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Lower Cretaceous Pearsall Oceanic Anoxic Events and Associated Development of Shale-Gas Reservoirs in South Texas

Hull, David C.1; Loucks, Robert *1; Milliken, Kitty 1
(1) The Bureau of Economic Geology, The University of Texas at Austin, Austin, TX.

The Lower Cretaceous Pearsall Formation in South Texas contains three calcareous, terrigenous, mudstone intervals that are productive shale-gas reservoirs or that have the potential to be shale-gas reservoirs. The intervals are the Pine Island Shale, lower Bexar Shale, and upper Bexar Shale Members. Each member is associated with the OAE-1 time interval, during which the Pearsall distal steepened ramp was transgressed and flooded. The Pine Island Shale Member contains the OAE-1A, the lower Bexar Shale Member contains a regional OAE, and the upper Bexar Shale Member contains OAE-1B. Secular carbon isotope curves record these OAEs and allow their correlation, not only within the GOM, but also worldwide. In the outer-ramp setting during OAEs, sedimentation was dominated by calcareous siliceous mudstone and argillaceous lime wackestone. Dysoxic to anoxic bottom conditions existed favoring the preservation of organic matter. TOC content increases in the offshore direction, reaching up to an average value of 1.8% (high single value of 2.8%) near the paleo-Sligo shelf edge. The reservoir may be composed of a dual pore network of open-mode, tectonic-related fractures and matrix interparticle and intraparticle nano- to micropores. Matrix porosity ranges between 4 and 8%, and matrix permeability ranges between 4 and 70 nd. The Pearsall outer-ramp facies belt throughout the GOM is expected to have environmental conditions similar to those in South Texas and therefore is expected to be a prospective shale-gas system. The limiting factor will be depth of burial and associated economics.


AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California