Albrecht, Tamee R. and Geoffrey D. Thyne
Colorado School of Mines, Golden, CO
Semi-arid western Colorado has vulnerable water resources, but is experiencing rapid growth in petroleum exploration and production activities. An increasing trend in average groundwater methane concentration is correlated to the increasing number of gas wells in the Mamm Creek area suggesting that increased well drilling has impacted water quality. Isotopic data shows that some methane is thermogenic, but other samples are derived from CO2-reduction of CO2, possibly from the production interval. More detailed statistical analysis of hydrochemical data produced other criteria to detect impact including elevated Fe-Mn, Na-Cl-SO4 or Na-HCO3-Cl water chemistry. Samples with high Fe-Mn as well as elevated benzene and methane concentrations are found primarily at methane seeps where reducing conditions dominate. Other impacted samples appear to be influenced by produced water from the gas-production interval. Inverse geochemical modeling shows that the impacted samples can result from mixing normal groundwater with 2-8% produced water. The absence of benzene in some impacted samples is probably due to rapid natural degradation. Reactive transport modeling shows that benzene travels less than 50 meters in 6 years under aquifer conditions. These models can be used to help regulate the density of petroleum development in areas where groundwater resources are actively utilized. This study demonstrates that there are discernable impacts from petroleum activities on water resources in the Mamm Creek field, although impacts are generally below actionable levels. Methane and salinity appear to be better indicators of subtle impacts, whereas benzene and high Fe-Mn define larger impact events.
AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah