Abstract Hydrocarbon gas composition and isotopic composition of methane were analyzed from cutting samples obtained from industry oil wells penetrating the Eileen and Tarn gas-hydrate deposits. These gas-hydrate deposits overlie the Prudhoe Bay and Kuparuk River oil fields and are restricted to the updip part of a series of nearshore deltaic sandstone reservoirs in the lower Tertiary (Eocene) Mikkelsen Tongue of the Canning Formation and the Tertiary Staines Tongue of the Sagavanirktok Formation, respectively. The Eileen and Tarn gas hydrates are thought to contain a mixture of deep-source thermogenic gas and shallow, microbial gas (methane carbon isotopic composition ranges from —54 to — 46% in the gas-hydrate zone). Thermogenic gases likely come either from existing oil and gas accumulations or from source rocks within the oil- and gas-generating window that have migrated updip and or upfault and formed gas hydrate. The timing of gas source mixing is unknown. The microbial gases likely have a source contribution from biodegraded oil or gas in the underlying oil fields, as evidenced by the carbon isotopic composition of methane, ethane, propane, and carbon dioxide. The distribution of the Eileen and Tarn gas-hydrate accumulations appears to be controlled in part by the presence of large-scale regional faults that may have acted as vertical and lateral gas migration conduits.