Diagenesis and Petrographic Characterization of Surma Group Sandstone in the South-Eastern Part of Bangladesh: Implications for Reservoir Quality Assessment

Abstract

The Neogene Surma Group sandstones exposed in the Sitapahar (Kaptai) and Sitakund structures of the Eastern Fold Belt, Bengal Basin, constitute important hydrocarbon-potential reservoirs whose quality is strongly influenced by depositional architecture and diagenetic evolution. This study integrates outcrop sedimentology, petrography, Field-Emission Scanning Electron Microscopy (FE-SEM), and Energy-Dispersive Spectroscopy (EDS) to evaluate lithofacies distribution, diagenetic alterations, and reservoir quality controls within these sandstones. Detailed facies analysis identified depositional associations related to delta front-shoreface, delta plain, tidal flat, prodeltaic, tidal-influenced channel-fill, and channel-bypass systems, indicating deposition within a tide-dominated deltaic-shallow-marine environment. Petrographic analyses reveal predominantly medium- to fine-grained, moderately to well-sorted sublitharenites composed mainly of quartz with subordinate lithic fragments and feldspars, reflecting moderate compositional maturity. Thin-section porosity ranges from 5–23%, with generally low matrix content (<4%). Diagenetic processes, including mechanical compaction, pressure solution, quartz overgrowth, calcite cementation, chlorite coating, and clay mineral authigenesis, significantly modified the primary pore framework and reduced reservoir quality. Conversely, dissolution of feldspar, lithic fragments, and carbonate cements generated secondary porosity and enhanced pore connectivity. FE-SEM observations confirm the presence of interconnected intergranular and intragranular pore systems, although pore-throat reduction by chlorite and illite-smectite coatings locally restricted permeability. Comparative analyses indicate better reservoir quality in the Sitapahar sandstones than in the Sitakund deposits, due to differences in depositional texture and diagenetic intensity. Overall, the Surma Group sandstones exhibit promising reservoir characteristics with significant implications for hydrocarbon exploration in the Eastern Fold Belt of Bangladesh. 

The Dhaka University Journal of Earth and Environmental Sciences, Vol. 15(1), 2026, P 79-105