Geochemical Characterization of Lower Wolfcamp Unconventional Reservoirs, Midland Basin (Texas)

Abstract

The Lower Wolfcamp play (Wolfcamp D and C2) in the Midland Basin represents an important shale oil prospect in the portfolios of many operators holding acreage in western Texas. A defining characteristic of the Lower Wolfcamp is high frequency vertical facies variability, comprised of a diverse suite of marine mudrocks and carbonates. Cyclic facies stacking in the Lower Wolfcamp most likely reflects dynamic climatic, tectonic, provenance, and oceanographic processes that affected the Midland Basin deepwater and platform environments during the late Pennsylvanian. Geochemical characterization of Wolfcamp D and C2 continuous drill cores using different analytical tools (x-ray fluorescence [XRF], LECO total organic carbon [TOC], Rock-Eval pyrolysis), supplemented by well-logs and thin section petrography, was performed to better understand unconventional reservoir characteristics along the strike of the basin. Major and trace elements derived from XRF scans revealed two distinct black mudrock facies, defined primarily by differences in silica and aluminum content. Although both black mudrocks are relatively rich in organic carbon (> 2.0 wt. %), the siliceous variant exhibited higher average concentrations of TOC, pyrite, and redox-sensitive trace metals, suggesting that preservation of Type II organic matter was enhanced by low sea floor oxygen availability and stagnant recharge. In many instances, thin section data appear to show that the origin of excess silica in these mudrocks is biogenic, an interpretation that is supported by the presence geochemical indicators with strong linkages to high surface water paleo-productivity. The thickness and distribution of silica-rich black mudrocks in Wolfcamp D is challenging to discern using well-log datasets or hand samples alone, thus validating the application of high-resolution chemostratigraphy for reservoir characterization in this petroliferous interval.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016