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Preliminary Investigation of the Thermal Maturity of Pearsall Formation Shales in the Maverick Basin, South Texas

Hackley, Paul C.1; Dennen, Kristin 1; Gesserman, Rachel 1; Ridgley, Jennie L.2
1 U.S. Geological Survey, Reston, VA.
2 U.S. Geological Survey, Lakewood, CO.

The Aptian Pearsall Formation, a regionally occurring limestone and shale package of 500-600 ft maximum thickness, is being evaluated as part of an ongoing U.S. Geological Survey assessment of undiscovered hydrocarbon resources in onshore Lower Cretaceous strata of the northern Gulf of Mexico. Pearsall Formation units are, in ascending stratigraphic order, the Pine Island Shale, James Limestone, and Bexar Shale Members. Currently, Pearsall shales are an active exploration target for independent oil and gas operators in the Maverick basin area of south Texas. Favorable initial gas flow rates from frac-treated horizontal Pearsall wells have been reported; however, to our knowledge there are no public data that document thermal maturity, organic richness, or other reservoir characteristics applicable to unconventional resource characterization.

Spontaneous potential-resistivity logs for twenty-six conventional wells in the Maverick basin were evaluated to correlate the stratigraphic positions of Pearsall Formation units. Cuttings and core available from these wells were sampled and prepared for petrographic, geochemical, and other analyses. Samples also were collected from Pearsall equivalent outcrops in western Travis County, Texas, on the San Marcos Arch (the Pearsall does not outcrop in the Maverick Basin).

Evaluation of a preliminary dataset derived from samples of core and cuttings from twelve wells in Maverick County (5,100-9,000 ft), cuttings from two wells in McMullen County (12,300-15,100 ft), and core from Bee County (15,900 ft) indicates low average total organic carbon (TOC) content of 0.80 wt.% (n=81) for Pearsall calcareous shale and mudstones from the Pine Island and Bexar Shales. The TOC content ranges between 0.17 and 2.97 wt.%. Rock-Eval pyrolysis typically yielded broad or low temperature S2 peaks; therefore Tmax and calculated Ro values are considered unreliable. Measured Ro values for a sample subset range from 1.5 to 2.3% (n=14), indicating that the samples are in the gas window. Low HI values and petrographic analyses indicate that gas-prone Type III organic matter dominates. Measured Ro values for overlying Upper Cretaceous Austin-Eagle Ford Group source rocks which range from 0.84 to 1.42% (n=7) further support our conclusion that Pearsall shales in the Maverick Basin are mature with respect to gas generation.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009