The development features and formation mechanisms of abnormal high formation pressure in southern Junggar region

Abnormal high formation pressure is an important component of fluid dynamics in petroliferous basins, and relates closely with origin, migration and accumulation of hydrocarbon, and drilling engineering as well. According to data of measured pressure in reservoir bed, vertical distribution of calculated pressure in mudstone, and plans of estimated overpressure in different strata, it is analyzed that the distribution features of abnormal high formation pressure in sandstone and mudstone in the southern Junggar basin, and the cause mechanisms of overpressure mainly by contrast of synthetical compaction curves. It is detected that the overpressure developed well in sandstones in sag, and the distribution of pressure in mudstone is either similar with measured pressure in sandstone, or is smaller than the latter, the abnormal high pressures in sandstones is not completed aroused by under-compaction in mudstone. By contrast of synthetical compaction curves in mudstone, it is affirmed whether or not the overpressure in different wells is caused by compaction. It is summed up that cause of abnormal formation pressure include factors of disequilibrium between compaction and expulsion of water in mudstone, tectonic compression, bearing pressure by high fluid potential, vertically hydrodynamic run-through by fault, etc, and their actions are of distinction respectively among different areas. It is generalized that the formation of abnormal pressure in western area is caused mainly by undercompaction, and in eastern area is from tectonic compression, hydro-dynamic transmission throngh faults, and so on.