Maturation of petroleum source rocks—II. Quantitative changes in extractable hydrocarbon content and composition associated with hydrocarbon generation

Abstract

Quantitative changes in extractable saturated and aromatic hydrocarbon compositions with increasing maturation have been examined in a natural source rock sample suite of uniform lithology and organic facies. The samples are from the Upper Devonian Duvernay Formation of Western Canada and range from immature to overmature with respect to hydrocarbon generation. Hydrocarbons used in conventional biomarker and aromatic hydrocarbon maturity parameters exhibit three widely varying quantitative trends: (1) selective loss of a single compound or isomer, while structurally-related isomers or homologues remain invariant or vary only slightly, (2) apparent interconversion of precursor and product, and (3) progressive increase in the concentrations of individual isomer(s) in association with bulk hydrocarbon generation. Of the various biomaker precursor-product reactions that have been proposed, only sterane aromatization exhibits quantitative product-precursor behavior. An increase in the widely-used sterane “isomerization” parameter at C-20 can be largely attributed to preferential loss of the ααα 20 R isomer. Most saturated hydrocarbon parameters examined vary to different extents with maturity; however, several of the parameters based on 2- and 3-ring aromatic hydrocarbon distributions do not vary closely with maturity level. The aromatic parameters which best reflect maturation level include those based on trimethylnapthalenes and methyldibenzothiophenes. The maturity dependence of molecular parameters used to characterize organic facies—pristane/phytane and hopane/sterane—is also demonstrated.