Natural gas found in potable water aquifers proximal to active hydraulic fracturing activities has resulted in fierce debate on the environmental and health impacts of the methodology. While studies have traced some stray gas to a thermogenic origin, the mechanisms for contamination (e.g., migration induced by hydraulic fracturing, natural migration, or wellbore integrity issues) remain poorly understood. Recently, the Colorado Oil and Gas Conservation Commission (COGCC) performed a baseline production gas study and enacted regulations requiring ongoing groundwater geochemical surveys to protect groundwater resources and mitigate potential contamination. These regulations include the measurement of hydrogen and carbon stable isotopes (δD and δ13C) of methane when dissolved gas concentrations exceed 1 mg L-1, which can identify the origin of stray gas as thermogenic or bacterial.
In this study, we use natural gas stable isotopes and water quality data, cataloged by the COGCC, to evaluate methane sources in the Wattenberg field of the Denver-Julesburg basin. We focused on 112 out of 897 (12%) groundwater samples that exhibited elevated methane concentrations (>1mg L-1). Within these wells, average methane concentrations from multiple sampling periods range from 1.1 to 38 mg L-1. Methane stable isotope data reveal the natural gas in 91 waters was of bacterial origin, while 12 wells contained thermogenic gas, and eight showed a bacterial-thermogenic mixture. No significant differences in average dissolved methane concentration between well waters affected by thermogenic and bacterial methane were observed. Water wells that showed elevated methane concentrations mainly occurred in the southern part of the Greater Wattenberg Area (GWA), however within that region no spatial significance was apparent for wells contaminated with thermogenic methane.
Bacterial methane concentrations exhibit a weak (R2 = 0.29), though significant (p<0.05), positive correlation with well depth, wherein wells penetrating deeper than 750 feet show methane concentrations of at least 5 mg L-1. We found no relationship between bacterial or thermogenic methane and chloride concentration or alkalinity. On the contrary, thermogenic methane concentrations did not correlate with depth, but showed a significant positive correlation with pH (R2 = 0.95).