A Compaction-Based Pore Pressure Model for Shales

Shales lose porosity through compaction. Compaction is the loss of porosity with increasing effective stress, increasing temperature, and diagenesis (smectite to illite). These specific types of compaction are mechanical, thermal, and chemical, respectively. Desorption isotherms of pure clays, clay mixtures, and shales illustrate the variables and the compaction types that ultimately control shale porosity. These isotherms are simply a measure of the mass of water per gram of dry clay (converted to porosity using the appropriate grain density), as relative humidity (p/po) is varied between 0 and 100%. These controlling variables are: CEC (cation exchange capacity), the specific exchangeable cation (Na is believed to be the most dominate species in the subsurface), effective stress, and temperature. Isotherms show that the amount of water per gram of dry clay increases with increasing CEC values for a given p/po. A plot of p/po vs. mass of water normalized by meq (a unit of CEC measurement) results in a general mechanical compaction curve when a thermodynamic relationship is used to convert p/po to effective overburden stress. This calculated effective overburden stress is at room temperature, the temperature of the normalized isotherm trend. The technique of normalizing water content per meq also allows for identifying the temperature effects of thermal compaction via another thermodynamic equation. This permits the mechanical compaction curve to be corrected for thermal compaction with the application of the insitu geothermal gradient. The effects of diagenesis (i.e. smectite to illite) and variable mineralogy are incorporated in the compaction model via the bulk CEC parameter, calculated from well logs using an adapted published algorithm. This thermodynamically-based, geologic compaction model allows density log derived porosity to be used to properly isolate the mechanical compaction component and hence quantify pore pressure.