IDENTIFICATION AND OCCURRENCE OF THERMOGENIC STRAY GAS IN GROUNDWATER WELLS OF THE DENVER-JULESBURG AND PICEANCE BASINS OF COLORADO

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 27,000 samples from 4,756 unique surface and groundwater locations, and 2,000 oil and gas production wells, collected since the year 1990. This represents one of the most extensive databases of water quality in relation to oil and gas development anywhere. Following rigorous data QA/QC, we identified groundwater samples containing microbial versus thermogenic hydrocarbon gas. Except in the case of shallow coal seams, the presence of thermogenic gas in groundwater aquifers may indicate natural or artificial migration pathways originating from deep oil and gas reservoirs. Based on data from nearby oil and gas production wells, we define thermogenic gas as having a carbon isotope (δ¹³C) value > -55 per mille and gas “wetness” value > 4% C₂+ alkanes. In the oil and gas producing Denver-Julesburg (DJ) Basin of eastern Colorado, 24 groundwater wells contained thermogenic gas. As a lower estimate of occurrence rate, this represents 0.06 % of the total number of groundwater wells in the basin; as an upper estimate, it represents 2.30 % of groundwater wells sampled by the COGCC in the basin. In the primarily dry gas producing Piceance Basin of western Colorado, 4 wells contained thermogenic gas, with estimated upper and lower occurrence rates of 0.10 to 0.55 %, respectively. Dissolved methane concentrations were higher when the methane had a microbial origin, compared to a thermogenic origin. Investigation of natural and artificial causes for the presence of thermogenic stray gas is ongoing, using the full suite of water quality parameters contained in the COGCC database, combined with information on underlying geological frameworks and oil and gas 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.