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Some anomalous pressure and salinity data observed within oil- and water-bearing reservoir rocks can not be explained by prior theories in hydrodynamics and geochemistry. Laboratory evidence has shown that compacted clay minerals act as semipermeable membranes and thereby exhibit osmotic-pressure and salt-filtration effects. Osmotically induced pressure and salt filtering occur in reservoir rocks adjacent to shales presumably serving as semipermeable membranes. Osmotic conditions might result from differences across a shale of salt concentration. Pressure would tend to increase in the reservoir rock on the emergent side of the shale membrane and decrease on the influx side under osmotic conditions. Cross-formational flow through a shale membrane may also cause salt filter ng and thereby increase the salinity on the influx side of the membrane.
Three widely separated areas in North America (central Alberta, Canada; San Juan Basin, New Mexico and Colorado; and Wheeler Ridge anticline, San Joaquin Valley, California) have anomalous potentials and salinities that may be explained by the movement of water cross-formationally through shales acting as semipermeable membranes. Pressure and salinity anomalies from other areas possibly may be explained by shale-membrane phenomena.
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