| Summary: | The Qiongdongnan (QDN) basin in the northwestern South China Sea has been shown to be rich in petroleum resources and has significant exploration potential for deepwater oil and gas resources. Therefore, it is of great significance to clarify the gas accumulation process in the deepwater area of the QDN basin, specifically the Y8 area. In this study, fluid inclusion analysis, including the identification and localization of fluid inclusions under a microscope, laser Raman spectrum analysis and homogenization temperature (T<sub>h</sub>) measurement, reconstruction of the thermal evolution burial history, geochemical analysis, and typical seismic section analysis were comprehensively used to interpret the gas accumulation process of the Y8 area. The laser Raman spectrum analysis results of fluid inclusions verified the existence of methane and confirmed the hydrocarbon charging event in this area. However, the Th of fluid inclusions in the reservoir, the mixed compositions of hydrothermal compositions (H<sub>2</sub>S + SO<sub>2</sub>) and hydrocarbons (methane) in the fluid inclusions, and the low T<sub>max</sub> values of some wells in the Y8 area comprehensively confirmed that the coincidence of hydrothermal activity with the accumulation process affected the long-distance gas accumulation of the Y8 area. The hydrothermal activity was also confirmed by the presence of fluid diapirs in seismic profiles of the Y8 area. Furthermore, the hydrocarbon accumulation process was reconstructed using the modified thermal evolution history considering the effect of hydrothermal activity. Overall, this study attributed the high T<sub>h</sub> of fluid inclusions in the reservoirs of the Y8 area to hydrothermal activity and proposed a long-distance migration model of hydrocarbon accumulation coinciding with hydrothermal activity, both of which are beneficial to understanding the accumulation process of the Y8 area.
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