From Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
For more information, please contact the author.

2004 West Texas Geological Society Fall Symposium, Midland, Texas, October 27–29.

Evidence of Post-Wolfcampian Fault Movement and Its Impact on Cleark Ford Reservoir Quality: Fullerton Field, West Texas

Rebecca H. Jones and Stephen C. Ruppel

ABSTRACT

An integrated study of seismic data, cores, and wireline logs from the Fullerton Clear Fork field, Central Basin Platform, West Texas, suggests that deep-seated faults influenced reservoir architecture, facies patterns, diagenesis, and ultimately porosity distribution in this Leonardian reservoir. These faults, mapped from recent 3-D seismic and older 2-D lines, provided a mechanism, in addition to eustasy, for changes in accommodation and circulation, as well as postdepositional sediment transport.

Whereas most major faults and associated erosion occur below the middle Wolfcampian unconformity—a widely recognizable downlap surface in the Permian Basin—a number of faults show continued movement into the basal Leonardian composite sequence 1 (L1: Abo and Wichita) and in some cases Leonardian composite sequence 2 (L2: Wichita and lower Clear Fork). L1 (Abo) clinoforms initiate close to fault boundaries and follow conduits coincident with deeper downthrown normal fault blocks toward the basin. Stratigraphic thinning of the lower Clear Fork is apparent on the high side of older fault blocks, suggesting continued or reactivated motion of older faults during L2 deposition. Slump and downslope transport features were observed in core located very close to the boundary of a downthrown fault block, suggesting that reactivation of this fault may have been the cause of sediment transport. Porosity trends in the reservoir share common boundaries with some of these structural elements, suggesting fault movement may also have exerted small but significant controls on paleotopography during deposition and postdepositional diagenesis of rocks as young as the Clear Fork.

These observations challenge previous assumptions of tectonic quiescence following the middle Wolfcampian unconformity and instead suggest continued, albeit diminished, tectonic activity at least into the Leonardian.