Pore pressure prediction in organic shales

Abstract

Organic shales, defined as mudrocks with total organic carbon (TOC) exceeding 1.5%, are prone to overpressure generation when buried well into the oil window temperature/depth range because the original solid organic matter is characterized by density greater than the liquid hydrocarbons produced from it during their thermal maturation and oil generation. This process of overpressure generation results in a significant reduction of the mean effective stress the rock has been subject to prior to its maturation, commonly referred to as unloading. We combine the Vernik shale normal compaction model with the Vernik-Kachanov rock-physics model adapted for organic shales to invert for the vertical effective stress. When combined with the total vertical principal stress, this yields a reasonable pore pressure estimate wherever the accurate information about the sonic (seismic) velocity, mineralogy, and TOC content is available. Although similar in nature to the empirical Bowers unloading-related pore pressure prediction model, our approach is based on the solid rock-physics understanding of the organic shales and their velocity controls. The model presented was tested against measured pore pressure data available in the Marcellus Shale.