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