Most structural models utilizing 3-D visualization software are based on seismic data sets with sparse wall control, yet many mature fields of the Gulf Coast basin contain numerous wells and limited seismic control. Using well data only, we have constructed a 3-D model of the Gilbertown graben of southwest Alabama, which is associated with oil fields developed in Jurassic and Cretaceous strata. Our strictly well-based model has proven valuable for visualization of the Gilbertown graben, yet limitations of the data set necessitate extreme care in model construction.

The Gilbertown graben is visualized in GeoSec3D with thirteen stratigraphic horizons in 625 wells. The structure is an east-west trending full graben, 4 km wide and 3 km deep, formed by extension near the updip limit of the Louann Salt. Faults bounding the graben are growth structures with up to 800 m of vertical displacement in Jurassic strata and less than 150 m in Upper Cretaceous and younger strata. Gilbertown field is a fault trap on a slight footwall uplift along the southern margin of the graben; reservoirs are in the Upper Cretaceous Eutaw sandstone and in fractured Selma chalk. Jurassic Smackover limestone reservoirs are in footwall uplifts on both sides of the graben.

Sequential area-balanced restorations suggest that the Gilbertown graben began growing as opposed, partially overlapping half grabens during the Jurassic and evolved into a full graben during Cretaceous time as the rate of slip on antithetic faults became equal to that on the master faults. The master faults of the Gilbertown graben are curved in plan and section, and map-scale grooves in the dip direction indicate dip slip. Average fault dip is about 60° in deep stratigraphic units but is closer to 45° in the Upper Cretaceous Selma chalk and younger units, probably due to compaction. Plan-view fault curvature occurs in accommodation zones that formed within the graben and by interaction of major faults of the Gilbertown graben with other fault systems. Small, isolated salt pillows are developed in the Jurassic section below both master faults, and small faults are associated with these structures.

The greatest problem in model construction is ensuring that bed surfaces do not cross the faults. This requires careful editing of fault shapes, fault-bed intersections, and the elimination of edge effects in constructing the fault planes. Wells are rare in the deeper part of the graben, and so the positions of stratigraphic horizons are inferred from fault cuts and stratigraphic thicknesses. Bed geometry changes across the master faults. Therefore, beds must be contoured independently on both sides of the faults.

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