Tectonic controls on Permian tight gas accumulation: Constrains from fluid inclusion and paleo-structure reconstruction in the Hangjinqi area, northern Ordos Basin, China

The development of tight gas reservoirs within the Ordos basin is closely related to the tectonic evolution. In this study, homogenization temperature of aqueous inclusions and the burial and thermal history of single well were combined to determine the tight gas charging time in the Permian Lower Shihezi sandstones in the Hangjinqi area, northern Ordos Basin. The paleo-structural characteristics and tectonic evolution, and their controls on source rock, gas migration and accumulation were discussed based on detailed 3D seismic interpretation. Results show that the natural gas was charged during the Middle Jurassic to the Early Cretaceous (170-140 Ma) in the southern part of the Porjianghaizi fault, while in the northern part, the gas charging time was the Early Oligocene to the Late Miocene (31-8 Ma). Paleo-topography in the Late Paleozoic controlled the coal beds thickness with generally decreasing trend to northward. Different burial depths led to the variation of thermal evolution, hydrocarbon generation time and intensity of source rocks between the northern and southern part. The results of this study indicate that the Hangjinqi area was evolved into a tectonic pattern with higher elevation in the south than the north in the Late Jurassic. The gas reservoirs in the southern part were mainly distributed in the paleo-slopes and paleo-highs, where were favorable areas for the gas accumulating in the Late Jurassic. Afterwards, the study area began to uplift since the late Early Cretaceous and made the Hangjinqi area a southwest inclined monocline. The rapid tectonic uplift event since the Early Oligocene in the northern Ordos Basin destroyed and reallocated the primary gas reservoirs in the southern part and prompted the gas migrating and accumulating into the structural traps in the northern part. The variation of natural gas compositions in both northern and southern parts verified the gas migration process.