General Ramifications of Abnormal Pore Pressures
Abnormally high pore pressures imply abnormally low effective or matrix stresses. Yet at the same time at least according to current experience high pore pressures are usually associated with abnormally high porosities. Thus some of the effects described in chapter 3 are a consequence of the higher than normal porosity only, whereas others must be ascribed to the abnormally low matrix stress and some are simply caused by a combination of both.
One may be tempted to ignore the effect of the low matrix stress since its effects are most pronounced at very low stress levels. However, in many parts of the world pore pressures approaching total overburden pressure have been measured, resulting in very low matrix stresses even at considerable depth. In many cases low matrix stress and high porosity affect a physical property in the same manner. Thus both tend to lower the sonic velocity. In the literature often little distinction is made between the two causes for abnormal behaviour of overpressured materials.
However, this dependence of certain physical properties on a number of factors which are likely to change simultaneously has already been recognized by Hottman and Johnson (1965). In particular the electrical resistivity is not only porosity dependent but also varies with temperature, salinity and clay content (Hottman & Johnson,1965,p. 721, table 2). Fertl (1976,p. 195) shows that at least in same cases the transition from hydro- to geopressures is accopanied by an abrupt decrease in the formation water salinity. The relationships