Superseals are qualitatively different from conventional hydrocarbon seals because they confine water and abnormal pressure, not merely fluids that are immiscible with water. In the normal course of dewatering, common shales are largely invisible to expelled water. In some basins, sediment can accumulate so fast that thick common shales can interfere with dewatering sufficiently to cause transient overpressure. Superseals, such as the Woodford Shale, are impermeable to water and can retain overpressure indefinitely. The identification of superseals may have an impact on both the sequestration of carbon dioxide and the isolation of certain waste materials. Common shales are able to trap hydrocarbons because the pore entry pressures for fluids other than water are very high (a capillary seal), but the shale remains transmissive to water and to some solutes. A common shale seal is unsuitable as a barrier to carbon dioxide. A shale superseal will retain CO2. Horizontal pressure compartments are also very common and must also be accounted for in basin models. The sealing capacity of a fault is determined by the capillary entry pressure of the sealing rock unit (or fault gouge zone). The sealing capacity is increased where there is a pressure differential across the fault.


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