Abstract:

Evolution of abnormal pressure was quantitatively determined by basin numerical simulation system, based on the analyses of current reservoir pressure, disequilibrium compaction and inclusion paleo-pressure of the Upper Paleozoic in Hangjinqi area, Ordos Basin. The study shows that current pressure in the reservoir is of under-pressure, whose coefficient decreases gradually from south to north. The target zone is the deepest by the end Early Cretaceous, meanwhile, the maximum over-pressuring of mudstone is concentrated in the lower Upper Shihezi Formation to the third member of lower Shihezi Formation. The maximum over-pressuring in mudstone varies in different zones. The maximum over-pressuring is of 15 to 20 MPa (locally >20 MPa) in west of Hangjinqi, medium over-pressuring occurs in the east between 10 MPa and 15 MPa, while the minimum over-pressuring (5 to 10 MPa) occurs in the north. The numerical simulation results show that the evolution of abnormal pressure is syn-tectonic. Residual pressure (RP) of the Upper Shihezi Formation began to form in the end Triassic under the Indosinian orogenic influence with a maximum of >10 MPa. During the Middle Jurassic, the RP decreased slightly under the influence of the early Yanshanian Movement. During the Cretaceous, the RP was mostly affected by late Yanshanian Movement with >20 MPa. Subsequently, the RP decreased rapidly with the structural uplift and denudation, and abnormally low pressures in the current reservoirs and partly residual pressures in caprocks were formed. The main factors for the formation of under-pressuring in Hangjinqi area are the gas diffusion, temperature drop and tectonic unloading caused by uplifting and denudation. The paleo-pressure genesis varies between different source rocks, caprocks and reservoirs. Generation of organic materials in the source rocks is the main factor for the RP formation, while disequilibrium compaction plays an important role in caprocks. The configuration of sand and mudstone is essential in the paleo-pressure formation in reservoirs. The third member of the Xiashihezi Formation is mainly composed of sandstone surrounded by thick mudstone, which is greatly affected by disequilibrium compaction. The lower part is composed by mudstone-sandstone interbeds or mudstone surrounded by thick sandstone. Effect of the disequilibrium compaction is weakened or even disappeared, and structural extrusion is a dominant control for reservoir paleo-pressure.

Key words: disequilibrium compaction, residual pressure, basin numerical modeling, abnormal pressure evolution, Upper Paleozoic