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

Abstract: Relationship between Drag Fold Geometry and Fracture Production in the Selma Chalk, Gilbertown Oil Field, Southwest Alabama

The Gilbertown oil field is in an east-west trending full graben, detached in the Jurassic Louann Salt at the updip limit of salt in the Gulf of Mexico basin. The hydrocarbon production in the Cretaceous Selma chalk is from a rider block above the segmented, north-dipping master fault. Fluid production is proportional to the fracture abundance, which is directly related to the dip of bedding.

The drag-style geometry of the hanging wall is shown by regional structure contour maps and two key dipmeter logs. The dipmeter logs show hanging wall dips of 10°-40° away from the master fault, suggesting that they formed in a drag fold. Most secondary faults dip consistently north-northeast, synthetic and subparallel to the master fault, although they may diverge by up to 20° in strike. The associated drag fold axes are usually east-southeast trending and virtually horizontal.

Fractures are interpreted from resistivity, FIL, seismogram, caliper logs, and SCAT analysis of the dipmeter logs. Fractures and secondary faults are found only in the drag zone in the hanging wall right above the master fault. The abundance of fractures and/or minor faults is proportional to the magnitude of the hanging wall dip. The hydrocarbon and/or water production corresponds well with the identified fracture zones and fault zones. All hydrocarbons from the Selma chalk are produced from fractured zones and the rate of hydrocarbon production is proportional to the fracture abundance. 

AAPG Search and Discovery Article #90924©1999 GCAGS Annual Meeting Lafayette, Louisiana