Eastern Section Meeting

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The Stable Isotope and Noble Gas Geochemistry of Point Pleasant, Utica, Tuscarora, and Trenton/Black River Formation Production Gases, Northern Appalachian Basin

Abstract

The Point Pleasant and Utica Formations are important shale resources in the Appalachian basin, and source rocks for the conventional Tuscarora and Trenton/Black River formations. Here, we present molecular and isotopic analyses of 18 production gases from Point Pleasant, Utica, and Tuscarora intervals in Tioga County, Pennsylvania, and from Trenton/Black River reservoirs developed between Bradford County, Pennsylvania and Steuben County, New York. These data constrain the origin and thermal maturity of the gases, and have useful applications in defining reservoir quality, producibility, and risks associated with the non-hydrocarbon impurities (N2, CO2, and H2S).

All samples are post-mature thermogenic gases with isotopically heavy CH4 that exhibit full isotopic reversals with respect to carbon number. Some samples have reversals with respect to hydrogen number. Most CH4 is primary and generated from refractory organic matter at maturities equivalent to VR ~2.1 to 2.6%. Ethane and propane are post-mature secondary gases.

Helium isotopes range from 0.01–0.022Ra for most gases indicating crustal origins; some exceptions in the Trenton/Black River (Glodes Corners) range up to 0.16Ra indicating minor mantle contributions. Elevated CH4/3He ratios in Glodes Corners gases (4.4×109 to 11.5×109) preclude mantle-derived, abiogenic hydrocarbon sources. The 4He/40Ar* ratios of Trenton/Black River gases are within 1σ of crustal production values indicative of thermally mature source rocks, while the Point Pleasant, Utica, and Tuscarora gases show elevated 4He/40Ar*, 4He/CH4 and 20Ne/36Ar suggesting 4He and 20Ne accumulation by advection of gas-phase or multi-phase fluids following migration into openly fractured formations.

CO2 and N2 comprise 0–1.08 and 0.57–2.96 mol% of the gases, respectively. δ13CO2 values of −6.7 to −0.39‰, and crustal R/Ra results, suggest that the CO2 was generated by thermal destruction of carbonate minerals. Similarly, δ15N ranges from −9.2 to −11.4‰, suggesting that N2 is derived from the thermal breakdown of organic matter.

Trenton/Black River gas produced from the Wolpert # 1 well in Bradford County, Pennsylvania contains 14.14 mol% N2. The gas isotope data indicate a crustal N2 source and residual CH4 component. The gas contains 0.51 mol % H2S. δ34S is 14.7‰ indicating a sulfate source. We postulate that the high concentration of N2, originally generated through denitrification of post-mature kerogen, developed as a result of H2S generation and hydrocarbon oxidation associated