Clay Minerals in Interbedded Sandstones and Shales of the Miocene Surma Group, Sylhet Trough, Bengal Basin (northeastern Indian Plate): Implications for Future Hydrocarbon Exploration

Clay minerals are common constituents of the Miocene Surma Group reservoir rocks in the Sylhet Trough, Bengal Basin, and may exert significant controls on reservoir quality. The relationship between diagenetic clay minerals and reservoir quality in the petroliferous Sylhet Trough is poorly understood, however. The current study was aimed at the origin and diagenetic pattern of clay minerals in interbedded sandstones and shales using thin-section petrography, scanning electron microscopy (SEM), and X-ray diffraction (XRD), and understanding their diagenetic effects on reservoir quality. The results showed that the clay mineral cements in sandstones comprise mainly chlorite, illite/illite-smectite, and minor smectite and kaolinite. In the early diagenetic stage, clay rims and growth of vermiform kaolinite occur and partly occlude the pore throats. Deep burial effects include pore-filling, pore-lining, and grain-coating authigenic clays (mainly chlorite and illite). Diagenetic clay minerals and mechanical clay infiltration showed a systematic distribution in sandstones lying in the vicinity of sequence and parasequence boundaries. In a lowstand systems tract (LST), clay minerals within the sandstones commonly include mechanically infiltrated smectitic clays that eventually evolved to grain-coating chlorite and/or illite during the meso-diagenesis stage. The presence of clays/clay minerals has no significant impact upon reservoir quality of sandstones. The Surma Group shales are enriched in illite with significant proportions of chlorite and kaolinite and are likely to be mainly detrital, with diagenetic changes of smectite to illite.