Pore pressure prediction in shales is quantified by relating the porosity, using wireline data as a proxy, to the vertical effective stress. However, the pore pressure predicted in shales may be in error because of variation in both the clay content and organic content. For example, TOC creates an apparent variation in porosity which can lead to erroneous pressure predictions. The critical impact of this study is that variation in TOC will lead to non-unique solutions for the vertical effective stress as a function of density and as TOC tends to soften/slow the elastic response this will lead to elevated pore pressure predictions that may not be valid.

To illustrate the workflow, a case study from an immature section of Kimmeridge Clay Formation in the North Sea will be presented. A rock physics model linking TOC to bulk density is used to generate a corrected bulk density log. The corrected bulk density will then be transformed into compressional velocity using a clay-dependent rock physics model as velocity data remain the most common data type for pore pressure prediction. The final results are more realistic pore pressure prediction magnitudes.