Limitations on Disequilibrium Compaction as a Mechanism of Overpressure Generation

When thick mudrock sequences undergo rapid burial, overpressure is generated because pore fluid cannot escape rapidly enough through the low-permeability overburden for the pore pressure to remain hydrostatic. This mechanism of overpressure generation is known as disequilibrium compaction. It is a common cause of overpressure generation in sedimentary basins, but new data and recent advances in understanding have shown that it would be more accurate to think of disequilibrium compaction as a common cause of overpressures only down to depths of 2–3 km. At greater depths, disequilibrium compaction does not occur because diagenesis causes the sediment to become overconsolidated. Methods of pore pressure estimation based on seismic data tend to assume implicitly that overpressure is due to disequilibrium compaction, with high porosities being retained in mudrocks and consequent low sonic/seismic velocities. In fact, porosities are in the chemical compaction regime are independent of overpressure until extreme overpressures are reached, yet fortuitously sonic velocity in mudrocks is reduced by unloading mechanisms of overpressure generation, and presumably seismic velocities are similarly affected. Nevertheless, it is still challenging to use sonic or seismic data to quantify the amount of overpressure that is present because of uncertainty in velocity-effective stress relationships.