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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 59 (1975)

Issue: 6. (June)

First Page: 957

Last Page: 973

Title: Abnormal Formation Previous HitPressureNext Hit

Author(s): John S. Bradley (2)

Abstract:

Abnormal formation Previous HitpressureNext Hit 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 Previous HitpressureNext Hit resulting from temperature change caused by change of overburden thickness must be corrected for inherited Previous HitpressureNext Hit and change of hydrostatic Previous HitpressureNext Hit 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 Previous HitpressureNext Hit at present drilling depths. The loss of porosity with depth in all sedimentary rocks appears to be a chemical process rather than mechanical compression.

Previous HitPressureNext Hit differentials between wells may indicate ambiguously either no fluid flow (wherein the Previous HitpressureNext Hit difference is maintained by a seal) or flow (wherein the Previous HitpressureNext Hit 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 Previous HitpressureNext Hit problems must be aware of (1) the many variables involved in subsurface pressures, (2) the low number and ambiguity of Previous HitpressureNext Hit measurements, (3) the need to establish what constitutes "normal" Previous HitpressureNext Hit to determine abnormal Previous HitpressureTop, and (4) the possibility of uniqueness in any field situation.

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