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Geochemical Characterization of Post Mature Gases from Middle Devonian Marcellus Formation, Northern Appalachian Basin, USA – Insights into Deep Basin Hydrocarbon Generation, Alteration, and Productivity


The Marcellus Formation in northeast Pennsylvania produces gas from post mature marine mudrocks. Organic matter reached the metagenetic stage of thermal evolution. Vitrinite reflectance ranges from 2.9 to 5 percent and kerogen H/C and O/C ratios are 0.4 and 0.06, respectively. The isotope composition of production and mud gases collected from 24 wells implies high thermal stress. Production d13C1 ranges from −28.69 to −24‰. Mud gas d13C1 ranges from −27.3 to −21.5‰. All gases exhibit carbon isotope reversals with respect to carbon number. d13C1 approaches, or is heavier than, d13CKEROGEN. Production gases exhibit hydrogen isotope reversals (methane d2H > ethane d2H). d13CO2 ranges from −19.7 to −10.2‰. The d13C and d2H isotope reversals may be interpreted in one of four ways: (1) mixing of gases from different sources, (2) a combination of mixing, Rayleigh fractionation of C2 and C3, and exchange of methane hydrogen with formation water, (3) water-reforming followed by Fischer-Tropsch synthesis, and (4) destruction of C2+ alkanes followed by reforming via gas-phase radical recombination reactions. Strong correlation between d13C1 and d13C2 (R2 = 0.986) and d13C1 and d13C3 (R2 = 0.923), and light d13CO2, support the reforming hypotheses. Post mature Marcellus prospects are limited by thermal maturity, geologic structure, and loss of organic porosity. Gas isotopes help to predict well performance. Productive wells have d13C1 = d13CKEROGEN, d13C2-3 > d13CKEROGEN, and high d13C1-3 correlation. Marginal wells have d13C1 = d13CKEROGEN, d13C2-3 > d13CKEROGEN, and variable d13C1-3 correlation. Unproductive wells have d13C1 > d13CKEROGEN, d13C2 = d13CKEROGEN, trace C3+, and poor d13C1-3 correlation.