H23C-0894:
Occurrence and Origin of Methane in Relation to Major Ion Concentrations in Groundwater Wells of the Denver-Julesburg and Piceance Basins of Colorado
Tuesday, 16 December 2014
Jessica D Rogers1, Owen Sherwood2, Gregory Lackey1, Troy L Burke1, Stephen G Osborn3 and Joseph N Ryan4, (1)University of Colorado at Boulder, Civil, Environmental and Architectural Engineering, Boulder, CO, United States, (2)University of Colorado at Boulder, Institute of Arctic and Alpine Research, Boulder, CO, United States, (3)California State Polytechnic University Pomona, Geological Sciences, Pomona, CA, United States, (4)Univ Colorado, Boulder, CO, United States
Abstract:
The rapid expansion of unconventional oil and gas development in North America has generated intense public concerns about potential impacts to groundwater quality. To address these concerns, we examined geochemical data from a publicly available Colorado Oil and Gas Conservation Commission (COGCC) database. The data consist of over 17,000 samples from 4,756 unique surface and groundwater locations collected since 1990, representing one of the most extensive databases of groundwater quality in relation to oil and gas development anywhere. Following rigorous data QA/QC, we classified groundwater samples with respect to major ion composition and compared the assigned water “types” along with other geochemical parameters to methane concentrations and carbon isotopes in the Denver-Julesburg (DJ) and Piceance Basins in Colorado. 88% of samples with elevated methane (defined as > 1 mg L-1) were classified as Na-HCO3 type in the DJ basin and 78% were classified as either Na-HCO3 or Na-Cl type in the Piceance basin. Of the elevated methane samples, 96% and 69% in the DJ and Piceance basins respectively had microbial gas signatures, as determined by d13C values < - 60 ‰. Samples with elevated methane concentrations had higher pH, higher concentrations of chloride and sodium and lower concentrations of calcium in both the DJ and Piceance Basin. Elevated methane concentrations were predominately microbial in origin and correlated to indicators of increased water-rock interactions and anaerobic groundwater conditions, indicating that methane observed in these groundwater samples are largely a result of natural processes. Rare occurrences of stray thermogenic gas (d13C > 55 ‰, gas wetness > 5 % C2+ hydrocarbons) were most frequently associated with the Na-HCO3 water type in the DJ basin (67% of occurrences) and were randomly distributed across water types in the Piceance Basin. Investigation of natural and anthropogenic causes for the presence of methane is ongoing, using the full suite of water quality parameters contained in the COGCC database, combined with information on underlying geological frameworks and industry infrastructure. With these data, we are working towards a more complete and objective understanding of the effects of oil and gas operations on regional groundwater quality.
