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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 64 (1980)

Issue: 12. (December)

First Page: 2179

Last Page: 2183

Title: Mechanical Versus Thermal Cause of Abnormally High Pore Pressures in Shales: GEOLOGIC NOTES

Author(s): Richard E. Chapman (2)


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, w ich is the stratigraphic context of most abnormally pressured shales.

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