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Abnormal formation pressure requires a seal; without a seal pressures would equalize to normal hydrostatic. Abnormal pressures originate from several interrelated processes, but temperature change appears to be the principal cause.
Both epeirogenic movements with associated erosion and deposition and long-term changes in climate can alter the temperature of a sealed formation at depth. Abnormal pressure resulting from temperature change caused by change of overburden thickness must be corrected for inherited pressure and change of hydrostatic pressure related to elevation difference. Osmosis, precipitation, or solution by trapped pore fluid and carbonization effects are minor in comparison with temperature effects.
Overburden stress cannot cause abnormally high pressure at present drilling depths. The loss of porosity with depth in all sedimentary rocks appears to be a chemical process rather than mechanical compression.
Pressure differentials between wells may indicate ambiguously either no fluid flow (wherein the pressure difference is maintained by a seal) or flow (wherein the pressure drop is from fluid friction in the permeable medium). A relatively small amount of flow across a seal can equalize pressures, retarding further flow. The fluid expelled as a result of loss of porosity during geologic time also flows at a low rate.
The geologist concerned with pressure problems must be aware of (1) the many variables involved in subsurface pressures, (2) the low number and ambiguity of pressure measurements, (3) the need to establish what constitutes "normal" pressure to determine abnormal pressure, and (4) the possibility of uniqueness in any field situation.
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