けつ岩の流体排出機構と異常高圧の発生について

抄録

Abnormal fluid pressures in shale-sandstone sequences occur commonly in the relatively deeper parts of the sequences. The causes of their anomalous pressures can be explained by the fluid release mechanisms related to compaction of shales.
The possibility of the formation of anomalous pressures due to retardation of expulsion from shales increases with depth, because in this situation the fluid must entirely support the added weight of overburden and the rock density tends to increase with depth.
The order of magnitude of the amount of fluid which should be expelled from shale in unit time to reach compaction equilibrium is determined, based on Dickinson's shale porosity-depth curve of the Gulf Coast region. The minimum permeability values necessary for compaction equilibrium to be reached are calculated according to Darcy's law. These calculated minimum permeability values, however, may be smaller than the true values in hydrostatic pressure zone, and must be greater in abnormal pressure zone.
If the calculated permeability at 7, 000ft, corresponding to the boundary surface between hydrostatic and abnormal pressure zones in the Gulf Coast region, approximates true value, the permeability at other depths can be determined by Kozeny's and Archie's porosity-permeability relations.
As the permeability based on Kozeny's and Archie's relations above 7, 000ft is greater than the minimum permeability necessary for compaction equilibrium to be reached, sufficient upward fluid expulsion would take place to maintain hydrostatic pressure above 7, 000ft. On the other hand, the permeability being smaller than the minimum permeability below 7, 000ft, the sufficient water could not pass through and abnormal pressure would occur.