A study of the coupling relationship between overpressure evolution and formation water of Xujiahe Formation in Xincang structure area, west Sichuan depression

Overpressure systems in sedimentary basins not only affect the migration and accumulation of geological fluid but also bring potential hazard for exploration. Formation water salinity, pressure coefficient, characteristic coefficient and ions relationships of the second and fourth member of Xujiahe Formation (T₃X², T₃X⁴) in western Sichuan Basin were integrated and utilized to study the coupling relationship between overpressure system and formation water. The results show that the development of overpressure is closely related to the formation water: (1) the formation pressure varies from weak overpressure to super overpressure in Xujiahe Formation. T₃X² belongs to medium overpressure, while T₃X⁴ is super overpressure. The fluid salinity of exploration wells in T₃X² decreases with increments of pressure coefficient, which yields opposite relationship in T₃X⁴. (2) Fluid expulsion leads to "undercompaction" overpressure during compaction, resulting in the occurrence of fractures and pronounced mixture and migration of formation water. (3) The maturation of organic matter results in the increment of pressure and the composition difference between fluid inclusions and formation water due to extensive water-rock reaction. (4) The scarcity of fractures in T₃X⁴ results in the increment of formation pressure, while the condensate water and water invasion in T₃X² results from the development of fractures during uplift. Thus, T₃X² and T₃X⁴ display nonidentical characteristics of formation water.