Compound-Specific Stable Isotope Analysis of Natural and Produced Hydrocarbon Gases Surrounding Oil and Gas Operations

Production of oil and gas from shales and other unconventional hydrocarbon-bearing reservoirs has increased dramatically in recent years, largely as a result of technological developments in horizontal drilling and hydraulic fracturing. This increase has been accompanied by growing awareness of the environmental impacts of drilling and production activities, with a particular emphasis on groundwater quality. The presence of natural gas in groundwater is a health and explosion hazard. It also may indicate hydraulic connectivity with deeper hydrocarbon reservoirs, along which formation and drilling fluids that could migrate upward. Gas in groundwater occurs naturally through microbial activity and connection with microbial or thermogenic hydrocarbon accumulations. More rarely, hydrocarbon gases can migrate upward along the annuli of well bores and into shallow aquifers. The ability to distinguish between in situ and migrated natural gas underlies recent baseline water quality regulations in jurisdictions such as the state of Colorado. Stable isotope analysis forms a critical element of natural gas identification and fingerprinting protocols. This chapter reviews analytical and interpretive methods of compound-specific stable isotope analysis of natural gas.