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Abstract: The Mystery of the Escaping Gas: Forensic Geology in the Northern San Juan Basin, La Plata County, Colorado


Coal-bed methane seeps have been observed along the Fruitland Formation outcrop in the northern part of the San Juan Basin, La Plata County, Colorado. The Upper Cretaceous Fruitland Formation underlies the basin's 7,500-square-mile extent in New Mexico and Colorado to depths slightly in excess of 4,000 feet; the Fruitland contains 200 billion tons of subbituminous to bituminous coal in the basin. During the last ten years, thousands of new gas wells have been completed in Fruitland coal beds, mostly in the northern part of the San Juan Basin, making it the world's largest coal-bed methane field with resources estimated to be between 50 and 100 trillion cubic feet of gas. La Plata County officials and land developers have become concerned about the Fruitland coal-gas seeps both as a hazard and because of their potential detrimental effect on property values. Coal gas is produced by pumping water out of the fractures in the coal thereby lowering the pressure on the coal and allowing gas to desorb from the coal's carbon molecules. One of the key questions that has emerged is whether or not production of coal-bed methane from wells relatively close to and down dip from the seep areas has mobilized coalbed methane that is migrating up dip to the outcrop thereby increasing the rate of gas seepage from old seeps or creating new gas seeps. Geochemical and geologic studies are ongoing to try and answer this question and the evidence thus far is equivocal. Geologic studies involving very detailed coal-bed correlations indicate that in at least one area, large gas seeps apparently originate in thinner, discontinuous coals that do not correlate with thicker, more continuous coal beds that are producing large volumes of gas in the subsurface; whereas in other seep areas, thick coal beds that produce methane down dip can be correlated directly to the outcrop. Clearly, more work must be done to resolve this problem.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado