Hydrocarbon generation capability of Sinian–Lower Cambrian shale, mudstone, and carbonate rocks in the Sichuan Basin, southwestern China: Implications for contributions to the giant Sinian Dengying natural gas accumulation

Abstract

The giant natural gas accumulation recently discovered within the (Neoproterozoic) Dengying Formation in the Sichuan Basin of southwestern China represents a new and important supplement to the ancient Neoproterozoic–Cambrian petroleum systems worldwide. However, the source of this gas is controversial because there are five possibilities based on geology, from bottom to top including the Lower Sinian Doushantuo mudstone, the Upper Sinian Dengying algal dolomite, the mudstone of the third member of the Dengying Formation, the Lower Cambrian Maidiping argillaceous dolomite, and the Lower Cambrian Qiongzhusi shale. To improve the understanding of this issue, 33 samples from the five possible source rock units in the study area were investigated for their character and maturity via petrology, total organic carbon (TOC), pyrobitumen reflectance, kerogen carbon isotope, and biomarker analyses. Based on the results, we address their hydrocarbon generation capabilities and contributions to the giant gas accumulation. Results show that present-day TOC values of the samples range from 0.03% to 2.36% and vary both regionally and stratigraphically, with the Qiongzhusi shales in the central basin and the Weiyuan–Ziyang area ranking the highest. In contrast, the organic matter source and maturity within these rocks are relatively homogenous with slight variation throughout the study area. The organic matter is predominantly derived from algae and bacteria with different compositions deposited in marine reducing environments. The rocks range in thermal maturity from postmature to overmature. These results imply that the generation capability of source rock had been high in the past, and the gas in the study area is most likely originated from oil cracking and the late generation directly from the kerogen of source rocks, with the Qiongzhusi shales being the dominant source rock. Thus, the hydrocarbon migration pattern in the study area is a lateral migration from deep-seated and younger Cambrian source rocks in sags to shallow-seated and older Sinian dolomite reservoirs in uplifts. The migration process occurred in the oil phase with the oil being later cracked. The differences in natural gas geochemistry may be controlled by complex factors, e.g., thermal maturity, the migration–accumulation process, and differing contributions from source rocks besides the dominant Qiongzhusi. Regionally, the central basin and the Weiyuan–Ziyang area have had high hydrocarbon generation capability and thus should be considered as favorable targets for natural gas exploration in the future. This study of China provides an additional example of Neoproterozoic–Cambrian petroleum systems worldwide and thus can help to synthesize the systematics of such systems and understand the sedimentary transition across the Precambrian–Cambrian boundary.