Geochemical mudstone study of Late Pennsylvanian to early Permian strata in the southern margin of the Delaware Basin (Wolf Camp Hills, Texas) and its depositional implications

Abstract The Wolf Camp Hills outcrops along the southern margin of the Permian Basin expose mudstone beds coeval with the subsurface Late Pennsylvanian to early Permian reservoirs in the Delaware Basin. This paper uses X-ray fluorescence (XRF), X-ray diffraction mineralogy (XRD), principal component analyses (PCAs) of elemental data, and textural variability in mudstone and siltstone strata from a composite stratigraphic section 234 m (768 ft) thick in the Wolf Camp Hills outcrops to characterize and differentiate these mudstone-rich units and infer changes in depositional conditions. Geochemical analyses from these outcrops indicate distinct compositional differences between Late Pennsylvanian (Cisco Group) mudstone beds and lower Permian (lower Wolfcamp) mudstone beds. These variations are controlled by influx and maturity of the siliciclastic sediment, biogenic input, changes in depositional settings, and sea-level variability. Mudstone strata from the Upper Pennsylvanian Cisco Group are mostly quartz-rich, with moderate feldspar content, have high Zr/Al and Zr/Rb ratios, and have low to absent total organic carbon (TOC), and often they are interbedded with sandstone beds. These characteristics indicate that during the Late Pennsylvanian, siliceous mudstone beds at this locality were deposited under oxygenated conditions, during periods of increased weathering and low sea level. Lower Wolfcamp mudstone units are predominantly siliceous and mixed siliceous to carbonate. The source of silica is from extra-basinal siliceous grains and biogenic siliceous grains. Lower Wolfcamp mudstone strata in this area were deposited in slope to basinal settings, with strong siliciclastic input, recorded by its high Rb/Al ratio, moderate mica content, and common silt-size siliciclastic grains. The lower Wolfcamp mudstone beds contain up to 1.1 wt.% TOC, and excess silica is present within the transgressive system. The low to fair TOC, high Rb/Al ratio, textural character, and lack of covariance between trace elements and TOC of the lower Wolfcamp mudstone beds at this locality indicate that they were deposited under oxic to suboxic conditions. We interpret that organic matter (likely mixed terrestrial and marine) was partially diluted by high sediment input during deposition. The results of this study offer a valuable chemostratigraphic record that can be tied to subsurface wells and cores in the southern Permian Basin and improve stratigraphic correlations of the coeval hydrocarbon reservoirs. It also helps to obtain a better understanding of these units to improve paleogeographic reconstructions of this region.