Reevaluation of Montmorillonite Dehydration as Cause of Abnormal Pressure and Hydrocarbon Migration

Powers proposed “montmorillonite dehydration” (montmorillonite-illite conversion) as a cause of abnormal pressures in the deep subsurface of the Gulf Coast area. This paper compares his hypothesis with “compaction disequilibrium” and concludes that, whereas the latter mechanism by itself can account for abnormal pressures, montmorillonite dehydration by itself cannot, though it may act in a secondary capacity to increase abnormal pressures. As to Burst’s hypothesis that montmorillonite (“second stage”) dehydration is an important factor related to hydrocarbon migration from shales into reservoirs, the conclusion here is that second-stage dehydration may be important in moving hydrocarbons out of source rocks when it is followed by the proper fluid expulsion from shales (similar to Burst’s first-stage dehydration), but that montmorillonite dehydration alone without good drainage may not be significant. The low frequency of producing zones below dehydration level in the Gulf Coast is probably the result simply of the poor drainage and reservoirs in this section. Montmorillonite dehydration probably is effective in releasing hydrocarbon sources from clay-interlayer to interparticle locations (shale pore space), but may not in itself suffice to move large amounts of hydrocarbons out of shales into nearby permeable beds; for hydrocarbons to be pooled, the shale pore water subsequently must be squeezed out by overburden pressure.