Stable carbon isotope systematics of methane, ethane and propane from low-permeability hydrocarbon reservoirs

Abstract

We have reassessed the stable carbon isotope systematics of methane (C1), ethane (C2), and propane (C3) in more than 500 natural gas samples from low-permeability hydrocarbon reservoirs around the world, with the purpose of providing new tools for thermal maturity assessment of natural gas that is increasingly produced from such reservoir types world-wide. A low-permeability reservoir resembles a semi-closed system and we found that the stable carbon isotope distribution in C1-C3 alkanes differs partially from what has been previously observed in natural gas from conventional hydrocarbon accumulations (e.g. C isotope distributions dominated by Rayleigh distillation and kinetic isotope effects). In a low-permeability reservoir, isotope exchange may play a more prominant role in the carbon isotope distribution, driving the system towards an even isotopic distribution (EID) of 6‰ between methane and ethane, and ethane and propane, at Ro of 1.5%. At higher maturity, ethane and propane depleted in 13C are formed as a consequence of thermal cracking of wet-gas components and possibly free radical decomposition/polymerization reactions, which leads to the occurrence of isotope reversals (δ13CC2 < δ13CC1 < δ13CC3, δ13CC2 < δ13CC3 < δ13CC1 and δ13CC3 < δ13CC2 < δ13CC1) at thermal maturity higher than 2.0%Ro. The diagram of the isotopic differences ∆13CC1-C2 (δ13CC1-δ13CC2) versus ∆13CC1-C3 (δ13CC1-δ13CC3) provides a new tool for classification of natural gas from low-permeability hydrocarbon reservoirs according to thermal history in three main regions: normal trend (subdivided into immature, oil/wet-gas window, and dry gas window), ethane reversal, and propane reversal with respect to methane. This new tool can be used to assess the maturity of the petroleum fluids and can assist in identifying hydrocarbon mixing.