Modeling thermogenic gas generation using carbon isotope ratios of natural gas hydrocarbons

An isotopic model to predict δ13C values for methane (C1), ethane (C2), and propane (C3) as a function of fractional conversion of kerogen and/or oil to gas is proposed for gas generation from Type II kerogen. Fractional conversion is then correlated to gas generation temperature by coupling the isotopic model to a multicomponent compositional kinetic model for gas generation. The relationships between δ13C1, δ13C2 and δ13C3 are constrained by the natural gas plot model of thermal cracking. The isotopic model has been tested and calibrated using a data set of gas samples derived from sapropelic marine shales (Type II kerogen) in the Delaware and Val Verde basins, West Texas. These gases are relatively unaffected by migration or mixing of biogenic gas, so that changes in carbon isotope ratios of the gas components primarily reflect the effect of maturation. This data set is therefore useful for illustrating the relationship between gas generation temperature and δ13C values for gases derived from Type II kerogen. l applications of the isotopic /compositional kinetic model in petroleum system evaluation are discussed. The effect of burial history on the relationship between δ13C1δ13C3 and gas generation temperature can be determined from the coupled models. Also, δ13C values of C1C3 observed in the reservoir will depend on whether gas accumulates from source to trap, or whether it is expelled at various stages into several traps. Both trapping scenarios can be examined using the isotopic/compositional kinetic model. Separate isotopic models are needed for gases derived from Type II versus Type III kerogens because of the larger difference between δ13C1 and δ13C2 observed for gases generated from terrestrial organic matter. This larger difference may result from greater molecular or isotopic heterogeneity in Type III kerogens, or may reflect differences in generation and/or accumulation of gases from the differing kerogen types.