PASHIN, JACK C.¹ and RICHARD H. GROSHONG, JR.^2
1Geological Survey of Alabama, Tuscaloosa, AL
²University of Alabama, Tuscaloosa, AL

Abstract: Hanging-wall Fracture Systems in Chalk -- Underdeveloped Reservoirs in the Peripheral Fault Trend of the Eastern Gulf Coast Basin

The peripheral fault trend of the eastern Gulf Coast basin comprises numerous extensional fault systems that are detached in Jurassic salt. Oil is produced locally from naturally fractured chalk of the Upper Cretaceous Selma Group within the fault trend, yet regional structural relationships indicate that only a small number of potential reservoirs have been tested.

Characterization of the fractured chalk reservoirs in Gilbertown Field of southwest Alabama and Langsdale Field of southern Mississippi emphasized geophysical log properties, seal distribution, and fault curvature. Dipmeter, FIL, and FMI analyses indicate that deformation is concentrated in hangingwall drag zones. Clay shale of the Porters Creek Formation forms the topseal for the reservoirs, and lateral seals are formed by poorly fractured chalk in the footwalls and in the tip regions of the faults.

Production is distributed along the full length of the shortest fault in the study area and is concentrated near the tips of faults extending laterally for more than 4 miles. Second derivative maps of fault surfaces demonstrate that drilling success and well performance are greatest in areas of high anti-listric dip curvature and high strike curvature; thus, transport of chalk through minor fault bends appears to have enhanced fracturing.

Wells along large segments of the productive faults have been completed more than 200 feet below the topseal, indicating that a substantial oil column remains untapped. Elsewhere, development is concentrated in footwall uplifts near the central parts of the faults, leaving the hanging-wall tip regions virtually unexplored.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas