ABSTRACT

Deep Wilcox sandstones in the Rosita field produce large quantities of gas from highly faulted reservoirs at depths of 9,500 to 15,300 ft (2,895 m to 4,660 m). The producing sandstones were deposited downdip from a large, active growth-fault system. Six full-diameter cores and twenty-five electric logs were used to interpret structure and environment of deposition.

Structural movement along listric normal faults resulted in structural closures and stratigraphic thickening. These faults and rollover structures form hydrocarbon traps. Thick sequences of sandstones and shales show a basal shear zone, overlain by a folded and contorted zone, and finally, a normal bioturbated section. The sequence ranges from 10 to 50 ft (3 to 15 m) in thickness and may indicate soft-sediment deformation by slip along nearly horizontal glide planes. The extensive deformation suggests that the entire Wilcox section moved intermittently shortly after deposition under the force of gravity. Numerous deformed slices are present within major fault blocks. High pore pressure probably facilitated movement of these slide blocks.

The depositional environment of Wilcox sandstones in the Rosita field was probably outer neritic to upper bathyal. This interpretation is supported by regional stratigraphic location and trace fossil morphology. The style of deformation also is compatible with an outer-shelf or upper-slope morphology. The producing sandstones are classified as lithic and feldspathic graywackes. Variation of texture and composition is small. Low permeabilities of 0.001 md to 2 md can be attributed to high authigenic clay content.