Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Md. Badrul Imam ¹

Affiliations
1 Department of Geology, University of Dhaka, IN

Abstract

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.

Full Text

     

Md. Badrul Imam ¹

Affiliations
1 Department of Geology, University of Dhaka, BD

Abstract

Strikingly similar diagenetic histories of the deltaic to shallow-marine sandstones from the Bengal basin, the Gulf Coast basin, the Denver basin and the North Central Texas, have been noted. The diagenetic events include an initial phase of compaction and cementation with sequentially developed authigenic clay rims, quartz overgrowth and calcite cements, a secoud phase of secondary porosity development by rock component dissolution and a final less important phase of recementation with kaolinite and 'other cements. All the sandstones have comparable detrital composition and textural properties. Similarities. in depositional environments have contributed to establish similar set of facies relationships among sandstones and shales in terms of sand body geometry. sand-shale ratio, sedimentary texture etc. in these sequences. Inorganic and organic diagenetic reactions in shales have influenced the composition of their contained water that migrated during compaction into the adjacent sandstone into which mass transfer from shales is supposed to have taken place. The above factors presumably . lead to similar sort of fluid flux and fluid composition in the sandstones, meaning that the pore fluid evolution in the above sandstones have common links. It is envisaged that these common links are perhaps responsible for the similarities in the diagenetic events in all these sandstones.

Full Text