Abstract

Many of the largest Permian Basin fields are associated with folding that appears to be post-Permian in age, the result of early Tertiary reactivation of deeper fault blocks. The processes of differential compaction and drape over paleotopography are not sufficient to explain the larger folds.

The Wasson San Andres field is a good example of a commercially important post-Permian fold. Study of Wasson depositional facies indicates that there was no topographic high in the area of the structural crest during San Andres time. This rules out drape over preexisting topography as a cause for the structural high. There is evidence for differential compaction at Wasson, but compactional thinning over the structural crest reduced the height of the structure. Therefore, compaction could not have produced the structure. The best hypothesis is that the folding formed as a result of reactivation of deeper faults. This reactivation was probably part of the early Tertiary Laramide orogeny.

San Andres dolomites on the structural crest of Wasson are fractured. Vertical uplift of fault blocks when the San Andres reached its maximum burial could have added additional vertical stress to the overburden pressure, producing localized areas of elevated pore pressure within the San Andres. Such increased pore pressure may have been sufficient to allow fracturing to occur as a result of northeasterly-directed Laramide compression.

Reservoir development is affected by localized fracturing of San Andres and Clearfork strata. Fracturing provides locally-enhanced permeability that can have different production characteristics such as higher primary recovery and early water breakthrough during secondary recovery.

Reactivation of Pennsylvanian faults has significant implications for pre-Woodford structural plays and for the Pennsylvanian and Early Permian synorogenic detrital reservoirs. The orientation of the pre-Tertiary structural grain has a primary control on how individual fault blocks were reactivated. Northwesterly-trending faults had reactivation that was dominantly vertical so large anticlinal folds formed in the overlying, unfaulted section. Such large anticlinal reservoirs were found early in the exploration history of the Permian Basin. Reactivation of easterly-trending fault zones, however, probably produced movement with a strike-slip component and less vertical uplift. These zones have complex structure with numerous small fault blocks. Much of the future potential in the pre-Woodford plays lies in areas such as these where 3-D seismic can be used to image these small but profitable traps.