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Coal Bed Methane Exploration in Zavala County, Texas


Warwick, P.D.1, R.W. Hook2, A.C. Clark3, M.D. Corum1, J.C. McIntosh4, C.R. Burr1, P.C. Hackley1, D.B. Jensen5 (1) U.S. Geological Survey, Reston, VA (2) Consultant, Austin, TX (3) U.S. Geological Survey, Lakewood, CO (4) University of Arizona, Tucson, AZ (5) Genesis Gas & Oil LLC, Kansas City, MO


Data from conventional oil and gas wells in Zavala and southeastern Uvalde counties, South Texas, define a 150-200-ft-thick (46-61-m-thick) coal-bearing zone in the lower part of the Indio Formation (Wilcox Group, Paleocene-Eocene). Individual coal beds generally are less than 5 ft (1.5 m) thick and cumulative coal thicknesses are up to 25 ft (7.6 m). More than 28,000 mcf (793,000 m3) of coal gas has been produced intermittently since 1999 from the Indio coal zone by a single shallow well (Kincaid, E.D., No. 10A, Woodley Field) in southeastern Uvalde County. Isotopic data from the methane produced by that well indicate a mixed biogenic and thermogenic origin (d13CCH4 = -56.68‰ VPDB; dDCH4 = -195‰ VSMOW).


To test the distribution of coal gas in the Indio Formation, an exploration well (Arroyo Roble No. 1) was drilled in north-central Zavala County by the U.S. Geological Survey and an industry partner in February, 2006. A total of 16.7 ft (5.09 m) of coal was cored from three coal zones that spanned a 193.2 ft (58.88 m) interval ranging from depths of 899.5 to 1092.7 ft (274.17-333.05 m). The coal core was sealed in fifteen canisters to measure desorbed gas over time (several weeks). Average desorbed gas content was negligible at 1.14 SCF/ton (0.03 cm3/g; as analyzed basis) or 1.70 SCF/ton (0.05 cm3/g; dry, ash-free basis). Equilibrium moisture and ash values for the coal samples averaged 15.92 and 15.46 percent (equilibrium moisture basis), respectively. Estimated coal rank for these samples is subbituminous A based upon the ASTM Parr Formula.


AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California