Notes on the origin of formation fluid pressure: well-logging methods aspect

Current methods of pressure determination using well-log data are mostly based on porosity vs. pressure correlation and on the concept of pressure dependence on compaction processes due to overburden weight. However, natural compaction is a complex process, and many geologic and physical phenomena and processes other than compaction are known to generate pressure abnormalities. For these cases, pressure—porosity correlation may not be definitive, other parameters may correlate better, or no correlation may exist. For example in basins with no compaction or any other geologic activity, distribution of pressure deviations from hydrostatic pressure is solely determined by surface relief and by the 3-D distribution of water density and rock permeability. If a consolidated formation has fracture permeability and vertical fluid migration is a mechanism of abnormal pressure generation, usually no correlation exists between the vertical distribution of pressure and rock properties. normal and subnormal pressure scenarios should be differentiated. Criteria to recognize each case and sets of parameters for indirect determination and/or prediction of formation pressure should be developed separately for each scenario. This will allow the maximum use of information pertinent to pressure distribution in each particular region or field and, therefore, more accurate pressure determination and prediction. In this paper, we formulate this problem and describe ways to develop new accurate methods of pressure determination using well-logs with other geologic information.