Potential Underestimate in Reported Bottom-up Methane Emissions from Oil and Gas Operations in the Delaware Basin

Methane is a greenhouse gas and identified as a key driver of near-term climate change. Bottom-up approaches estimate annual methane loss from US natural gas production and transport at 6 Tg, but recent studies suggest this may be an underestimate. To investigate this possibility, an equipment-based emissions inventory, using EPA emission factors, was developed to calculate methane emissions from oil and gas operations in the Delaware basin, USA. Emission factors and activity data were then updated using contemporary and region-specific measurement data. The original inventory estimated emissions at 315 Gg CH₄ y⁻¹ (gas production-normalized rate of 0.6% loss), while the updated inventory estimated emissions of 1500 Gg CH₄ y⁻¹ (2.8% loss). The largest changes resulted from large fugitive emissions from oil production (+430 Gg CH₄ y⁻¹), updating maintenance activity emissions (+214 Gg CH₄ y⁻¹), considering flaring inefficiency (+174 Gg CH₄ y⁻¹), and the inclusion of associated gas venting (+136 Gg CH₄ y⁻¹). This study suggests that a systematic underestimate probably exists in current bottom-up inventories and identifies sources currently missing or may be incorrect. We also strongly recommend that emission factors should be validated through direct comparison against measurement campaigns that include long-tail distributions typical of oil and gas activities.