PHYSICAL-PROPERTIES OF SHALES AND POSSIBLE ORIGIN OF HIGH-PRESSURES

Data presented in this study indicate that many U. S. gulf coast shales underwent normal compaction with burial and maintained normal pressure by losing fluids. After burial to depths of about 8,000 ft (2440 m) or more, the dewatering of montmorillonite provided additional pore water that could not be drained off as fast as it was released. Therefore, high pressures were generated in these formerly normal shales. Such shales may be leaking fluids but not fast enough to keep up with dewatering. Presumably other shales that originally contained montmorillonite also have undergone dewatering but have remained at or near normal pressure by losing fluids fast enough to keep up with dewatering. Still other shales that now have normal pressures may have developed high pressures due to dewatering of montmorillonite but since have leaked off enough fluids to recompact and re-establish normal pore pressures.