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

Abstract

ABSTRACT The Gilbertown oil field is located along the southern boundary fault of the Gilbertown graben (Fig. 1). The Gilbertown graben trends east-west and is detached in the Jurassic Louann Salt at the updip limit of salt in the Gulf of Mexico basin (Pashin et al., 1998). The Gilbertown fault system forms the south side of the graben and the Melvin fault system forms the north side. The hydrocarbon production from the Cretaceous Selma chalk is from the rider blocks between both the East Gilbertown A-B and the West Gilbertown A-B fault segments (Fig. 1). Figure 1. Structure contour map of the top of the Selma Chalk, Gilbertown Field and vicinity (unsmoothed triangulation contouring), contour interval is 20 ft. The drag-style geometry in the rider block has been determined from regional 3-D subsurface mapping, area balancing (Pashin et al., 1998a, b), and two key dipmeter logs. The interpretation is illustrated (Fig. 2) by a cross section through a well (4195) which has a dipmeter log. The dipmeter shows hangingwall dips of 10† - 40† away from the master fault and horizontal footwall dips, indicating a drag fold confined to the hangingwall. Nearby wells have been projected onto the composite cross section according to their perpendicular distances from the East Gilbertown A fault at the top of the Selma in order to determine the probable extent of the oil production in the drag zone. Fractures and minor faults are interpreted from resistivity, FIL, and seismogram logs, caliper logs, and SCAT analysis (Bengtson, 1981) of the dipmeter logs. Fractures and minor faults are abundant in the drag zone in the hangingwall right above the master fault. 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. All hydrocarbons from the Selma chalk are produced from fractured regions within the drag zone. The proven productive area within the drag zone above the East Gilbertown A fault (Fig. 2) extends about at least ft vertically from the middle to the top of the Selma, and about 250 ft perpendicular to the master fault. End_Page 17------------------------ Figure 2. Composite cross section with drag fold interpretation in the vicinity of the master fault. Horizontal scale = vertical scale, A - G are faults interpreted from SCAT analysis. Wells are identified by their Alabama Oil and Gas Board permit numbers.