Abstract

In recent years, the Lower Permian Wolfcamp Shale of Texas and New Mexico has become one of the most prolific onshore unconventional hydrocarbon plays in the United States. Understanding how sea level change, paleoclimate, and tectonic subsidence interact to control these basinal facies can enhance the predictability of both lateral and vertical changes in reservoir attributes. In this study we develop a depositional facies and sequence stratigraphic framework for the up dip equivalent Hueco Formation which can be utilized to increase predictability of subsurface play elements in the Wolfcamp Shale.

The Upper-middle member and Gastropod member are age-equivalent subunits of the Hueco Formation that form a mixed carbonate and siliciclastic succession found in the Robledo and Doña Ana mountains of southern New Mexico. These members contain depth-sensitive lithofacies that allow paleogeographic reconstruction and delineation of sea-level cycles on the Robledo Shelf and the western margin of the Orogrande Basin at the time of deposition. A broad range of carbonate and siliciclastic lithofacies are documented including fluvial siltstones, supratidal to open marine carbonates, and basinal shales. A total of 19 fifth-order cycles were correlated out of a thickness of 50m in the Robledo Mountains and 65m in the Doña Ana Mountains. The duration of each cycle was approximately 100,000 years and deposited an average of 2.5 m of carbonate and siliciclastic sediment. These fifth-order cycles took place between two third-order cycles which saw subaerial exposure and fluvial siliciclastic deposition. The frequency and amplitude of these sea-level changes is evidence of a glacio-eustatic control for both third and fifth-order cycles.

Despite their relatively similar position on a carbonate ramp depositional profile, the Upper-middle and Gastropod members have different facies in outcrop. The Upper-middle member is dominantly restricted tidal dolomites and semi-restricted limestones with some normal marine limestone and thin shale units. The Gastropod member is mostly basinal shale and normal marine limestones, with occasional semi-restricted limestones and fluvial silt-stones, no dolomite was found in the Gastropod member of the Doña Ana Mountains. The presence of both carbonate and siliciclastic lithofacies is likely related to glacio-eustatic sea-level changes, a difference in the restriction each location had with the rest of the basin, and paleoclimate shifts whereby carbonates were deposited during arid periods associated with sea-level lowstands and siliciclastics during more humid periods associated with sea -level highstands.