IDENTIFYING THE SOURCE OF METHANE EMISSIONS FROM LEAKING ORPHAN WELLS ON THE KEVIN-SUNBURST DOME, TOOLE COUNTY, MONTANA

The U.S. Geological Survey and the Well Done Foundation measured methane emissions and collected gas samples from 16 orphaned wells on the Kevin-Sunburst dome in Toole County, Montana, in July of 2023. The wells in this study were drilled in the 1920s and 1930s into the same producing formation, and share similar drilling and production histories. Some wells are physically located in very close proximity, as near as a few hundred feet apart. The producing formation for the wells is the Mississippian Madison Limestone, which primarily produced oil in the northern part of the dome where the samples were collected. The goal of our study is to better understand the geologic framework that leads to methane emissions by incorporating information on the subsurface sources of methane and the engineering factors constraining well emissions.

Stable carbon isotopic analysis was performed on methane in the gas samples collected from each well, along with gas composition analysis that included C₁−C₅ hydrocarbons, air components, and helium. δ¹³C values for methane were in the -50s ‰ VPDB (relative to Vienna Peedee Belemnite) and gas wetness ratios were around 10 for all the wells. Based on both the gas composition and reservoir properties, we attribute the methane emitted from these wells to be a mixture of oil associated gas and gas generated from secondary biodegradation of the residual oil in the reservoir. Comparing these samples to previous geochemistry studies on the dome with isotopic data on gases from actively producing wells, we confirm that the gas emitted from the orphan wells is essentially the same composition as the gas emitted from nearby production wells collected in recent years. Comparing our data with those from the U.S. Bureau of Mines collected in the 1980s, confirms that the geochemistry has not changed over the last 40+ years, even as the well status changed from active to abandoned, and eventually, orphaned. Knowledge of the sources of emissions from geochemical data is important for developing emissions budgets, understanding how wells decay over time, and identifying the processes that govern the emissions lifecycle of unplugged abandoned and orphaned wells.