CYCLES IN THE DEEP: AN INTEGRATED STRATIGRAPHIC ANALYSIS OF THE MIDLAND BASIN IN THE LATE PENNSYLVANIAN USING SUBSURFACE DATASETS

Four drillcores and associated petrophysical datasets of the Wolfcamp D shale (Cline shale) from the Midland basin in west Texas have been studied using high-resolution lithostratigraphic and chemostratigraphic analyses. Based on these data, inferences about paleoenvironmental conditions within the basin were made within a sequence stratigraphic framework. Intrabasinal correlations via well logs to biostratigraphically dated (fusulinids) shelf carbonates suggest that the age of the Wolfcamp D is late Pennsylvanian (309-299 Ma). During this time, the Midland basin was a mixed siliciclastic-carbonate depositional system influenced by a global icehouse climate and large eustatic sea level fluctuations. These environmental dynamics produced the classic Pennsylvanian cyclothems that have been extensively studied across the U.S. Midcontinent. Our results indicate that the Wolfcamp D is the condensed, deep basinal stratigraphic interval that correlates with up-dip shelf cyclothems. Detailed lithostratigraphic analysis revealed repetitive stacking of black organic-rich siliceous shales, grey organic-poor clayey mudrocks, grey carbonate gravity flow deposits, and brown-grey dolomite cement grounds. Total organic carbon (TOC) measurements demonstrated that the siliceous shales frequently exceeds the 2.0 weight percent threshold for a petroleum source rock, and stable isotope analyses confirmed that marine algae dominated the organic composition. Energy dispersive x-ray fluorescence data (ED-XRF) collected at two inch intervals demonstrated cyclic variability in aluminum, silica, and calcium. Trace metal data from the ED-XRF exhibited a positive correlation with TOC, which suggests anoxic and possibly euxinic bottom waters. However, comparative analysis of facies and trace element data among the cores indicates the potential for along strike variability in the basin axis, which may be attributable to differences in depositional environment, proximity to sediment sources, and the position of the chemocline. Developing an enhanced understanding of how the late Pennsylvanian cyclothems are expressed in a deep basinal environment may aid in development of the Wolfcamp D as an unconventional petroleum play, and our understanding of Paleozoic paleoceanography in the southwestern USA.