As sediments are buried to greater and greater depth, the weight of the overlying rocks increases and the increasing stress acting at the grain contacts leads to rearrangement of the grains, resulting in lower porosity. If the rate of sedimentation exceeds the rate at which fluid can be expelled from the pore space, or if dewatering is inhibited by the formation of seals during burial, the pore fluid becomes overpressured and thus supports part of the overburden load. Overpressure generated in this way is said to result from disequilibrium compaction or undercompaction, this being the most common mechanism for generating overpressure in deepwater sediments.
Log based methodologies for pore pressure estimation are only relevant where overpressure has developed through disequilibrium compaction in shale/clay sequences. Compaction represents a reduction in porosity with increasing depth, and will produce a linear relationship on a logarithmic plot of porosity versus depth. Under normal compaction, porosity is reduced at the same time as pore fluid is expelled.
The workflow to perform log based pore pressure analysis includes: determination of the overburden gradient (OBG), discrimination of shale intervals, definition of a normal compaction trend to allow identification of compaction anomalies, pore pressure analysis calibrated where possible to measured data or well response, fracture gradient (FG) analysis.
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