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 (d¹³C_CH4 = -56.68‰ VPDB; dD_CH4 = -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.