Pressure evolution of shale gas reservoirs in Wufeng-Longmaxi formations, Lintanchang area, southeast Sichuan Basin and its geological significance

Abstract

The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in Lintanchang area of the southeastern Sichuan Basin has good exploration and development conditions, and clarifying the paleo-fluid pressure evolution is of great significance for revealing the process of shale gas accumulation and fugitive emission. Taking the shale tectonic fractures and fluid overpressure fracture veins of the Wufeng-Longmaxi formations in Lintanchang area as the research object, the paleo-fluid pressure evolution process in this area and its controlling impact were investigated using cathodoluminescence, inclusion temperature measurement, laser Raman analysis, and basin simulation. The study shows that: (1) Lintanchang area has experienced four stages: normal pressure, mild overpressure to normal pressure, overpressure, and pressure relief. Overpressure is mainly due to hydrocarbon generation. Pressure relief is mainly caused by shale gas fugitive emission. The gas reservoir pressure relief reaches 54% of the initial pressure during the pressure relief stage. (2) There are two stages of vein filling in the fractures at the bottom of the black shale in the Wufeng-Longmaxi formations. The first stage is formed in the sedimentation and burial stage at 131-118 Ma, the temperature is 178-205 ℃, and the trapping pressure of methane inclusions is 105.6-119.8 MPa. The second stage is formed in the tectonic uplift stage at 25-18 Ma, the temperature is 150-175 ℃, the methane inclusions have relatively low salinity and its trapping pressure is 80.8-92.1 MPa. The low pressure coefficient (1.37-1.49) indicates that mass fugitive emission has occurred. (3) The late tectonic movement, especially the rapid uplift in the late Himalayan period, is the root cause of the fugitive emission and pressure relief of the gas reservoirs. The decrease of the roundness and pore size of the organic pores indicates the deterioration of reservoir physical properties. However, due to the long formation and preservation time of the gas reservoirs, the shale in the Wufeng-Longmaxi formations in Lintanchang area still has good exploration potential. This study is helpful to deepen the understanding of the accumulation mechanism of the normal-pressure shale gas reservoirs in Lintanchang area and can provide theoretical guidance for optimal selection of favorable shale gas exploration areas.

Keywords

• pressure evolution
• fluid inclusion
• wufeng-longmaxi formations
• lintanchang area
• southeast sichuan basin