Changes

===Detection of overpressure with well logs===

===Detection of overpressure with well logs===

Undercompacted shales associated with overpressured zones have a much lower electrical resistivity than normally compacted shales ([[:file:pressure-detection_fig3.png|Figure 3]]). According to the Archie formula, doubling the porosity of a shale from 10 to 20% should cause its resistivity to drop to one-fourth. As a result, it is possible to determine accurately the degree of undercompaction of a shale from its resistivity and to estimate the pore pressure ([[:file:pressure-detection_fig4.png|Figure 4]]) [[(Hottman and Johnson,1965)]]{{Citation needed}}.

Undercompacted shales associated with overpressured zones have a much lower electrical resistivity than normally compacted shales ([[:file:pressure-detection_fig3.png|Figure 3]]). According to the [[Archie equation|Archie formula]], doubling the porosity of a shale from 10 to 20% should cause its resistivity to drop to one-fourth. As a result, it is possible to determine accurately the degree of undercompaction of a shale from its resistivity and to estimate the pore pressure ([[:file:pressure-detection_fig4.png|Figure 4]]) [[(Hottman and Johnson,1965)]]{{Citation needed}}.

Because undercompacted shale has slow seismic velocity and low density, a high pressure zone can also be identified from sonic and density logs.<ref name=pt03r33>Magara, K., 1978, Compaction and fluid migration: New York, Elsevier Scientific Publishing Company, 319 p.</ref>

Because undercompacted shale has slow seismic velocity and low density, a high pressure zone can also be identified from sonic and density logs.<ref name=pt03r33>Magara, K., 1978, Compaction and fluid migration: New York, Elsevier Scientific Publishing Company, 319 p.</ref>