Organic geochemistry of Carboniferous-Permian sources rocks in Qiangtang Basin, Tibet

The Jurassic source rocks have long been one of the research hotspots, but the Carboniferous-Permian source rocks' capability of hydrocarbon generation has not aroused much attention due to the scattered distribution in Qiangtang Basin. Organic geochemical characteristics of carbonate rock and mudstone samples from eight sections in the Carboniferous-Permian strata of Qiangtang Basin were analyzed for the organic matter abundance, types and thermal evolution. The authors found that source rocks, including carbonate rock and mudstone, were influenced by the evolution of sedimentary environment. The carbonate source rocks might have been the micrite limestone of the carbonate platform, and the mudstone source rocks were mainly the dark mudstone of delta and slope facies and tuffaceous mudstone. The organic carbon content of Carboniferous-Permian mudstone is higher, suggesting hydrocarbon-generative potential; most sections have moderate to high content of organic carbon except Rejuechaka section which has no or poor source rock. Kemo section and Gongri section have mainly reached the standard of mid-good hydrocarbon source rock. However, the Carboniferous-Permian carbonate rock shows a lower content of organic carbon, with no or poor source rock. The type of organic matter of Carboniferous-Permian carbonate rock is mainly Ⅱ₁, but the type of organic matter of Carboniferous-Permian mudstone is mainly Ⅱ₂ and Ⅲ. The degree of thermal evolution of Carboniferous Permian source rock is generally high, and all the source rocks, except for the Kemo areas where the source rock is at the mature stage, are mostly in the high mature and over-mature stage. Such a situation is very favorable for the generation of natural gas and has a good prospect of natural gas exploration.