In the last decades, exploration for new and additional oil and gas reserves has greatly expanded laterally, both on- and off-shore, and at the same time has also greatly progressed downward to investigate the hydrocarbon potential of deep and superdeep targets. In these activities, abnormal formation pressures; i.e., pressures other than hydrostatic have been encountered on all but one continent. The present discussion attempts to compile information on the worldwide occurrence of abnormal formation pressures, which are equally important in the pressures, which are equally important in the search for and drilling and production of oil and gas.
In the worldwide search for oil and gas, formation pressures other than hydrostatic have been encountered on all but one continent (Figure 1). Such abnormal pressures, defined by departure from hydrostatic at any given depth, may be excessively high or low, the possible modes and causes of origin being numerous.
The present discussion, unfortunately, is limited by (1) data available to the author or (2) their proprietary nature (North Sea, South China Sea, Arctic Islands). Since each country had to be treated rather briefly, selected references will assist in pursuing the subject further. All pressure pursuing the subject further. All pressure data shown in graphical rather than tabular form. Each graph includes three trend lines representing the overburden gradient of 1.0 psi/ft, which corresponds to the weight of the overlying rock series (i.e., weight of both the rock matrix and fluids) with an average density of 2.30 gm/cc, (2) the hydrostatic pressure gradient of 0.465 psi/ ft, which corresponds to the average hydrostatic gradient as found in the U.S. Gulf Coast area, and (3) the hydrostatic gradient of 0.433 psi/ft, representative of the domestic Mid-continent area.
