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Chemostratigraphy of the Pearsall Formation, Lower Cretaceous, South Texas

Rolando Castilleja¹, David Hull², Harry Rowe¹, and Robert Loucks³
¹ University of Texas at Arlington, Arlington, Texas 76019
² University of Texas at Austin, Austin, Texas 78713
³ University of Texas at Austin, Austin, Texas 78758

Oceanic anoxic events (OAEs) of the Cretaceous represent unique episodes in Earth history. The Aptian-age Pearsall Formation of South Texas represents sedimentation in a mixed carbonate-siliciclastic system, and may preserve a record of paleoceanographic conditions during a portion of OAE-1. The objective of the current project is to develop and interpret stratigraphic changes in the geochemistry of the Pearsall Formation for the purposes of 1) understanding the paleoceanography of South Texas during a unique period in Earth history, 2) developing regional correlation using multiple cores, and 3) refining the stratigraphy for the oil and gas industry. Notably, unlike some other total organic carbon (TOC)-enriched intervals, the Pearsall is not enriched in redox-sensitive trace elements (RSTEs). Further characterization and interpretation of the high-TOC, low-RSTE Pearsall will be undertaken, as well as an in depth comparison between the core chemostratigraphy and well log results. A preliminary comparison of well log and chemostratigraphy data from a Maverick County well demonstrates a strong correlation between the gamma log response and the %K, suggesting the gamma contribution is largely held by clay or some other potassium-bearing phase. A cross-plot of %K and %Al confirms that the potassium is almost exclusively associated with clay minerals. Additionally, a strong correlation between the resistivity log and the %Si/%Al ratio may indicate how the quartz/clay ratio can be used to further interpret resistivity.

 

AAPG Search and Discovery Article #90158©2012 GCAGS and GC-SEPM 6nd Annual Convention, Austin, Texas, 21-24 October 2012