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  • Mechanisms of overpressureMechanisms of overpressure

  • Disequilibrium compactionDisequilibrium compaction Ongoing sedimentation increases overburden (vertical stress) faster than �uid di�uses out of zone

  • Characteristic time of di�usion in porous mediumCharacteristic time of di�usion in porous medium

    τ = (ϕ + )ηβf βr l



    low-permiability sand (~1 md) on the order of years for kmτ l = 0.1

    low-permiability shale (~10 nd) on the order of 100,000 years for kmτ l = 0.1

  • Common in Gulf of MexicoCommon in Gulf of Mexico

    © John Wiley & Sons, Inc. Gordon and Flemmings, 1998

  • Techtonic compactionTechtonic compaction

    Occurs in areas where large-scale tectonic stress changes occur over geolocgically short periods of time.

  • Hydrocarbon column heightsHydrocarbon column heights

    Image Source

  • Hydrocarbon column heightsHydrocarbon column heights

    © Cambridge University Press (Fig. 2.11, pp. 42)Zoback, Reservoir Geomechanics

  • Centroid e�ectsCentroid e�ects

    © Cambridge University Press (Fig. 2.12, pp. 43)Zoback, Reservoir Geomechanics

  • Other Other mechanismsmechanisms

    Aquathermal pressurizationAquathermal pressurization Temperature increases due to radioactive decay and upward heat �ow from mantle

    Hydrocarbon generationHydrocarbon generation From thermal maturation of kerogen

  • Direct measurement of pore pressureDirect measurement of pore pressure Via wireline samplers that isolate formation pressure from annular pressure in a small area at the wellbore wall. Mud weight

  • Estimation of pore pressure at depthEstimation of pore pressure at depth

  • Con�ned compaction experimentCon�ned compaction experiment

    © Cambridge University Press (Fig. 2.13, pp. 46)Zoback, Reservoir Geomechanics

  • Shale compaction relationShale compaction relation

    ϕ = ϕ0e −β( − )Sv Pp

  • Use with caution!Use with caution!

    © Cambridge University Press (Fig. 2.14, pp. 48)Zoback, Reservoir Geomechanics

  • Porosity inference from Porosity inference from -waves-waves

    Compaction -wave Pore pressures

    © Cambridge University Press (Figs. 2.16a,b, pp. 48, Fig. 2.8b, pp. 36)


    = + ( ln( )) ϕ = 1 −Pp Sv 1β ϕϕ 0

    ( ) Δtma Δt




    Zoback, Reservoir Geomechanics