ABNORMAL FORMATION PRESSURE OF QAIDAM BASIN AND DISCUSSION ON ITS CAUSE

Qaidam basin is the Mesozoic and Cenozoic inland basin, where the Tertiary strata of developing abnormal formation pressure is mainly a suit of lake facies deposits of semi-alkaline water-alkaline water under dry climate. During the Ter-tiary period, this basin was an area of continuous deposition in a wide range, in the center of which shore-deep lake facies was developed, and the maximum area of lake basin was 50000 km}. "1'he rate of deposition generally was about 0.1-0.2mm/yr, the maximum being 0.56mm/yr.The abnormal formation pressures are found in、arious areas of relatively depressed region, In these areas the Pressure gradient increases with the depth.Vertically the formation pressure may be divided into three zones the normal pressure zone, the transitional zone and the abnormal pressure zone.The abnormal pressure is found below 3200 m depth, and its pressure gradients are from 0.14 to 0.22atm/m. With regard to the cause of abnormal formation pressure of this region. the authors' ideas are as follows:(1)Compaction unequilibrium caused by lack of permeable bed and rapid loading:In the process of sedimentation, the gravity load causes sediments to compact and lose porosity. The porosity reduces rapidly with the depth during the early stage of compaction and then retards. Until the porosity of argillaceous or Shales reaches about 6%,the expulsion of the pore fluid is resisted and the sediments become overpressured.On lithology,abnormal pressure primarily occurs in the gone consisting chiefly of fine-grained rocks and its upper half part rich in marl in No. 1 Yuejin area.In the interbedded strata of sandstone and shale, the excess pressure may also occur as long as the overlying sediments have a low permeability and the rate of deposition is rapid enough when the abnormal pressure is produced.(2)The effect of temperature:Within this basin, the actual average rate of pore-pressure increase since isolation is 0.31 arm/m, significantly higher than 0.23 atm/m of the average overburden-pressure gradient. This shows obviously that there is the aquathermal pressure effect. Owing to the incompletion of the isolation,the escape of pore water may occur, and the actual amount of pressure increases since isolation is greatly lower than the value calculated according to the temperature pressure-density diagram by Barker.(3)The montmorillonite-illite conversion and dehydration are not the prerequisites for the cause of abnormal pressure:.}16normal pressure, even super pressure, occurs below the depth of 4000m of Han 2 Well, but in the sequence there is not any montmorillonite content from shallow to deep. In contrast with this,some montmorillonites occur above 2400 m depth of Yue 110 well, where the abnormal pressure can also be found. The most abundant content of montmorillonite in this sequence, however, is only 20%,so the volume increase and pressure increase of pore fluid caused by dehydration are limited. These facts have shown that the dehydration of montmorillonites is nei ther a prerequisite nor an important factor in the generation of abnormal pressure.(4)High pressure zone is not always characterized by undercompaction in the shales. This might be caused by the fact that the match relation of lithological characters of abnormal pressure zone and its overlying strata is different from that of the general case.(5)The comparison of depth and age of the strata with abnormal formation pressure between No. 1 Yuejin area and Han 2 Well indicates that the compaction degree of pelitic rocks is affected by the age. At the same depth, the older the strata age is, the greater the compaction will be.In addition, the possibility can not be ruled out that the growth fault may cause the isolation in lateral for abnormal pressure zone, and the volume expansion produced by thermal-degradation of organic matter after the source rock entering the threshold depth of oil may also enhance the formation pressure.