X-Ray Diffraction Study on Neogene Shales from the Patharia Anticline, Eastern Bangladesh, with Emphasis on Smectite Clay Dehydration and Implications

Mineralogical composition of Neogene shales from the Patharia structure, as determined by X-ray diffraction analysis, includes quartz, illite, kaolinite, clllorite and mixed layer illite/smectite as major and feldspars as minor components, with calcite, dolomite and siderite in tsace amount. High degree of tectonic stress rather than the above mineralogy is believed to be responsible for high density of shales from the patharia-5 well. Progress illillitization of randomly interstratified illite/smectite is the rnost important diagenetic change observed with increasing burial depth. Transition from random to ordered inter layering in illite/smectite is suggested to have taken place at about 3000 m in the Palhuria- 5 well. Considering the huge volume of shale in the Neogene Group, sliicctitc dehydration, as a result of diagenetic conversion to illite, appears to have made available significant volume of interlayer water for migration into the pore water system in the subsurface. This may have played important role in the generation of overpressure at depth as well as structural shaping of the Patharia anticline.