The heterogeneous characteristics and their influencing factors of organic matter of saline lacustrine hydrocarbon source rocks

The heterogeneous characteristics and their influencing factors of organic matter are still the research focuses of saline lacustrine hydrocarbon source rocks. This paper examined the heterogeneous characteristics of organic matter richness and type. The effects of tectonic activities, sedimentary facies, climate, biological sources, and preservation conditions on organic matter richness and type were examined in the Yiliping Depression, Qaidam Basin, northwestern China. Organic matter enrichment models for the Yiliping Depression were developed. Total organic carbon (TOC) content, extractable organic matter (EOM) content, and free hydrocarbon and kerogen thermal cracking hydrocarbon yield of mudstones are higher than 0.5%, 0.05% and 0.5 mg/g in the condensed sections and the early stage of high stand systems tracts while lower in other intervals. The heterogeneity of organic matter richness and type in the Pliocene is displayed. With the uplift of the A-erh-chin mountains, influenced by the Himalayan tectonic movement during the Neogene, the sedimentary facies were mainly saline lakes with braided river deltas, and their depocenters gradually migrated eastward. Climatic conditions changed from humid in the lower Pliocene to dry in the middle and upper Pliocene. Multiple water transgressive and regressive cycles formed multiple condensed sections and high stand systems tracts, where enough accommodation space, organic matter supply, favorable preservation conditions resulting from relatively deep and quiet water brought more organic matter accumulation. Mixed input of aquatic algae and terrestrial plants formed type II2 and type III kerogen in the depression, and there are more type II1 and type I hydrocarbon source rocks around lacustrine centers. The saline lacustrine evolution and organic matter accumulation models were developed, showing that the peak saline lacustrine period in the middle Pliocene was the most favorable for organic matter enrichment. The results indicate that there are active hydrocarbon source rocks (TOC content more than 0.4% and reflectance of vitrinite more than 0.5%) from the Neogene around the Jianshishan and Hongsanhan structures, and the Yikeyawuru and Taijinaier structures in the Yiliping Depression.