Maturity and thermal evolution differences between two sets of Lower Palaeozoic shales and its significance for shale gas formation in south-western Sichuan Basin, China

Abtract The Lower Cambrian Qiongzhusi shale and the Upper Ordovician-Lower Silurian Wufeng-Longmaxi shale are major source rocks and important shale gas exploration targets in the Sichuan Basin. Cutting and core samples from two shale gas wells in south-western Sichuan Basin have been selected for organic matter (OM) reflectance measurement and Raman spectra analysis with the aim to determine maturity. The results were incorporated for basin modelling to construct the thermal evolution of this area. Graptolite fragments and pyrobitumens are the dominant form of OM in Wufeng-Longmaxi shale. A limited number of the OM as pyrobitumens and vitrinite-like particles is presented in Qiongzhusi shale. According to the measured reflectance of OM in conjunction with the maturity calculated using Raman spectra, the equivalent vitrinite reflectance (EqVRo) values based on solid bitumen and graptolite is 2.53-2.81% for the Wufeng-Longmaxi shale. The EqVRo value is 2.85-3.53% within 300 m depth intervals for the Qiongzhusi shale. A significant difference in oil generation occurred before the Late Permian between Qiongzhusi shale and Wufeng-Longmaxi shale based on basin modelling and reassessment of previous published data. Compared with the Qiongzhusi shale, the Wufeng-Longmaxi shale would have lower maturities before Late Permian and less hydrocarbon could be generated/expelled at that time, therefore, this shale remained with a large amount of oil, which could crack into gas. This difference had shown a favourable time relationship for generation and accumulation of shale gas in Wufeng-Longmaxi shale.

Automated summary

Organic matter reflectance measurement and Raman spectra analysis were conducted on shale samples from two wells in the Sichuan Basin, revealing that graptolite fragments and pyrobitumens are dominant in the Wufeng-Longmaxi shale, while a limited amount of pyrobitumens and vitrinite-like particles are present in the Qiongzhusi shale. Based on the results, it was found that the Wufeng-Longmaxi shale had lower maturities before the Late Permian compared to the Qiongzhusi shale, indicating a favorable time relationship for generation and accumulation of shale gas in the former.