A Cold Phosphatic Woodford Shale

Michael Aufill, Newfield Exploration

Anna M. Cruse, Oklahoma State University

Stanley T. Paxton, United States Geological Survey Oklahoma Water Science Center

Gamma-ray and spectral gamma-ray analysis (SGR) have long been used as correlation tools in both subsurface and outcrop studies. Among the most useful correlation markers are “hot” units that have calculated API values above 250 units. Such high API values are driven by high concentrations of U, which is sequestered in shales in association with organic carbon. In addition to organic carbon, phosphate is also recognized as a chemical host for U, likely through diagenetic adsorption and recrystallization reactions. Thus, it is commonly assumed that a “hot” SGR marker will indicate high TOC and/or high phosphatic concentrations. Recent data from outcrops of the Woodford Shale indicate that these assumptions may not always be valid. The Woodford is informally divided into three members, based on its spectral gamma ray (SGR) response. The middle member has the hottest SGR, highest TOC concentrations, and minimal concentrations of phosphate. Ubiquitous spherical phosphate nodules and laminations are common in the upper member in association with some of the lowest SGR readings, the lowest concentrations of uranium, and the lowest TOC. This inconsistency likely reflects the mineralogy and origin of Woodford phosphate nodules. Previous petrographic studies indicate that nodules in the Woodford are derived from diagenetic inorganic phosphate rather than recrystalization of organic phosphate (e.g. fecal pellets). As such, diagenetic transformations related to uranium adsorption and fixation were likely inhibited. Ultimately the recognition of a “cold” phosphatic, black shale has implications for the assumptions underlying the use of SGR in correlation and basin reconstruction studies.

AAPG Search and Discover Article #90097 © 2009 Tulsa Geological Society, Tulsa, Oklahoma