Abstract
The purpose of this paper is to present an updated overview on the coalbed methane potential of the Raton Basin of southern Colorado and northern New Mexico, in which coal measures underlie approximately 2000 square miles. Prior to this and related studies by us, the most recent public assessment of Raton Basin coalbed methane resources was published in 1984, which was extracted from another study completed in 1980. The Raton Basin is one of several basins in the U.S. that has undeveloped coalbed methane potential and is presently undergoing active coalbed methane exploration.
The geology and measured and inferred reservoir characteristics of Raton Basin coalbed methane are, by analogy, essentially a combination of commercial Black Warrior Basin (Alabama) coalbed methane reservoir thicknesses, lateral continuity, vertical distribution, drilling depths, pressures, and gas and water productivity, and commercial northern San Juan Basin (Colorado and New Mexico) coalbed methane reservoir gas contents, diffusion characteristics, and permeability. Updated subsurface maps and cross sections which incorporate new geophysical log data and outcrop observations indicate that potential Raton Basin coalbed methane reservoirs are typically 2 to 10 feet thick at depths of up to 2500 feet, and have cumulative net thicknesses of 15 to 70 feet. Net coal isolith, depth, structure, coal characterization, depositional environment and hydrogeologic maps and data, along with published information indicating gas contents from negligible to approximately 500 standard cubic feet (SCF) per ton, suggest that these potential reservoirs may contain a nearly pure methane natural gas resource of up to 40 billion cubic feet (BCF) per square mile. A significant portion of the Raton Basin is considered prospective for coalbed methane in the long term. Potentially commercial areas contain an estimated ultimate natural gas reserve of up to roughly 1 trillion cubic feet (TCF). An "average" Raton Basin coalbed methane well (30 feet of producible coal with a gas content of 250 SCF/ton, 1 to 5 millidarcy absolute permeability, normally- to somewhat under-pressured, somewhat damaged, and with very fast diffusion characteristics) is estimated to have recoverable reserves of 1.1 BCF per one-quarter of a square mile (160 acres) over a 10-year period. This recoverable reserve estimate is equivalent to average gas recoveries in commercial coalbed methane producing areas of the northern San Juan Basin over an approximately 10-year period, and thus the Raton Basin coalbed methane well reserve estimate is considered optimistic. Published chemical data indicate that central Raton Basin coalbed reservoir waters are almost fresh, which opens up the possibilities of low-cost surface water disposal, and use of the water by ranchers in this semiarid, agriculturally-based area.
Unfortunately, Raton Basin coalbed methane delineation and production are currently hampered by multiple, thin coal zone hydraulic fracture completion problems, weak gas markets and lack of a pipeline infrastructure. However, coalbed methane and conventional reservoir drilling activity in the Raton Basin is on the strong upturn, and it is possible that the collective potential reserve base of several operators may justify gas pipeline installation. We predict that development of improved completion techniques, along with a long term increase in domestic gas prices and demand, will favor large scale commercial development of Raton Basin coalbed methane after payout starting in about 1995.
