COAL BED METHANE FORMATION AND WATER MOVEMENT: APPLICATIONS OF THE I-129 AND CL-36 SYSTEMS

A crucial question for the formation of methane from coal beds is the continuity of water coverage of the formation. Long-lived cosmogenic isotopes such as Cl-36 (half-life 300,000 a) and I-129 (half-life 15.7 Ma) have the age range and geochemical characteristics to provide important information in this context. We present here results of an extensive study of the hydrologic situation in the Fruitland Formation, CO, a well-studied formation with significant production potential for methane. I-129 content was determined in 80 wells, and Cl-36 in a subset of 40 wells from that area. The central part of the study area has I-129 ages up to 50 Ma, only slightly younger than the formation age in this region. Younger ages were found predominantly at the edges of the study area. Cl-36 ages are in good agreement with the general observations from the I-129 results, but provide more specific information on the time and direction of fluid movement in the last few Ma. The study shows that a large part of the hydrologic system has remained essentially undisturbed over a long time period and that the intrusion of younger waters is limited to marginal regions of the study area.