In July 1990, the U.S. Geological Survey began a study of the occurrence of natural gas in near-surface ground water in the Animas River valley in the San Juan Basin between Durango, Colorado, and Aztec, New Mexico. The general purpose of the study was to identify the sources and migration pathways of natural gas in nearsurface ground water in the study area. The purpose of this report is to present interpretive conclusions for the study, primarily based on data collected by the U.S. Geological Survey from August 1990 to May 1991.

Seventy of the 205 (34 percent) groundwater samples collected during August-November 1990 had methane concentrations that exceeded the reporting limit of 0.005 milligram per liter. The maximum concentration was 39 milligrams per liter, and the mean concentration was 1.3 milligrams per liter. Samples from wells completed in bedrock have greater mean concentrations of methane than samples from wells completed in alluvium. Correlations indicate weak or nonexistent associations between dissolved-methane concentrations and concentrations of dissolved solids, major ions, bromide, silica, iron, manganese, and carbon dioxide. Dissolved methane was associated with hydrogen sulfide.

Soil-gas-methane concentrations were measurable at few of 192 ground-water sites, even at sites at which ground water contained large concentrations of dissolved methane, which indicates that soil-gas surveys are not useful to delineate areas of gas-affected ground water. The reporting limit of 0.005 milligram per liter of gas was equaled or exceeded by 40 percent of soil-gas measurements adjacent to 352 gas-well casings. Concentrations of at least 100 milligrams per liter of gas were measured at 25 (7 percent) of the sites.

Potential sources of gases in water, soil, gas-well surface casings, and cathodic-protection wells were determined on the basis of their isotopic and molecular compositions and available information about gas-well construction or leaks. Biogenic and thermogenic sources of gas exist in the near-surface environment of the study area. Biogenic gas is present locally in the near-surface Animas and Nacimiento formations, and biogenic gas has been detected in water wells completed in those rocks. Most gas probably is thermogenic gas from deep reservoirs, including the Dakota Sandstone, Mesaverde Group, Lewis Shale, Pictured Cliffs Sandstone, and coals in the Fruitland Formation. Less important sources include sandstones in the upper Fruitland Formation and the Kirtland Shale.

Although migration of gas by diffusion or through natural fractures is possible, manmade conduits probably account for most of the upward migration of gas to the near-surface environment of the study area. Primary migration pathways largely consist of 1) leaking, conventional gas wells and 2) uncemented annuli of conventional gas wells along coals in the Fruitland Formation. Secondary migration pathways are gas-well annuli, cathodic-protection wells, seismic-test holes, and bedrock water wells.