Distribution, origin and evolution of overpressure in the Paleogene and Neogene in the Western Qaidam Basin, northwestern China

Abstract

Based on pressure test data, well logging data and geological conditions, the distribution and cause of overpressure in the western Qaidam Basin are analysed. The contribution of different overpressure causes is quantified, and the main controlling factors of overpressure at different evolution stages are further divided. This is useful for analysing the pressure state in different geological historical periods and indicating the direction of oil and gas migration. The research results show that the formation pressure coefficient in the western part of the Qaidam Basin is mainly in the range of 0.5–2.1, and the pressure coefficient generally decreases from the depression to the edge. According to the stress variation characteristics and logging response of overpressure, two models of acoustic travel time-vertical effective stress and electrical resistivity-vertical effective stress are established to identify and quantify the cause of overpressure for loading and unloading. Through the analysis of logging curves, acoustic velocity-density cross-plot and geological conditions, the causes of overpressure in western Qaidam Basin was clarified. The overpressure calculation results of different origins show that the main controlling factors of overpressure in the Kunbei fault stage are disequilibrium compaction and tectonic extrusion, with contribution rates of 38% and 52%, respectively. The overpressure in Mangya depression is caused by disequilibrium compaction, tectonic extrusion and hydrocarbon generation, with overpressure ratios of 30%, 32% and 38% respectively. The overpressure of the Dafengshan uplift can contribute up to 53% of the disequilibrium compaction, and the contributions of tectonic extrusion and hydrocarbon generation are 28% and 19%, respectively. Finally, the evolution of residual pressure in the upper segment of the Xiaganchaigou Formation (${E_3}^2$) in western Qaidam Basin can be divided into four evolution stages: undercompaction stage (42.8–40.5 MPa), normal compaction stage (40.5–12.0 MPa), hydrocarbon generation pressurization stage (12.0–2.8 Ma) and pressure release stage (2.8 Ma-present).