Characteristics of Underpressured Gas Pool in Upper Paleozoic Shiqianfeng Formation of Eastern Ordos Basin

The gas source in Ordos basin mainly comes from the alternate marine-continental coal-bearing measures of Shanxi and Taiyuan formations. The Upper Shihezi Formation with a vast thick mudstone layer can be taken as a very good seal, so it is difficult for natural gas to pass through. Recently, some secondary gas pools have been found in Shiqianfeng Formation lying above the Upper Shihezi Formation in eastern Ordos basin. In this paper, according to the properties of secondary gas pools and the inclusion data, the secondary gas pools are thought to be formed due to the leakage of the original gas pools penetrating through the regional caprock when the overpressures within lower overpressured strata release. The forming process of gas reservoirs includes three stages, that is, the formation of the primary gas pools with high temperature and high pressure, the leakage of the primary gas pools and the formation of the secondary gas pools, and late adjustment and maintenance of the secondary pools.