Stratigraphy and Influence of Effective Porosity on Ground-Water Flow in the Wolfcamp Brine Aquifer, Palo Duro Basin, Texas Panhandle

In the Palo Duro Basin, the Wolfcampian Series (lower Permian) overlies Pennsylvanian (primarily Virgilian) strata and underlies the Wichita Group (Leonardian). Tectonic activity from late Pennsylvanian through early Wolfcampian deposition included basement subsidence that resulted in localized basin development. The basin evolved into a restricted, shallow-marine, evaporitic environment that prograded southward, resulting in deposition throughout most of the basin of evaporite and red-bed sediments of Leonardian age and younger.The Pennsylvanian-Wolfcampian contact is mostly conformable, but a few erosional unconformities or depositional hiatuses occur in peripheral areas of the basin. The upper Wolfcamp-Wichita lithologic boundary is time-transgressive, conformable throughout the basin, and marked in geophysical well logs by responses that indicate a downward change in lithology from a low-porosity dolomite/anhydritic dolomite to a high-porosity granular dolomite.Lower Wolfcamp lithofacies consist of shales, shallow-marine carbonates, and deltaic sands. Upper Wolfcamp lithofacies are primarily shallow-shelf carbonates, but the uppermost facies, the Brown Dolomite, reflects basinwide dolomitization of carbonates deposited in the late Wolfcampian.The predominantly carbonate Deep-Basin Brine aquifer in the Wolfcampian Series lies 2,100 to 3,100 ft (640 to 945 m) beneath bedded San Andres (Guadalupian) salt, a potential host rock for isolated high-level nuclear waste. Knowing effective-porosity distributions within the Deep- Basin Brine aquifer is therefore important for estimating ground-water flow velocities and the frequency of basin flushing. Maps of effective-porosity distribution generated in this study are based on cross-plotted neutron-density log responses.In the lower Wolfcamp, effective porosity is highest in coarse sands and granite-wash sediments adjacent to the Amarillo Uplift and the Bravo Dome and in thin carbonate units throughout the basin. In the uppermost Wolfcamp, the Brown Dolomite is basically a diagenetic unit characterized by dolomite lithology and relatively high effective porosity. The Brown Dolomite is a hydrocarbon reservoir in areas peripheral to the basin; it is also the upper part of the Deep-Basin Brine aquifer system, which extends throughout the Palo Duro Basin. The axes of thickening and relatively high effective porosity in the Brown Dolomite coincide with the early Wolfcampian shelf-margin positions and associated carbonate buildups. The Brown Dolomite thins basinward, where relatively low effective porosity developed, and landward, where clastic sedimentation prevailed until the late Wolfcampian.Integrity of a geologic containment system at a potential repository site depends on hydrologic conditions and the local resource potential of the site. In this study, therefore, mapped effective-porosity distributions are used to (1) define porosity fairways and their relation to Wolfcampian shelf-margin positions, major lithofacies, and zones of diagenesis; (2) refine ground-water travel-time estimates; and (3) provide a foundation for estimating total effective pore volume, which equals the quantity of mobile fluids.Average frequency of basin flushing (measurements taken from the middle of the basin) is estimated to be once every 2.0 m.y. by a typical effective-porosity model and once every 1.2 m.y. by a log-derived effective-porosity flow model. Planimetry-based analyses of log-derived effective-porosity distributions show that total effective pore volume is 2.9 x 1013ft3 (8.2 x 1011 m3).