The Middle Devonian Marcellus Formation is the most prolific hydrocarbon play in the Appalachian basin. The Marcellus is a prospective CO2 sequestration reservoir and a potential seal for CCUS and NGL storage. Regional differences in Marcellus fluid chemistry reflect variations in thermal maturity and hydrocarbon alteration. These variations define specific wet gas and dry gas production in the basin. This report integrates results from a decade of geochemical studies of Marcellus natural gas chemical and isotope composition, including noble gas systematics for dry gas samples. The interpretation of the chemistry of wet gas samples is supplemented by condensate High Resolution Gas Chromatography and API gravity data.
Dry Marcellus gases produced in northeast Pennsylvania and northcentral West Virginia are mixtures of over-mature methane, mostly cracked from refractory kerogen, and ethane and propane cracked from light oil and wet gas. Natural gas plots reveal reversed carbon and hydrogen isotope trends which reflect (1) mixing of hydrocarbons of different thermal maturities, (2) Rayleigh fractionation of wet gas during redox reactions with formation water at high temperatures, and (3) isotope exchange between methane and formation water. Noble gas systematics supports the interpretation of hydrocarbon-formation water interactions. Noble gas data constrain the maturity of the hydrocarbon gases and provide a method of quantifying gas retention versus expulsion in the reservoirs.
Marcellus gases co-produced with condensate in southwest Pennsylvania and northwest West Virginia are mixtures of residual associated gases generated in the late oil window and post-mature hydrocarbons generated from oil cracking in the wet gas window. The δ13C of ethane and propane are congruent and reflect effective maturation trends within the source rocks. Correlation of API gravity and C7 expulsion temperatures, high heptane and isoheptane ratios, and the gas geochemical data confirm that the Marcellus condensates formed through oil cracking. Respective low toluene/nC7 and high nC7/methylcyclohexane ratios indicate selective depletion of low-boiling point aromatics and cyclic light saturates in all samples, suggesting that water washing and gas stripping have altered the fluids. These alterations may be related to deep migration of hot basinal brines.
Marcellus hydrocarbon composition, isotope, and noble gas data provide a methodology independent of rock data, choke management and completion design for defining and mapping economic production limits. Development decisions and production forecasts are better informed when resource maturity and relative fluid retention controls on gas/liquid ratios supplement petrophysical and production data.