DEPOSITIONAL FACIES, SEDIMENT ORIGIN, AND DIAGENESES OF PALEOZOIC SHALE HORIZONS FROM THE MIDLAND BASIN, WEST TEXAS, USA: INSIGHTS INTO THERMAL EVOLUTION AND PROVENANCE HISTORY USING CLAY MINERAL DIAGENESIS

Unconventional reservoirs are a major source for hydrocarbon exploration and production; however the majority of studies on these reservoirs focus on porosity and reservoir characteristics affecting hydrological fracturing practices and remain limited on clay mineralogy or sediment origin. This study focuses on three objectives 1) determine depositional environment of fine-grained lithologies of Paleozoic strata along the northern Midland basin 2) identify clay mineral phases and subsequent diagenesis to understand burial-thermal history of fine grained siliciclastics 3) determine major trace elements and REE concentrations for provenance analysis. Samples were collected from core in three intervals spanning Early Permian to Early Carboniferous within the northern Midland Basin. Petrology and whole rock XRD show three major lithologies; fine-grained siliciclastic-clay rich, fine-grained calcareous-rich, and carbonate. Clay fraction phases are illite, chlorite, interstratified illite-smectite, and interstratified chlorite-smectite. XRF of major oxides are consistent with lithology changes across the study interval. REE concentrations normalized to chondrite and show similar trends indicating an intermediate igneous source material suggested by the enrichment of LREE, negative Eu anomaly, and low HREE concentrations. Data suggests a depositional environment along marginal marine setting under anoxic conditions due to the occurrence of intermixed eroded limestone lithoclasts and fine-grained siliciclastics and preservation of organic matter. Negative Ce and positive Gd concentrations indicate seawater and organic matter influence on sediment during deposition. Cross plot data of Zr/TiO₂vs. Nb/Y ratios also suggest an intermediate igneous source of possible rhyodacite/dacite/andesite origins. Clay mineral assemblages indicate an increase in thermal development after burial during mesogenesis due to the change from smectites to interlayered illite-smectite and chlorite-smectite. Understanding clay mineralogy can not only effect reservoir characteristics but can offer insight to the thermal evolution within sedimentary basins and furthermore, when paired with organic matter data can be a useful exploration tool to understand the maturation of hydrocarbons.