Pore Pressure Modeling and Lithologic Discrimination

The most common inadequacy I have observed when reviewing PPFG models is insufficient effort applied to the selection of shale intervals for input into the PPFG model.

The Basic Principle of Lithologic Discrimination for Pore Pressure Modeling

Pore pressure is assessed by analysis of “impermeable” rock formations subject to the effects of compaction due to increased burial.

“Impermeable” is a time-dependent term. As burial progresses so, too, does compaction. With increased compaction the porosity is reduced. As the porosity is reduced, the water in the pore spaces being reduced must move. (If water did not move out of the decreasing pore space the pore pressure would always be equal to the maximum stress, the overburden stress in an isotropic condition.) ?The flow rates of this movement are relatively very low, referenced to geologic time. Formations with flow rates referenced to clock time are considered “permeable.” Formations with flow rates so low it is impractical to reference to clock time are considered “impermeable.”

Permeable formations are not included in the PPFG model.

Selection of Lithology

Commonly pore pressure analysts use only the Gamma Ray log to discriminate lithology and select shale intervals for input into their models. Using only one log to discriminate lithology often causes invalid (permeable) depth intervals to be included in the model.

Using only the Gamma Ray log to determine shale intervals does not provide the analyst with sufficient information to determine lithology.

Figure 1.?Gamma Ray log by itself

Using the Gamma Ray log with the resistivity log yields more information about the formation to evaluate the lithology.

Figure 2.?Gamma Ray log with Resistivity log

Using the combination of Gamma Ray and Resistivity logs allows the PPFG analyst to more precisely discriminate lithologies and avoid erroneously including permeable strata in the PPFG model.