Key Controlling Factors of Hydrocarbon Accumulation of Fine-Grained Mixed Sequence in a Saline Lacustrine Basin: An Integrated Research of Petroleum System in the Northwestern Qaidam Basin, Qinghai–Tibet Plateau

Fine-grained carbonate-siliciclastic mixed sequences, formed in saline lacustrine settings, constitute substantial unconventional and conventional hydrocarbon resources. Clarifying the key controlling factors of hydrocarbon accumulation is pivotal for predicting potential resources and enhancing exploration strategies. However, there is still a lack of research on how hydrocarbons accumulate in fine-grained carbonate-siliciclastic mixed sequences. Here, we present integrated research on a unique saline lacustrine petroleum system with fine-grained mixed deposits in the northwestern Qaidam Basin based on geochemical analysis, reservoir data, fluid inclusion analysis, and basin modeling. The saline lacustrine source rocks have low organic abundance, with type II–III organic matter. The high content of soluble organic matter and large thickness of saline lacustrine source rock provided sufficient hydrocarbon for the petroleum system. The reservoir rocks exhibit unusual mixed characteristics of carbonate and siliceous minerals. Dissolution and microfracture development are critical for the formation of high-quality reservoirs. Hydrocarbon charging began during the Middle Miocene, and initially, it occurred in those areas where early traps were formed. By comparison, hydrocarbon began to charge late traps during the Late Miocene or Pliocene. The crucial controlling factors of hydrocarbon accumulation in the saline lacustrine basin include (1) adequate hydrocarbon supply, (2) high-quality fine-grained mixed reservoirs, (3) favorable source–reservoir–caprock assemblage, (4) many anticlinal traps generated by tectonic movements in the central lacustrine basin, and (5) suitable matching relationship of geological elements. This research also established hydrocarbon accumulation models of early trap and late trap to promote future exploration. This research provides new insights into a saline lacustrine petroleum system, which may serve as an efficient template for other saline lacustrine basins worldwide to promote future petroleum exploration.