PRELIMINARY GEOLOGICAL ANALYSIS OF WOLFCAMP D BASINAL CYCLOTHEMS, 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 shelf carbonates suggest a Pennsylvanian age for the Wolfcamp D. The Pennsylvanian was a dynamic interval of earth history, and the imprint of tectonic, climatic and eustatic changes was strong on sedimentary processes and 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 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 deepwater 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 response. Basinal cyclothems are particularly characteristic of Wolfcamp D but may also extend upward into the 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 core are organic-rich siliceous shales, organic-poor clayey shales, carbonates and dolomites. Regional isopach mapping indicates multiple provenances for detrital clays, with predominate sources located along the Eastern Shelf. Ongoing research will clarify microfacies characteristics and integrate new geochemical information to better constrain the evolution of basinal cyclothems, which may ultimately have implications for completion strategies.