging wall is juxtaposed with chalk in the footwall and that fracture permeability is developed principally in the hanging-wall blocks of the faults.

Wells are completed as much as 250 ft below the seal along a 2 km-long segment of one fault, and a large segment of this fault remains to be drilled. Results of structural modeling indicate that infill drilling is a viable option for revitalizing the field and that horizontal drilling has potential for success in areas with prospective untapped oil.

AAPG Search and Discovery Article #90932©1998 GCAGS/GCS-SEPM Meeting, Corpus Christi, Texas