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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 the northeastern Pennsylvania core area produces natural gas from post mature marine mudrocks. Modeled VRe in the region is between 2.0 and 4.6%. In selected samples from Sullivan, Susquehanna, Wyoming, and Luzerne counties, measured VRo ranges between 3.2 and 4.5%, respective kerogen H/C and O/C ratios are 0.4 and 0.06, the Kübler Index is 0.428, and CAI values are between 4 and 5. These data indicate that the Marcellus Formation in northeast Pennsylvania belongs to the prehnite-pumpellyte metamorphic facies and the organic matter in the rocks is at the metagenetic stage of thermal maturity.

The isotope composition of production and mud gases collected from 24 wells in this area also indicates high thermal stress. Production gas δ13C1 ranges from −28.69 to −24‰. Mud gas δ13C1 ranges from −27.3 to −21.5‰. All gases exhibit carbon isotope reversals with respect to carbon number. δ13C1 approaches, or is heavier than, δ13CKEROGEN. Production gases exhibit hydrogen isotope reversals (methane δ2H > ethane δ2H).

While several processes proposed in recent literature are plausible, the δ13C isotope reversals are most likely due to mixing of primary gases cracked from refractory post mature organic matter and secondary gases cracked from residual oil, condensate, or light gases. The δ2H isotope reversals may indicate hydrogen isotope exchange between hydrocarbons and formation water, and reflect the role of H2O in secondary cracking of C2+ hydrocarbons to methane. Primary gas generation in the Marcellus Formation mudrocks in this study area was complete by thermal maturations equivalent to VRo ~ 2.0%, and secondary gas generation was complete by VRo ~ 3.0%. EUR and well productivity are functions of the amount of trapped hydrocarbon gases retained in the shales, reservoir properties, tectonics, seal integrity, and regional fluid migration.

Gas isotope data help to predict well performance. Productive wells have δ13C1 ≤ δ13CKEROGEN, δ13C2-3 < δ13CKEROGEN, and high δ13C1-3 correlation. Marginal wells have δ13C1 ≥ δ13CKEROGEN, δ13C2-3 < δ13CKEROGEN, and variable δ13C1-3 correlation. Unproductive wells have δ13C1 > δ13CKEROGEN, δ13C2 ≥ δ13CKEROGEN, trace C3+, and poor δ13C1-3 correlation. Noble gas data help to constrain gas migration mechanisms in the Marcellus Formation and distinguish between residual shale-gas and hydrocarbons lost from reservoirs through diffusion or advection of gas-phase or multi-phase fluids.