High-Resolution Chemostratigraphic and Paleoenvironmental Analysis of the Wolfcamp Shale, Eastern Midland Basin, Texas
Ratliff, Meri 1972-
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Elemental chemostratigraphy is a geological discipline that functionally combines petrology and stratigraphy. Vertical and lateral changes in whole-rock elemental composition can decipher stratigraphic, diagenetic, and/or paleoenvironmental information. This study focuses on the Wolfcamp Shale source and reservoir rock of the Permian Basin, Texas. One 1600-foot core, from Glasscock County in the Midland basin, provides a sample set that includes 1,064 samples. Energy dispersive X-ray fluorescence (ED-XRF), X-ray diffraction (XRD), and Rock Eval measurements provide the data for this analysis. Major elements define dominant lithologies on a first-order scale. Certain minor and trace elements are studied as proxies for clastic influx, paleoredox conditions, and paleoproductivity. Several geochemical indices (calculated DOP, S/TOC, U/Th, authigenic U, V/Cr, V/V+Ni) suggestive of paleoenvironmental oxygenation are also assessed. The purpose is a multi-proxy chemostratigraphic approach undertaken to differentiate mineralogy and stratigraphic contacts that characterize pre-defined operational subdivisions (Wolfcamp A – Wolfcamp E) in order to describe laterally extensive attributes that may assist in future well correlations in the basin and evaluate currently preferred horizontal targets with the intention of suggesting possible additional target zones based on paleoredox environmental indicators suggestive of organic carbon preservation. The Wolfcamp Shale formation exhibits an overarching, low-order cyclicity of alternating carbonate-rich mudstone to limestone and siliceous mudstone. Siliceous mudstone lithologies exhibit consistently high TOC values and increased trace metal enrichment throughout the Wolfcamp formation, with variability in characteristic paleoredox proxies. The Wolfcamp A represents the highest quality source rocks in the study area, exhibiting the highest free hydrocarbon content, remaining hydrocarbon potential, and the thickest interval (about 170 feet) showing a variety of paleoredox indices indicative of a dysoxic to anoxic depositional environment conducive to the preservation of organic material. Increased Ni and Cu may be considered paleoproductivity proxies and suggest that the mode of oxygen depletion in these intervals was the result of high primary productivity. While Rock Eval shows slightly decreasing TOC, S2, and maturation with depth, the base of the Wolfcamp D contains a 75-foot interval exhibiting good to moderate values of paleoredox indices suggestive of a dysoxic depositional environment and the Wolfcamp E exhibits a similar 45-foot interval.