Origin mechanism of overpressure in saline lacustrine formation of the Paleogene and Neogene in the Western Qaidam Basin, NW China

Abstract

Previous research shows that the origin and distribution of formation overpressure are governed by several dominant factors including disequilibrium compaction, hydrocarbon generation, tectonic compression and diagenesis, influenced by salt components and their concentration, but it is unclear how salts affect formation overpressure in saline lacustrine basins. This paper investigated the effect of salts on formation overpressure based on organic geochemistry, rock mineralogy, logging curve comparison and wave velocity-density cross-plot by combining the sedimentary and structural background. In the Western Qaidam Basin, the proportion of abnormally high pressures rises from the Neogene to the Paleogene. The top surface of the overpressure is between 2300 and 2500 m deep. As the subsidence and sedimentary centers of the basin moved eastward, the centers of the overpressure migrated from west to east. In the Upper Oligocene, the overpressure is developed in the deep and semi-deep lacustrine facies, and the pressure coefficient is 1.8–2.0. Disequilibrium compaction is the primary control factor with a contribution rate of more than 60% in the intersalt and subsalt strata, followed by tectonic compression with a contribution rate of 20–30%. Fracture reducing by salt filling and fluid volume expanding by gypsum dehydration increase the fluid volume in the formation, which promotes formation overpressure. The gypsum salt rocks also have strong plasticity and sealing effect, thus providing a closed environment for the formation overpressure. Through providing the primary migration driven force and sealing conditions for oil and gas, the overpressure is meaningful to petroleum accumulation and preservation in saline lacustrine formation of the Paleogene and Neogene in the Western Qaidam Basin, NW China.