ABSTRACT

Gilbertown Field, the oldest oil field in Alabama, has produced oil from fractured chalk of the Cretaceous Selma Group since 1944 but is now in danger of abandonment. The field is at the southern margin of the Gilbertown graben, which is part of the peripheral fault trend of the Gulf Coast basin. 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 near zero in Cretaceous strata, indicating that small-scale deformation is localized within the fault zones. Resistivity, dipmeter, and fracture identification logs indicate that this deformation is mainly in the hanging walls of the faults, and curvature analysis suggests that the distribution and openness of fractures is influenced by subtle fault bends. Juxtaposition 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. Wells are completed as much as 400 ft below the seal along a 1.5 mi-long segment of one fault. Therefore, infill drilling has potential for reviving production, and horizontal drilling may be the preferred method in areas with prospective untapped oil.