Mechanical versus thermal cause of abnormally high pore pressures in shales

The formation of a seal to shale pore fluids is essential to both the thermal and the mechanical processes proposed for the generation of abnormally high pore-fluid pressures. The thermal hypothesis requires a more perfect seal than the mechanical. Inferred gradients of pore-fluid pressure with depth in shales are commonly greater than the overburden pressure gradient. Such gradients are unstable and can only be maintained with a perfect seal if the shale is also impermeable. Shales have measurable permeability. It is therefore concluded that these pore-pressure gradients are the result of pore-fluid flow and an imperfect seal, and that the thermal contribution is minor. This conclusion is important for the understanding of the petroleum geology of regressive sequences, which is the stratigraphic context of most abnormally pressured shales.