Preliminary geologic and chemostratigraphic analysis of the Wolfcamp D shale, Midland Basin, west Texas

The geological characteristics of the Wolfcamp D shale (Midland Basin) are unique relative to overlying Wolfcampian and Leonardian basinal deposits. Well log-based intrabasinal correlations to biostratigraphically dated (fusulinids) shelf carbonates suggest that the age of Wolfcamp D is late Pennsylvanian. The late Pennsylvanian was a dynamic interval of Earth's history, and the imprint of tectonic, climatic, and eustatic changes was strong on sedimentary processes and the organization of depositional environments in the Midland Basin. Orogenic belts flanked the Midland Basin to the south and east, and together with basement uplifts to the north and west, provided sources of siliciclastic sediment to available accommodation space. Because much of Gondwana was positioned over the South Pole, icehouse climatic conditions prevailed, which led to large-scale sea level fluctuations. Cyclic variability in sea levels helped to produce shelf cyclothems that have been recognized throughout much of the U.S. Midcontinent and Appalachian regions. Wolfcamp D represents a deep basinal expression of time-equivalent deposits which are an important unconventional reservoir target. At least 11 basinal cyclothems ranging from 7-10 m thick have been identified and correlated based on gamma ray and resistivity log response. Basinal cyclothems are particularly characteristic of Wolfcamp D but may also extend upward into the overlying Wolfcamp C2. Wolfcamp D cyclothems in the basin axis consist of lithofacies that stack in a repetitive and frequently predictable arrangement. Common lithofacies encountered in the cores are black organic-rich siliceous shales, grey organic- poor clayey shales, grey carbonates and brown-grey dolomites. Energy dispersive x-ray fluorescence profiles collected on core from the across Midland Basin provide further insights on mineralogy and paleoceanographic transitions during Wolfcamp D time. High frequency variability in aluminum, silica, calcium and iron likely reflect the influence of sea level on depositional patterns and processes. Concentrations of certain trace elements (including molybdenum and chromium) exhibit a positive correlation with total organic carbon, which suggests the potential for restricted bottom water circulation and a redox front positioned above the sediment-water interface. Future research will clarify microfacies characteristics and integrate additional geochemical information to better constrain the evolution of basinal cyclothems, which may ultimately have implications for completion strategies and well performance.