THE EVALUATION OF MULTIPLE ISOTOPIC GROUND-WATER TRACERS, AND IMPLICATIONS FOR FLOW PATTERNS IN THE FRUITLAND FORMATION, SAN JUAN BASIN, COLORADO

The occurrence of four environmental isotopic tracers, He-4, Cl-36, Ar-40, and I-129, have been evaluated in the Fruitland Formation in order to assess regional ground-water flow patterns. The Fruitland Formation is located within the San Juan Basin in southwestern Colorado and northwestern New Mexico, and is the primary reservoir in one of the largest coal bed methane producing regions in the United States. Interbedded lenses of Late Cretaceous sandstone, siltstone, mudstone, and coal complicate the hydrogeology and may act as compartmentalized hydrologic units. Therefore, the regional hydraulic impact resulting from dewatering of the Fruitland for methane recovery is uncertain. Potential impacts resulting from decrease of hydraulic head include depletion of rivers and other surface waters, and the development of methane seeps at outcrop areas.

Isotopic analyses were used to characterize the regional hydrogeology so that the long-term hydraulic impact of methane recovery from the Fruitland can be determined. Each isotope was evaluated to determine the distribution of hydraulic age. The age distributions were then used to establish implications for flow velocity, flow paths, and hydrologic continuity of the Fruitland Formation in the study area. The results also provide a basis for comparison of the isotopic methods and an assessment of advantages and disadvantages of each technique in the Fruitland Formation.