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.