Implication of Shale Diagenesis on Cementation of Reservoir Sandstones in the Neogene Surma Group of the Bengal Basin, Bangladesh

Late Fe-calcite and quartz overgrowths are the two most widespread and important cements in the Surma sandstones and are most responsible for damaging their reservoir properties. This paper suggests that the burial diagenesis of Surma shales has significantly influenced the cementation of adjacent sandstones by supplying ion charged water expelled during shale compaction and dewatering. The diagenetic conversion of smectite to illite layer in the illite/smectite mixed layer clay mineral in the Surma shales release Si⁴⁺ and Ca²⁺ that migrate up with expelled water and form important sources of quartz overgrowth and late calcite cement respectively in the overlying sandstones. This is compatible with the correlatable relation between the higher degree of illitization plus greater volume of shale and the zone of increased sandstone cementation in the deeper subsurface. The ferroan nature of the calcite cement may have been related to the availibility of Fe²⁺ ions, also released by the illitization reaction. The diagenetic dissolution of calcite in shales in the subsurface and its migration upward may have been an additional source of the late Fe-calcite cement in the Surma sandstones.