ABSTRACT: Chemistry of Western Interior USA Coal-Bed Gases Based upon Desorption of Subsurface Coal Samples

William B. Hanson

Coal-beds produce major amounts of gas in the Western Interior U.S. basins, primarily the San Juan basin of Colorado and New Mexico. Desorption is the process of allowing coal samples recovered from depth to naturally emit gas at ambient surface conditions. Desorbed gases from five western U.S. basins provide a baseline to which production samples can be compared.

Desorbed gases show that large variations in coal gas chemistry are natural phenomena related to thermal maturity, organic matter type, burial depth, lateral in-seam migration, and possibly water washing and alteration. Coal gases are generally chemically dry (C₁/C_1-5 > 0.95) but show a total range from very wet (C₁/C_1-5 value 0.81) to very dry (C₁/C_1-5 > 0.99). Chemically wet gases (C₁/C_1-5 < 0.95) have been desorbed from Late Cretaceous-age coals from four western U.S. basins. Energy contents greater than 1150 Btu (gross, dry) have been recorded.

Carbon dioxide, generally second to hydrocarbons in abundance in desorbed gases, varies from a trace to 17.5% by volume. Carbon dioxide abundance varies as a result of subsurface processes and sometimes as a result of reactions within desorption canisters but does not appear to originate from intrusives. Samples from the pervasively intruded and sometimes coked coals of the Raton basin are low in CO₂ (average 0.6%) as are samples from within the northeastern San Juan basin dike swarm.

Nitrogen is not a significant component of thermogenic coal-bed gas. High nitrogen values from desorption canisters result from air contamination, oxygen consumption, and the standard air-correction algebra.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990