Undercompaction: Pressure from Burial

Burial Too Fast to Drain

Most of the world's overpressure is made one way: disequilibrium compaction, or undercompaction. Bury mud normally and it compacts, grains packing closer, water squeezing out, porosity falling along a smooth trend that Athy captured in 1930 as an exponential, $\phi = \phi_0\,e^{-cz}$. That orderly compaction depends on the water having somewhere to go. Bury the mud faster than it can drain, pile sediment on a low-permeability shale quicker than the pore water can escape upward, and the water gets trapped. Trapped water cannot leave, so it takes up the new overburden load itself, and its pressure climbs above hydrostatic. The rock stops compacting because the fluid is holding the grains apart: the effective stress freezes at whatever value it had reached when drainage failed.

Drive the burial in the figure. At low sedimentation rate, or high shale permeability, the porosity follows Athy's normal trend and pressure stays hydrostatic. Crank the sedimentation rate up or the permeability down and the porosity curve departs the trend, holding anomalously high porosity at depth while the pore pressure ramps toward lithostatic. That departure is the signature the next sections exploit: a shale carrying more porosity than its depth warrants is a shale whose effective stress froze, and effective stress is exactly what velocity and density respond to.

Why the Porosity Anomaly Is Readable

The reason undercompaction is predictable, not just explicable, is that retained porosity leaves fingerprints on every log. High porosity means low bulk density and, crucially, low velocity, because the frame never stiffened. So an interval where the sonic slows down, or the velocity picked from seismic drops below its compaction trend, is announcing that its effective stress is lower than its depth implies, and lower effective stress at fixed overburden means higher pore pressure. This is the physical chain that Eaton's and Bowers' methods, two sections on, turn into a pore-pressure number: velocity below trend equals effective stress below trend equals overpressure. Undercompaction is the mechanism that makes velocity a pressure gauge, and it works because the same effective stress that Part 1 defined controls both the compaction and the sound speed. One caution the course will honor: velocity below trend can also mean unloading, a different mechanism the Bowers section handles, so the chain has a branch. But for the ordinary overpressure of rapidly buried basins, undercompaction is the story, and porosity is its ink.

References

• Athy, L. F. (1930). Density, porosity, and compaction of sedimentary rocks. AAPG Bulletin, 14(1), 1-24.
• Swarbrick, R. E., & Osborne, M. J. (1998). Mechanisms that generate abnormal pressures: An overview. AAPG Memoir 70, 13-34.
• Zoback, M. D. (2007). Reservoir Geomechanics. Cambridge University Press.