MATURATION OF PETROLEUM SOURCE ROCKS .1. CHANGES IN KEROGEN STRUCTURE AND COMPOSITION ASSOCIATED WITH HYDROCARBON GENERATION

Kerogens ranging from immature to overmature with respect to hydrocarbon generation have been analyzed by various solid-state spectroscopy techniques, by pyrolysis/gas chromatography, and pyrolysis/gas chromatography/mass spectrometry in order to characterize changes in molecular composition. The kerogens analyzed were isolated from organic-rich calcareous shales of the Upper Devonian Duvernay Formation of Western Canada and represent marine Type II kerogens. C-13 NMR, IR, EPR, and ESE (electron spin echo) spectroscopy results indicate that during thermal maturation kerogen is transformed from a macromolecule characterized by a significant aliphatic component containing more isolated, less condensed aromatic structures to one of shorter chain length dominated by more highly-associated, condensed polyaromatic structures. These changes in composition are reflected in pyrolysis/gas chromatograms by the progressive decrease in yields of aliphatic moieties and total pyrolysates per unit carbon. Various molecular parameters used for thermal maturity assessment were calculated from pyrolysate distributions. The parameters are based on distributions of isoprenoid hydrocarbons (Pristane Formation Index, measured on powdered rock samples) and polynuclear aromatic hydrocarbons (naphthalenes, phenanthrenes, and dibenzothiophenes, measured on isolated kerogens). The Pristane Formation Index increased monotonically with increasing maturity and shows the best agreement with maturity level of all parameters measured. The aromatic parameters based on trimethylnaphthalene and methyldibenzothiophene distributions also vary with estimated level of maturity. Parameters based on dimethylnaphthalene and methylphenanthrene distributions do not accurately reflect maturity level, in part due to interferences from coeluting pyrolysates (in the case of methylphenanthrenes). It is emphasized that these findings are specific to this kerogen type and lithofacies and may not apply to other sedimentary organic matter types.