Comparison of the Ediacaran and Cambrian petroleum systems in the Tianjingshan and the Micangshan uplifts, northern Sichuan Basin, China

Abstract

Several different types of petroleum occurrences (including solid bitumens, oil sands, and oil seeps) are observed in various strata (e.g., the Ediacaran, Cambrian, Devonian, Permian, Triassic, and Jurassic strata) of the Tianjingshan and the Micangshan uplifts, northern Sichuan Basin, China. However, the sources of these petroleum occurrences are still in dispute, because of the severe biodegradation, strong thermal alteration, and multiple tectonic movements. At present, such uncertainty in the potential source rock systems has hindered oil and gas exploration in the study areas. In this study, methods including catalytic hydropyrolysis and trace elements analysis were introduced into the oil-source correlation study, to avoid the influence of biodegradation and/or thermal alteration. The results show that the petroleum shows that occur in the Cambrian-Jurassic strata of the Tianjingshan area are characterized by low maturity (equivalent vitrinite reflectance (EqVRo) < 1.0%), light organic carbon isotopic ratios (δ13Corg values ranging from −34.0‰ to −38.4‰), a predominance of C29-steranes, enrichment in V and Mo, and strong depletion in Bi elements. In contrast, the solid bitumens in the Ediacaran Dengying Formation in the Micangshan area have high maturity (EqVRo > 2.0%), slightly heavier δ13Corg values ranging from −27.5‰ to −36.0‰, a predominance of C27-steranes, weak enrichment in Mo and strong depletion in Rb and Bi elements. In addition, the Ediacaran and the Lower Cambrian source rocks can be discriminated by the bound sterane distributions, trace element concentrations, and bulk δ13Corg values. Further oil-source correlation indicates that the petroleum shows in the Cambrian-Jurassic strata in the Tianjingshan area were dominantly derived from the Lower Cambrian Qiongzhusi Formation, while those in the Ediacaran Dengying Formation in the Micangshan area were mainly sourced from the Ediacaran Doushantuo Formation. Finally, the results of oil-source correlation, combined with the tectonic evolution history of the investigated areas further elucidate the occurrence and distribution of petroleum, which can assist in oil and gas exploration in the investigated uplifts and the adjacent areas.