Airborne ethane observations in the Barnett shale: Quantification of ethane flux and attribution of methane emissions

We present high time resolution airborne ethane (C₂H₆) and methane (CH₄) measurements made in March and October 2013 as part of the Barnett Coordinated Campaign over the Barnett Shale formation in Texas. Ethane fluxes are quantified using a downwind flight strategy, a first demonstration of this approach for C₂H₆. Additionally, ethane-to-methane emissions ratios (C₂H₆:CH₄) of point sources were observationally determined from simultaneous airborne C₂H₆ and CH₄ measurements during a survey flight over the source region. Distinct C₂H₆:CH₄ × 100% molar ratios of 0.0%, 1.8%, and 9.6%, indicative of microbial, low-C₂H₆ fossil, and high-C₂H₆ fossil sources, respectively, emerged in observations over the emissions source region of the Barnett Shale. Ethane-to-methane correlations were used in conjunction with C₂H₆ and CH₄ fluxes to quantify the fraction of CH₄ emissions derived from fossil and microbial sources. On the basis of two analyses, we find 71–85% of the observed methane emissions quantified in the Barnett Shale are derived from fossil sources. The average ethane flux observed from the studied region of the Barnett Shale was 6.6 ± 0.2 × 103 kg hr^–1 and consistent across six days in spring and fall of 2013.