Empirical carbon isotope/maturity relationships for gases from algal kerogens and terrigenous organic matter, based on dry, open-system pyrolysis

Pyrolyses experiments (dry, open-system) on algal kerogens and landplant material (Berner et al., 1995) have shown that carbon isotopic variations of methane, ethane and propane obtained from laboratory simulations mimic isotope variations of natural thermal gases. These isotope variations can be approximated through kinetic models (Berner et al., 1995). Also, maturity parameters like vitrinite reflectance and Rock-Eval Tmax are reliably calculated from kinetic models that are based on pyrolysis experiments. User-friendly versions of isotope/maturity models for methane, ethane and propane were obtained from application of statistical curve-fitting procedures to the results of instantaneous kinetic models of Berner et al. (1995). The resulting simple empirical functions relate carbon isotopic variations of light hydrocarbons directly to source rock maturity and can be applied where gases have accumulated instantaneously. They allow flexible calculations that account for carbon isotopic variability of precursor sites of the individual gases. Application of the model enables detection of mixing between bacterial and thermal gases, as well as mixing between thermal gases of different maturities. The application of the proposed models is demonstrated in two case studies (Green Tuff Basin, Japan; Cooper Basin, Australia) with data taken from the literature (Rigby and Smith, 1981; Sakata, 1991).