The identification of normal to underpressured formations in the Southeastern Sichuan basin

Abstract

The Sichuan Basin contains many overpressured formations with pore pressure coefficients as high as 2.3. However, in a Nanye 1 well found in the Southeastern (SE) Sichuan Basin, low-pore pressure coefficients as low as 0.9 have been recorded at a depth between 1544.3 and 1903.6 m, indicating the presence of underpressured formations. But the knowledge of normal and underpressured formations is not well studied in the SE Sichuan Basin. In this work, the normal to underpressured formations in the SE Sichuan Basin are identified, and their origins are discussed.We integrated the pore pressure measurement and estimation, pore pressure coefficient and pore pressure depth plot to identify the normal to underpressured formations. We also integrated basin modelling along with the faults system to determine the origins of underpressured formations. As a result, the Upper Permian Longtan and Middle Permian Maokou Formations (1906.5–2782.9 m depth) in the Nanchuan area, were identified as normal-to-underpressured gas deposits. Also, the study identified the Lower Silurian Longmaxi and Upper Ordovician Wufeng Formations (2072–2160 m depth) in the Pengshui area, SE of the Chongqing region as normal pressured gas formations. The study discovered a decrease in temperature, rock porosity rebound and gas migration out of the formations through faults as the reasons for underpressured formations in this area. The tectonic uplift and erosion resulted in a decrease in temperature and rebound of the rock porosity, which contributed to total pore pressure drop up to 46.62 MPa. The tight overlying seal rocks and underlying rocks helped to sustain the low pore pressures in these formations.