Abstract

Many major Permian oil and gas fields in West Texas produce from unfaulted anticlines and domes. Published studies commonly explain the folds as products of differential compaction or drape over paleotopography. Data from two of these fields, Spraberry Trend Area and Wasson, suggests that the folds formed by post-Guadalupian epirogenic uplift.

The Spraberry Trend Area field produces from Leonardian deep-water sandstones and siltstones deposited in the Midland Basin. In southwestern Upton County, the Spraberry has been folded into a series of plunging anticlines and monoclines with structural relief up to 400 ft. The folds occur along the margins of a large block of lower Paleozoic rocks. There is no evidence of thinning associated with differential compaction. Restoration of the Spraberry to the original depositional surface suggests that the lower Paleozoic faults were reactivated after Guadalupian time. This basement-involved uplift apparently produced forced folds in the Permian section. The Spraberry has a regional set of northeast trending fractures. The fractures occur in both folded and unfolded areas so they appear to have formed in response to a regional compressive event.

The Wasson San Andres field produces from Guadalupian ramp dolomites deposited on the Northwestern Shelf of the Midland Basin. The structural crest of Wasson is a dome with over 300 ft of closure. The carbonate lithofacies indicate that there was no depositional high in the vicinity of the present dome during San Andres time. There is thinning associated with differential compaction but it occurs over the dome. Structural closure would be even greater if the rocks had not differentially compacted. Therefore, compaction cannot have produced the dome. The domal geometry appears to be the result of basement-involved uplift of a lower Permian carbonate buildup. The San Andres dolomites are also fractured at Wasson. There is no orientation data at Wasson but northeast-trending San Andres fractures have been reported elsewhere.

There is no data to precisely date the timing of these uplifts. However, the northeast-trending fractures could have formed in response to northeastly-directed Laramide compression during the Eocene. The structural style of the folds also indicates northeast-directed compressional deformation. Therefore, it is logical to correlate the folding and fracturing with the Laramide deformation because it represents the only post-Guadalupian regional compressive tectonic event in West Texas.