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Under normal conditions most compaction current after deposition would move upward out of shales, because porosity and permeability of shales tend to decrease downward. Where a thick permeable sandstone bed underlies the shales, however, some downward compaction current should occur within the shales that are close to this sandstone. Therefore the boundary zone or boundary surface between the zones of upward and downward fluid movement must exist in the lower middle part of the shales. The level of the boundary surface or the thicknesses of the zones of upward and downward movement is related to minimum permeability, viscosity of water and pressure distribution. These zones can be determined in a schematic example of shale-sandstone sequence where the deposition is very rapid, by using a method based on Athy's porosity-depth curve as the relationship under the compaction equilibrium condition, and Kozeny's porosity-permeability relation. The downward compaction current could, moreover, be more important than the upward, because the shales which produce the downward current perform the role of the cap rock to the underlying sandstone reservoir.
In order to estimate the amount of the downward compaction current, the thickness of the downward zone is related to the porosity decrease during compaction. Calculations based on this schematic shale-sandstone example indicate that the main downward compaction current would occur in the relatively early stages of the rapid sedimentation. Later, another main downward current would take place just before the shales reach the equilibrium condition of compaction.
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