CAUSES OF SUBNORMAL PRESSURE IN THE SEDIMENTARY BASINS

The forming mechanisms of subnormal pressure can be divided into 4 types: 1)increase in pore volume caused by tectonic uplift and formation erosion, or chemical effects of pore water; 2)fluid flowing out due to the release of overpressure in seal formation, unhomogeneous flow, osmosis across semipermeable shale membranes and diffusion of light hydrocarbon; 3)deep burial of saturated gas reservoir; 4)piezometric surface lower than ground surface. In some sedimentary basins in which large-scale tectonic uplift has taken place, the main cause of low pressure may be the removal of overburden, which results in dilation of pore volume with the consequent reduction in stress and temperature. Subnormal pressure in the small-scale uplift area is mostly related to the diffusion of light hydrocarbon and the burial of saturated gas reservoir. The release of overpressure seems to be a probable mechanism; however, it is a question how low pressure can be preserved. The chemical effect and osmosis effect of pore water are self-limited, which may not be the major factors in subnormal pressure system. Aside from unhomogeneous flow, other mechanisms require good seal, otherwise pressure will be equalized to normal hydrostatic. The low pressure is not a true meaning subnormal pressure, because that the piezometric surface is lower than the ground surface, but people give a wrong assumption in the course of showing the relative size of pressure.