Proper interpretation of the occurrence of clay mineral assemblages in Tertiary argillaceous sediments, containing abnormally high pressure zones, depends on understanding their diagenetic history. Of particular interest to petroleum engineers and geologists is the observed diagenetic conversion of montmorillonite to mixed-layered clay mineral forms under conditions of deep sediment burial at low geothermal temperatures (less than 500 deg.F). The origin of these clay mineral transformations may be related to the existence of the overpressured zones.
A series of pilot temperature-pressure laboratory experiments were employed to investigate the proposed transformation of montmorillonite to an inter-stratified layer arrangement of illite and montmorillonite. Samples of natural and pure sprayed-dried sodium montmorillonite (Wyoming bentonite) were saturated with distilled and substitute sea water, and treated with various chemical solutions before being run at pressures from 1,000 to 200,000 psig, under pH conditions from 0.5 to 13, and at autoclave temperatures up to 380 deg. F. The clay samples were exposed to these environmental conditions in both closed and open chemical systems from 7 to 21 days. The modification of the sodium montmorillonite structure to an illitic form did not take place under the above stated conditions. Apparently, time is an important factor in this conversion process and will have to be taken into consideration in future experiments. Evidence for non-conversion is based on X-ray powder diffraction studies, infrared absorption scans, and chemical analysis of the samples before and after each run. Results of these experiments are discussed and compared with data from previously reported field data and previously reported field data and temperature-pressure laboratory investigations on clays naturally occurring in sediments.