The diagenetic history of the Neogene clastic sediments of the Bengal Basin is described based on petrological analysis of samples from four boreholes. In the shales evidence has been found of progressive illitization of the mixed-layer illite-smectite with increasing depth of burial. Unlike other studies, notably of the Tertiary Gulf Coast sediments, there is no evidence of the illite-smectite changing from a random to a regular mixed-layer phase during diagenesis. Other diagenetic changes in the shales include the decrease in the proportions of kaolinite and calcite with increasing burial. Diagenesis in the sandstones principally involves the progressive development of various types of cements in the following order: chlorite authigenesis, siderite crystallization, illite rim formation, quartz overgrowths, and late Fe calcite cements. After the formation of the cementation phases, there was a dissolution phase creating secondary porosity via the dissolution of carbonate and feldspars, and this was accompanied by the generation of a late kaolinite-cementing phase. The latter processes are thought to arise from the generation of acidic pore waters generated by the maturation of organic matter in the Neogene and Paleogene shales. In some of the sandstones, an early poikilotopic Fe calcite cement formed and halted further diagenesis beyond the formation of the chlorite and early siderite crystallization.

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