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Qaidam basin is a Mesozoic and Cenozoic inland oil basin where the abnormal formation pressure is as developed as in many offshore regions.
The abnormal formation pressures are found at depths below 3,200 m in relatively depressed regions. Their pressure gradients are from 0.14 to 0.22 atm/m.
The causes of abnormal formation pressure in this region are as follows.
1. Compaction unequilibrium is the principal cause of high pressure. Abnormal pressure occurs not only in the zone of fine-grained rocks, but also in the interbedded strata of sandstone and shales as long as the rate of deposition is rapid enough at the time when abnormal pressure is produced.
2. In this region, an aquathermal pressure effect is also apparent; the geotemperature gradient is 35°C/km, and the actual average rate of pore pressure increase since isolation is 0.31 atm/m, significantly higher than 0.23 atm/m of the average overburden pressure gradient.
3. Because there is not any montmorillonite content in some sequences developing abnormal pressure, montmorillonite-illite conversion and dehydration are not prerequisites for the abnormal pressure.
In addition, growth faulting of some regions and the volume expansion produced by thermal degradation of organic matter may also enhance the formation pressure.
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