Formation and mechanism of the abnormal pressure zone and its relation to oil and gas accumulations in the Eastern Jiuquan Basin, northwest China

Three abnormal overpressure zoes with a planar top at different depths occur in the Ying’er Depression in the Eastern Jiuquan Basin. The distance and the temperature difference between them are about 1 000 m and 30℃, respectively. The studies of sedimentary history, nature of formation water, variation of geothermal gradient and examination of thin sections, and the relationship between lithologic section and formation pressure show that there are conditions for formation of abnormal overpressure zones in the Ying’er Depression. Aquathermal pressuring and the overlying sediment load are main factors forming the abnormal overpressure zones. The study indicates that most of oil and gas in the Ying’er Depression accumulated in reservoirs above or under the seals or in the top of the compartments.