Origin and history of waters associated with coalbed methane: 129I, 36Cl, and stable isotope results from the Fruitland Formation, CO and NM

Abstract The Fruitland Formation of the San Juan Basin was deposited during the late Cretaceous and is associated with significant reservoirs of coalbed methane (CBM). The purpose of this study is to determine the origin and history of waters associated with the formation, using long-lived cosmogenic and stable isotope systems. Ratios of 129 I/I and stable isotope values (δD and δ 18 O) were determined in waters from close to 100 wells, 36 Cl/Cl ratios for a subset of these samples. A significant group of samples has 129 I/I ratios between 100 × 10 −15 and 200 × 10 −15 , indicating minimum iodine ages close to 60 Ma. If these ages are corrected for the addition of fissiogenic 129 I, they are compatible with the depositional age of the Fruitland Formation (Late Cretaceous). Several sets of waters are clearly present within the data. A group dominated by infiltration of recent surface waters is restricted to the uplifted basin margins, with a lateral extent of less than 5 km from outcrop, and is characterized by 129 I/I ratios in excess of 1500 × 10 −15 and meteoric δD, δ 18 O, and 36 Cl/Cl signatures. The rest of the basin is characterized by several subsets of formation waters which have undergone variable degrees of iodine enrichment through diagenesis as well as variable degrees of dilution. The first subgroup is found in coals of relatively low vitrinite reflectance and moderate enrichment of iodine. This subgroup predominantly consists of entrapped pore fluids, although it may also contain waters which infiltrated the coals at the time of the Laramide uplift, between 25 and 30 Ma. A second subgroup consists of formation waters associated with coals of high vitrinite reflectance. Despite subsequent uplift, the high iodine concentrations and low 129 I/I ratios of this subgroup, as well as a moderate depletion of deuterium relative to 18 O, suggest that these waters were not significantly altered since the time when diagenetic reactions occurred in the deepest portion of the basin. A third subgroup, with higher δD and δ 18 O values as well as higher 129 I/I ratios, extends roughly west to east at the New Mexico–Colorado state line and corresponds to a region of extensive fracturing of the coalbeds. In this case, the higher 129 I/I ratios are probably due to contributions of fissiogenic 129 I through fracture flow, perhaps from deeper formation waters. Our results do not support models of subsequent basin-wide groundwater migration in the Fruitland Formation. The combined use of 129 I and 36 Cl with stable isotope studies provides valuable information as to the hydrologic history of coalbed methane deposits, as well as their potential for commercial exploitation.