ABSTRACT

Density measurements of Oligocene shales from four wells in Matagorda County, Texas show that density there does not increase regularly with depth below 8000. In the sands associated with the low-density shale the pressure of the interstitial fluid is considerably above hydrostatic wherever data are available.

The high interstitial fluid pressures are caused by compaction where thick sections of low-permeability shale (clay) impede normal expulsion of interstitial water. Part of the weight of the rock frame is transferred to the water, thereby increasing its pressure; this increase is balanced by a corresponding decrease in the rock frame (compaction) pressure, which restricts the degree of compaction of the shales relative to comparable material in well-drained sections.

Interstitial water of salinity considerably less than normal sea water, ranging from 5000 ppm to 12,500 ppm, is present in the thin sands of the deepest low-density shale zone, below 11,000. The undiminished electrical conductivity of the associated shale demonstrates that the salts are held by the clay minerals so strongly that they do not accompany the expelled water.

Dynamic compaction phenomena have practical effects on the migration and entrapment of hydrocarbons, drilling technology, and gravimetric and seismic exploration. The high interstitial fluid pressure reduces the friction on grain-to-grain contacts, thereby facilitating deformation under small stress differences. Combined with density inversion this may produce shale anticlines; and with a mechanical potential energy gradient directed toward the Gulf of Mexico it may be responsible for extensive normal faulting, possibly injection of sandstone dikes that may breech oil and gas traps, and certain well-drilling difficulties.