Effect of tectonic evolution on hydrocarbon charging time: A case study from Lower Shihezi Formation (Guadalupian), the Hangjinqi area, northern Ordos, China

A giant gas field has been discovered in the Lower Shihezi Formation in the Hangjinqi area, northern Ordos Basin, China. Coal seams and mudstone in the Taiyuan and Shanxi Formation are considered as source rocks and sandstone in the Lower Shihezi Formation is treated as reservoirs for gas. The charging time in the sandstone reservoirs was studied based on the homogenization temperature of aqueous inclusions, thermal and burial history simulation and K-Ar dating of authigenic illite. Two episodes of gas charging in the northern part and one episode in the southern part were delineated in the reservoirs of the Hangjinqi area, evidenced by microscopic observation of the occurrence of fluid inclusions and the analysis of salinity in aqueous inclusions. In the southern part of the study area gas charging occurred between circa 171-162 Ma, whereas in the northern part gas charging was between 158-144 Ma and 120-109 Ma, respectively. The gas charging events appeared to be influenced by the tectonic evolution of northern Ordos Basin. Paleo topography caused reduction in thickness of source rocks northwards, and contributed to differences in hydrocarbon generation potential and thermal evolution between the northern and the southern part of the study area. Therefore, gas charging occurred in the southern part prior to that in the northern part. Furthermore, tilting occurred in Middle Jurassic and provided the driver and pathways for gas, leading to a first charging event in the northern part of the study area.