Quantitative Evaluation and Evolution of Overpressure in the Deep Layers of a Foreland Basin: Examples from the Lower Cretaceous Bashijiqike Formation in the Keshen Area, Kuqa Depression, Tarim Basin, China

The Kuqa area comprises a foreland basin located near the southern border of the South Tianshan Mountains which is considered as a major hydrocarbon-producing basin in NW China. The Keshen area is an important zone for hydrocarbon accumulation. The main oil-bearing reservoirs in the Keshen area are documented in the Bashijiqike Formation (Lower Cretaceous), located at depths ranging from 6000 to 8000 m, where overpressure (maximum up to 85 MPa) is prevalent. The origin of overpressure in the Bashijiqike Formation (Lower Cretaceous) includes mudstone disequilibrium compaction, tectonic compression, and fracture transfer overpressures. In this work, mathematical modeling is key to evaluate different types of overpressure quantitatively. The coupling evolution of different overpressures is also crucial. The results showed that the overpressures due to disequilibrium compaction, tectonic compression, and fracture transfer were 5–10, 25–30, and 10–15 MPa. The evolution characteristics of polygenic pressure are as follows: before 23.3 Ma, the overpressure was almost 0; from 23.3–10 Ma, the overpressure was mainly caused by disequilibrium compaction, and the residual pressure gradually increased to 18 MPa; and after 10 Ma, the overpressure was mainly caused by the combination of the disequilibrium compaction type of overpressure, tectonic extrusion type of overpressure, and fracture transfer overpressure. The residual pressure rapidly increased to 60 Mpa and then slowly released due to formation uplift. This research is of great significance for the quantitative evaluation of different origins of overpressure and the study of the evolution of multi-cause overpressure in deep layers of foreland basins. This research of deep oil and gas exploration provides the possibility to realize further sustainable oil.