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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Mixed layer smectite-illite clays comprise a significant fraction of fine-grained clastic sediments in many basins around the world. Abnormally high fluid pressure (geopressure) is associated with parts of these basins. The presence of smectite-illite is a necessary but not sufficient criterion for the existence of geopressure.
Smectite reacts with potassium feldspar producing illite, silica, sodium-calcium feldspar, and releasing loosely bound water. Observations in the northern Gulf of Mexico support an equilibrium model for the reaction, the shift in the logarithm of the equilibrium coefficient with depth being proportional to the product of the reaction enthalpy and the geothermal gradient. Reaction enthalpies range from 2,000 to 26,000 cal/mole, highest reaction enthalpies occurring along the south Texas coast, lowest in the Mississippi delta. Abrupt diagenesis-depth profiles are associated with geopressure, gradual reaction with depth associated with near hydro-static fluid pressure gradients.
Sediments compact over intervals where the effective pressure increases. Geopressure is associated with porosity increase and effective pressure decrease with depth. The top of the zone of sediment under-compaction coincides with change in sign of the effective pressure gradient from positive to negative. In this interval, fluid pressure increases with depth faster than does the overburden pressure. Very high fluid-pressure gradients are associated with the combination of low shale permeability, high shale porosity, and rapid basement subsidence. Because of the close connection between high fluid-pressure gradient and abrupt conversion of smectite to illite, we conclude that this reaction is responsible for abnormal loss of permeability, probably a result of the finely divided silica hat is produced.
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