Natural Gas Charging and Pressure Evolution in the Slope of the Yinggehai Basin

The abnormally high pore pressure is developed in the Huangliu Formation reservoirs of the Ledong X structure on the slope of the Yinggehai Basin. The present maximum pressure coefficient reaches 2.3, indicating a strongly overpressured system. The development and evolution of reservoir overpressure are closely associated with multi-stage natural gas charging. Based on fluid inclusion petrography and laser Raman spectroscopic analysis of gas inclusions, the pressure evolution of the Huangliu Formation reservoirs was reconstructed.

The results show that three types of gas inclusions occur in the Ledong X area: N₂–CH₄, CH₄–CO₂, and CO₂ inclusions. Three episodes of fluid charging were identified. The first stage was dominated by hydrocarbon gas charging at approximately 2.3 Ma. The second and third stages involved mixed CO₂–CH₄ charging, initiating at ~1.8 Ma and ~0.5 Ma, respectively.

The pressure evolution of the Huangliu Formation reservoirs can be divided into two main stages: (1) an early pressure build-up stage and (2) a re-pressurization stage following formation fracturing and pressure release. During 2.3–0.8 Ma, combined hydrocarbon gas and CH₄–CO₂ charging caused a rapid increase in reservoir pressure, with pressure coefficients exceeding 2.3. Subsequent formation fracturing led to pressure dissipation and partial loss of early-charged gas. Since approximately 0.4 Ma, renewed large-scale natural gas charging during the third stage has caused reservoir pressure to rise again, ultimately reaching the present overpressured state.

These results demonstrate that gas charging plays a key role in overpressure development and pressure evolution in deep, overpressured gas reservoirs, providing new constraints on gas accumulation and preservation in the Yinggehai Basin.

Keywords：fluid inclusion，Raman spectrum、paleo-pressure restoration、natural gas charging history、Yinggehai Basin