Fluid inclusions were used to determine gas accumulation period and accumulation process in Ledong 8-1 diapir area of Yinggehai Basin

Yinggehai Basin is an important high-temperature and high-pressure gas producing basin in the South China Sea, and Ledong 8-1 diapir is located in the southeast of the basin, which is the focus of natural gas migration and accumulation. However, the study of fluid inclusions in this gas-bearing structure is not enough, and the accumulation period and accumulation process are not clear, which restricts the progress of exploration in this area. Taking Ledong 8-1 as the research object, this paper uses microscopic lithofacies observation, microscopic temperature measurement of inclusions, laser Raman composition identification, in situ quantitative spectroscopy based on inclusion data to recover paleo-pressure and indirect projection dating of uniform temperature-burial history to identify gas inclusions of different composition types and recover the paleo-pressure evolution history of the Huangliu Formation. The accumulation period and process of natural gas were determined. The results show that there are four types of gas inclusions in Ledong 8-1 diapir, corresponding to three phases of gas charging. The charging time of the first phase of natural gas is 2.2-1.6Ma, the gas composition is mainly CH₄, and the pressure coefficient of Huangliu group is 1.13-1.42, indicating weak overpressure. The second phase of natural gas charging occurred at 1.6-0.6Ma, and the gas composition was mainly CO₂. Since 0.6Ma, the third stage of natural gas was charged into the formation, and the pressure coefficient of Huangliu formation was as high as 1.8, indicating strong overpressure. It is speculated that the diapiric was opened at this stage, and the natural gas formed in the early stage of Huangliu formation and the newly generated natural gas from deep source rocks migrated upward and adjusted. Studying the fluid inclusions in Ledong 8-1 area of Yinggehai Basin and clarifying the process of gas accumulation in this area is of great significance to the oil and gas exploration in the whole basin, and it is also the key to promote the increase of oil and gas storage and production in the whole basin.