Significant Reduction in Ethane Emissions in the Denver-Julesburg Basin From 2015 to 2021 From Oil and Natural Gas Operations.

Top-down studies of methane (CH4) emissions often use the average ethane-to-methane (C2/C1) ratio of wellhead gas in a basin for source attribution between thermogenic and biogenic sources. Biogenic (CH4) sources (ruminants, wetlands, landfills, and other methanogenic sources) do not co-produce ethane (C2H6); the presence of C2H6 indicates a thermogenic CH4 source. However, the C2/C1 ratio often varies within and across basins, as well as among emitting sources and facility configurations. This study uses Fourier Transform Infrared Spectroscopy (FTIR) data from exhaust stack tests of both four-stroke rich-burn and four-stroke lean-burn engines, comparing the relative “destruction efficiency” between the engines and across species. Results show that engines combust heavier hydrocarbons more efficiently than CH4, as evidenced by consistently higher destruction efficiencies for C2H6 across all engine types. The data indicates a preferential destruction of C2H6 (and higher hydrocarbons) relative to CH4; the exhaust gas C2/C1 ratio is consistently lower than the fuel gas C2/C1 ratio. Additionally, recent design modifications at O&G production facilities resulted in a reduction of both C2H6 and CH4 emissions with a more pronounced reduction in C2H6 emissions. In the Denver-Julesburg basin in Colorado, natural gas production increased by 73.9 % from 2015 to 2021. Despite these increases, the companion paper by Daley et al. (2025) reveals that top-down CH4 and C2H6 emissions from oil and natural gas facilities decreased by -25.3%% and 63.6 %, respectively. This work provides a possible explanation and may indicate a shift in emissions from the production sector to the midstream sector.