Overpressure Distribution and Affecting Factors in Southern Margin of Junggar Basin

As a part of Junggar basin, the southern margin region has received the sediments from Permian to Quaternary period. The tectonic activities, occurring intensively in the area during recent million years, cause strange tectonic stress in SN direction and result in a series of anticlines accompanied with different scales of faults. Very high overpressures, the largest one with the pressure coefficient of 2.4, are observed in the area. Pressures measured with DST/RFT methods and those estimated from acoustic logging data are used in this paper to characterize overpressure distribution in the basin. Pressuring effect of different geological factors in permeable sandy and sealing shaly formations are discussed. It indicated that the occurrence and distribution of overpressures in the southern margin have been seriously influenced by many geological factors, such as deposition facies, the tectonic stress action, deformation of sediments, as well as fractures occurrences and so on. Synthetically, the extremely high pressures encountered in the research area result from the effect of compaction caused by both the gravity and the tectonic stress, in combination with faulting that makes hydraulic connection among permeable formations, and with folding that makes the permeable formations incline.