Comprehensive thermal maturity assessment in shales: a case study on the upper cretaceous Qingshankou formation (Songliao Basin, NE China)

The Songliao Basin is the largest Early Cretaceous lacustrine rift basin of northeast China and hosts tremendous conventional and unconventional petroleum resources. However, the distribution of shale oil is not well constrained, as production from source rock reservoirs requires certain thermal maturity cut-offs (vitrinite reflectance of > 0.8%Rr). In this study, the thermal maturity of the Qiangshankou Formation along the Southern Uplift and in the Central Depression of the Songliao Basin was evaluated by combining traditional and advanced petrographic and geochemical maturity proxies with clay mineralogical investigations. The Qingshankou Formation is one of the major source rocks of the region, but previous studies encountered difficulties in obtaining reliable maturity data, mainly due to strongly shifted T-max values. Equally, T-max determined during this study showed apparent shifting likely caused either by retained solid bitumen or by variations in kerogen type. In contrast, the measured vitrinite reflectance shows a reliable depth trend, meeting the shale oil cut-off maturity of 0.8%Ro at a drilling depth of similar to 2300 m, which is only reached in the Central Depression of the basin. The vitrinite reflectance trend is supported by changes in clay mineralogy, such as the progressing ordering of I/S mixed layers (Reichweite), increasing illitization, decreasing detrital kaolinite and correspondingly increasing authigenic chlorite contents. Maturity-related biomarker proxies are only partly valid; while the methylphenanthrene index (MPI-1) and the trimethylnaphthalene ratio (TNR-1) show reliable maturity trends even over short depth intervals, other established proxies (e.g. based on alkylbiphenyls) proved insensitive to changes from immature to peak oil mature and are, therefore, not recommended for further use in studies on Qingshankou Fm. shales. In summary, the combination of various maturity parameters improves the understanding of shale oil distribution in the central Songliao Basin, providing valuable insights for studies on similar unconventional targets in the region.

Automated summary

The study evaluated the thermal maturity of the Qiangshankou Formation in the Songliao Basin by combining traditional and advanced petrographic and geochemical maturity proxies with clay mineralogical investigations. The vitrinite reflectance trend, supported by changes in clay mineralogy, provided valuable insights for understanding shale oil distribution in the central Songliao Basin. Maturity-related biomarker proxies showed partial validity, with some proxies proving insensitive to changes in maturity from immature to peak oil mature.