An improved technique to more accurately calculate pore pressure of sedimentary rock due to subsurface fluid expansion. The technique is built upon a combinations of known force balanced in situ loading limb mineralogical stress/strain relationships with locally variable unloading stress/strain relationships (S, P). In stress/strain space, the material properties governed loading limb is and upper limit for the many possible unloading limbs. A method for relating these different natural stress/strain paths and applying the correct path to calculate pore fluid pressure from in situ force balance is preferably calibrated with in situ stress/strain data which allows for a lithologic sealing caprock, shale (16) to be identified and the locally prevailing in situ unloading limb stress/strain relationship to be estimated. The forced balanced loading and unloading calibrations are applied to more accurately determine well casing depths while drilling petrophysical data. Loading and unloading power law linear stress/strain relationships are determined between effective stress and solidity for common sedimentary rocks.