AN EASILY USED MATHEMATICAL MODEL OF POROSITY CHANGE WITH DEPTH AND GEOLOGIC TIME IN DEEP SHALE COMPACTION

Abstract

Shale compaction behavior is complex and differs from sandstone. The compaction trend shows
marked changes in the physical properties with increasing depth. Therefore, the porosity-depth curve in the
shallow section is not well established. Several authors published porosity-depth curves in which vary
considerably, particularly in the shallow part of the compaction trend. Deep shale compaction curves show
deviations in the trend of porosity decreasing with depth, especially at shallow depths where mechanical
compaction is dominant and this suggests that mechanical compaction is not the only factor causing porosity
change and that geological time is another important factor to be considered. In this study twenty-two sets of
published data on deep shale compaction up to a depth of 5 km from various sedimentary basins around the
world were synthesized and analyzed to depict the effect of geological age on deep shale compaction curves.
A time-dependent solution of the compaction equation with porosity as the variable was revisited, based on
which and in combination with multivariate curve-fitting of the collected compaction data, a new and easilyused mathematical model of porosity-depth-geological time was derived. Finally, research results substantiate
that a burial depth (overburden pressure) cannot be considered only for studying mechanical compaction of
shales, the geological age (an effect of time) is necessary to take into account.