While sands and permeable-carbonate reservoirs once served as a convenient source of pressure measurements—and of some erroneous concepts of pressure "graients" as well—we now realize that most fluid pressures originate in the relatively-impermeable clays and shales from whence they are transmitted to the sands and carbonates, where they tend to gradually bleed off and reach "hydrostatic" equilibrium.
The water being released dynamically, continuously, and currently from the younger shales (Tertiary and into Cretaceous), during their compaction and diagenesis, appears to be primarily responsible for the generation of overpressures. This shale-pressuring follows no consistently "linear" gradient, showing many irregularities and pressure "reversals"—thereby causing, and pressure "reversals"—thereby causing, and explaining, many of oar complex drilling problems. problems. Since shales—quite thick in comparison to the relatively thin sands-are the predominant "rock" in most of our predominant "rock" in most of our sedimentary basins, their water content becomes a tremendous hydrodynamic reservoir for fluid pressures, and perhaps the origin of the pressures, and perhaps the origin of the very hydrocarbons for which we are drilling The diagenetic changes in shale mineralogy which accompany the release and subsequent movement of internal water from shales are being utilized worldwide in a convenient and reliable group of parameters for monitoring pressure changes (either up or down) from shale cuttings while drilling is in progress. When these shale mineralogy parameters are properly interted (to allow for such things as "jump shifts" in geologic age caused by folding and faulting; missing sediments; or for 11 strange" minerals squirted spasmodically by volcanic activity into the depositing shale sediments), they tend to fit the preceding generalizations and verify dynamic preceding generalizations and verify dynamic considerations of overpressuring. Four such case histories are presented here.
Dynamic mechanisms of pressure generation are not new to this group*, having been discussed in prior symposia by Jones, Burst and several others.