Abstract: A Strike-Slip Margin of the Block 31 Field, West Texas

Bob W. Krantz, Dennis Yarwood

The Central Basin Uplift in the Permian Basin displays a pattern of compressional blocks bounded by both reverse and strike-slip faults. Steep faults accommodate both lateral and vertical displacements. ARCO's Block 31 field occupies the crest of a large thrust-bound wedge. The wedge and its bounding thrusts terminate against a steep lateral fault zone with complex displacements. These vary along strike from dominantly lateral to dominantly vertical. We have interpreted this lateral fault as a compartmental strike-slip fault, separating two structural domains or compartments that have different magnitudes of compressional deformation and horizontal shortening.

Structures within the compartmental fault zone are complex, and difficult to interpret even with 3-D seismic data. En-echelon faults and restraining/releasing bend patterns suggest both left-and right-lateral offset. In section view, faults are steep and marge with depth. After detailed mapping, we recognized distinct domains of lateral offset that correspond to movements of the major blocks on either side of the compartmental fault.

We then reinterpreted individual faults and small fault-bound wedges with potential closure. One of these wedge "pop-ups," developed at a restraining bend in the fault, was tested in 1993. An existing well on the crest of the pop-up was deepened to test the reservoir interval that is the main producer in Block 31 field. The reservoir was encountered sat the prognosed depth with oil shows, but had insufficient permeability. Although the structure tested is small, understanding its geometry and development can help us appreciate the role of compartmental faults and lateral motions in Permian Basin geology, and may open new exploration opportunities along similar fault zones.

AAPG Search and Discovery Article #90960©1995 AAPG Southwest Section Meeting, Dallas, Texas