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Line 98: − − [[file:pressure-detection_fig4.png|thumb|{{figure number|4}}Shale resistivity parameter, resistivity of normally pressured shale divided by observed resistivity of abnormally pressured shale, plotted against formation pressure gradient (FPG) and equivalent mud weight.<ref name=pt03r24>Hottman, C. E., Johnson, R. K., 1965, Estimation of formation pressures from log-derived shale properties: Journal of Petroleum Technology, v. 17, p. 717–723., 10., 2118/1110-PA</ref>; by permission of SPE]]
→Delayed indications
* ''Shale density''—Undercompacted shales, characteristic of overpressured zones, have a lower density (because of abnormally high [[porosity]]) than normal shales at a given depth. The density of shale cuttings can be measured by several methods. Also, the shape of drill cuttings from undercompacted shales may be different than those from normally compacted shales.
* ''Shale density''—Undercompacted shales, characteristic of overpressured zones, have a lower density (because of abnormally high [[porosity]]) than normal shales at a given depth. The density of shale cuttings can be measured by several methods. Also, the shape of drill cuttings from undercompacted shales may be different than those from normally compacted shales.
* ''Temperature''—There may be an increase in the temperature of the mud returns. Although it has been widely claimed that the geothermal gradient is higher in overpressured shales because of their abnormally high porosity and lower thermal conductivity, a doubling of shale porosity from 10 to 20% should cause a decrease in conductivity of only about 1% (with a correspondingly small increase in geothermal gradient). Thus, the increase in temperature is probably due to faster drilling and increased cavings in undercompacted shales.
* ''Temperature''—There may be an increase in the temperature of the mud returns. Although it has been widely claimed that the geothermal gradient is higher in overpressured shales because of their abnormally high porosity and lower thermal conductivity, a doubling of shale porosity from 10 to 20% should cause a decrease in conductivity of only about 1% (with a correspondingly small increase in geothermal gradient). Thus, the increase in temperature is probably due to faster drilling and increased cavings in undercompacted shales.
===Detection of overpressure with well logs===
===Detection of overpressure with well logs===