Hydrocarbon Accumulation Processes and Model Controlled by Overpressure Evolution of the DF1-1 diapir in the Yinggehai Basin, South China Sea

The fluid diapir is formed through the upward intrusion of fluids along pre-existing fracture zones, with hydraulic fracturing and fluid charge occurring at the uplift point of the overpressure interface. The DF1-1 diapir is the most typical fluid diapir in the Yinggehai Basin, characterized by multi-layer gas accumulation. In this study, petrography observation, laser Raman spectroscopy analysis, micro-thermometry, approximate calculation of fluid inclusion capture pressure and natural gas characteristics have been integrated to delineate the natural gas dynamic accumulation process and summarize the accumulation model of DF1-1 diapir. Results suggest that four-episode natural gas with different composition have been documented in the DF1-1 diapir. The first and second episode of hydrocarbon gas-dominated charging occurred at 3.4-2.9Ma and 1.8-0.4Ma, respectively. The third and fourth episode were dry gas and inorganic CO₂, which occurred at 0.4-0Ma. The paleo-pressure evolution of HL₁ Formation was reconstructed following a model of “pressurization-release--pressurization”. And the coupling relationship between the Formation paleo-pressure evolution and the natural gas charge history was elucidated. Based on these analyses, this conformed to an overpressure-controlled episodic gas accumulation model, and the accumulation process of the DF1-1 diapir can be summarized as follows: initially, gas accumulated in deep reservoirs, with formation pressure increasing to fracture pressure, leading to diapir opening and subsequent gas loss or adjustment to shallower reservoirs along diapir faults for further accumulation. Simultaneously, gas filled the reservoirs and episodic diapir activity in the later stages resulted in rapid gas charging. This process is the primary factor contributing to the heterogeneity of gas distribution.