82-6 Insights into the characteristics of subsurface deposits of the Pennsylvanian Upper Strawn Group, Permian Basin, Texas from an outcrop analog in the contemporaneous Bug Scuffle Member of the Gobbler Formation in the Sacramento Mountains, New Mexico

Abstract:

The Desmoinesian Upper Strawn Group on the Eastern Shelf of the Permian Basin, Texas (TX), contains oil/gas reservoirs that have produced for over a century. The Strawn comprises mixed carbonate-siliciclastic deposits recording relationships between deltaic point sources, a tidally dominated siliciclastic shelf, and a carbonate ramp during high-amplitude (>100 m), high-frequency (20-100 kyr) sea-level fluctuations of the Pennsylvanian icehouse. Glacioeustatic fluctuations combined with internal complexities in the evolution of deltaic systems and carbonate platforms produced highly variable vertical and lateral facies mosaics and key surfaces/transitions. While deposits of mixed carbonate-siliciclastic systems may be common in the rock record, their internal characteristics remain poorly documented at high-resolution, and controls on complex stratigraphic stacking patterns and facies transitions remain somewhat enigmatic, particularly in icehouse systems.

Sparse, non-analogous Strawn outcrops in TX mean that in the subsurface only rare core, localized wireline logs, and isolated seismic profiles inform depositional models. To provide a deeper understanding of the characteristics of the subsurface Strawn along the Eastern Shelf we first examine a series of publicly available drill core and a high-density wireline log dataset from near the Katz Field in Stonewall County, TX to develop a new model of the paleoenvironmental evolution of Upper Strawn deposystems. Next we analyze an outcrop analog from the mixed carbonate-siliciclastics of the Bug Scuffle Member of the Gobbler Formation in the Sacramento Mountains, New Mexico (NM). The Strawn and Bug Scuffle both contain deposits of outer, middle, and inner ramp environments (e.g. wackestone/packstones, ooid and skeletal grainstones), mixed carbonate-siliciclastic tidal channel deposits (e.g. fossiliferous trough cross stratified sandstones, oolitic sandstones), and mixed-process delta deposits and associated sub-environments  Both successions record a siliciclastic wedge that prograded over a carbonate ramp. The Strawn and Bug Scuffle also contain select intervals of stratigraphy that are carbonate-dominated that alternate with more siliciclastic intervals. However, internal to these intervals carbonates and siliciclastics interfinger.

In both cases we interpret that stratal characteristics were governed by both allogenic processes, including high-frequency, high-amplitude sea-level fluctuations associated with the Pennsylvanian icehouse, and autogenic processes including delta lobe switching, delta front channel migration, variable sediment influx, and development of shorefaces or a tidal shelf and carbonates along the margins siliciclastic sources. Our results provide a test for pre-existing models for mixed carbonate-siliciclastic systems.

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