Abstract

The zone below 10,000 ft in the earth is dominated by ionic and surface phenomena. Whenever sufficient overburden stress and temperature are available, a mechanical model is incapable of explaining the behavior of sediments. This is especially true of shales, and it is noticed that clays undergo phase changes whenever active tectonics are evident.

Shales under sufficient compaction have such small pore size and surface charge that their gross behavior is dominated by the interaction of water, ions and membrane surfaces. The phenomena of geopressure in the deep crust is associated with low water salinity, due to ionic diffusion in shales at great depths.

The normal circumstance for marine sediments shallower than 10,000 ft is that of simple compaction. The porosity and permeability is high enough to allow water and salt to be squeezed from shales, to yield an increasing water salinity with depth. The sands become more saline and the shales become fresher under increasing load. Below 10,000 ft mechanical stress is offset by ionic gradients accompanied by incipient gas pressure. Both sands and shales are observed to freshen as pressure gradients increase.

Vertically, salinity gradients show the change of both solid stress and fluid pressure gradients. It is evident that mechanical stress, such as around faulting, disturbs the electrochemical equilibrium. Ions tend to move in the direction of decreasing salinity, bringing water of hydration with them. A new equilibrium is established whenever gas expansion is offset by influx of water under compression. One foot of dewatered shale will pressure up 150 ft of high water fraction pressure up 150 ft of high water fraction sediments an additional 1,000 psi. The cap zone above geopressure is the dewatered shale zone, which signals entry into high pressure gradients.

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