--> Abstract: USGS Assessment of Undiscovered Shale Gas Resources in the Lower Cretaceous Pearsall Formation, Maverick Basin, South Texas, by Paul Hackley; #90124 (2011)

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Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

USGS Assessment of Undiscovered Shale Gas Resources in the Lower Cretaceous Pearsall Formation, Maverick Basin, South Texas

Paul Hackley1

(1) USGS MS 956, Reston, VA.

The U.S. Geological Survey (USGS) completed a geologic assessment of technically recoverable undiscovered shale gas resources in the Lower Cretaceous Pearsall Formation in the Maverick Basin, south Texas, following established USGS methodologies for the assessment of unconventional hydrocarbon accumulations. This work was part of a comprehensive assessment of undiscovered hydrocarbon resources in Mesozoic strata underlying the U.S. Gulf of Mexico Coastal Plain and State waters. An assessment unit (AU) covering about 10.5 million acres was defined for Maverick Basin Pearsall Shale Gas following the international boundary with Mexico on the west, the Gulf Coast Total Petroleum System boundary on the north, the 200 ft Pearsall isopach on the San Marcos Arch, and the Lower Cretaceous shelf edge on the southeast. Within this area, Pearsall drilling depths range 5,000-16,000 ft with approximately 290 wildcat penetrations of the Pearsall. There are about ten recent horizontal wells drilled in Pearsall shales, from which four wells have data available to compute EUR volumes. Measured vitrinite and/or bitumen reflectance values indicate dry gas thermal maturity; therefore, no liquid hydrocarbons are expected in the play and there has been no reported production of condensate to-date. The highest IP rate reported has been about 8 MMCFGD. About 30 % of the AU area was calculated to have potential for additions to reserves based on the extent of the thickest Pearsall strata (~600 ft), the extent of the 200 ft isopach, the area of present drilling, and a predicted future success ratio of about 75 %. The best practice completion method is not yet determined with both open-hole and fully cased and cemented wells reported. All wells considered for the assessment are stimulated and horizontal, although anecdotal reports described marginal comingled production from the Pearsall interval in older vertical wells in the Los Cuatros field. The closest shale gas analog to the Pearsall is the overlying Upper Cretaceous Eagle Ford Formation, which has similar mineralogical content and also was deposited in a shallow marine carbonate platform environment. However, the Pearsall is overpressured and at higher thermal maturity than the Eagle Ford. Despite engineering challenges associated with developing the correct completion and stimulation methods for Pearsall shale gas wells, the USGS assessment estimates that significant undiscovered gas resources are present.