Carbon isotope fractionation during natural gas generation from kerogen

Abstract

In petroleum exploration it is important to be able to determine the origin of any gas which is found. This paper describes a new method of estimating the source-type and maturity of a gas based on a Rayleigh fractionation model. Kerogen is divided conceptually into a labile (dominantly oil-generating) fraction and a refractory, gas-prone, component. δ¹³C of methane from either kerogen type, and ethane, propane and butane for gases from labile kerogen, can be defined as a function of δ¹³C of the gas precursor groups in kerogen, a kinetic isotope fractionation factor, k, and the extent of gas generation. The isotopic ratio of the methane precursors relative to bulk kerogen, determined from laboratory pyrolysis, are −17.5‰ for labile kerogen and −1.4‰ for refractory kerogen. Values for ethane, propane and butane from labile kerogen, based on field correlations, are −4.9‰, −2.2‰ and −1.6‰ respectively. The corresponding fractionation factors are 0.9892, 0.9919, 0.9947 and 0.9975 for methane, ethane, propane and butane respectively from labile kerogen, and 0.9984 for methane from refractory kerogen. Using these parameters, summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil, and recognition of gases of mixed origin. If an independent estimate of δ¹³C for the source kerogen is possible, then δ¹³C of the gas components can be used to estimate maturity in terms of the Gas Generation Index, the fraction of gas potential which has been realized.