SANDSTONE DIAGENESIS OF THE NEOGENE SURMA GROUP FROM THE SHAHBAZPUR GAS FIELD, SOUTHERN BENGAL BASIN, BANGLADESH

This study examines the various diagenetic controls of the Neogene Surma Group reservoir sandstones encountered in the Shahbazpur-1 petroleum exploration well from the southern part of the Bengal Basin, Bangladesh. The principal diagenetic minerals in the Surma Group sandstones are quartz overgrowths, Fe-carbonates (mainly Fe-calcite) and authigenic clay minerals dominantly chlorite, illite-smectite and mnor kaolinite. Compaction played a more extensive role than cementation in destroying primary porosity. Cementation was important in drastically reducing porosity and permeability in sandstones at depths of 1796.5, 2015.5 and 3019 m and is typically poikilotopic, pore-filling blocky Fe-calcite cement. Illite-smectite and chlorite occur as pore-filling and pore-lining authigenic phases. Sandstones having good porosities (20% to 30%) and high permeabilities (20 mD to 415 mD) are well sorted, and tend to be relatively coarse grained and more loosely packed with better rounded grains. They are typically found at depths ranging from similar to 2290 m to similar to 3411 m. These high quality reservoir rocks are, however, not uniformly distributed resulting in compartmentalization of the reservoir-quality units, which are interbedded with sandstone layers showing low to moderate porosity (1 to <20%) and low permeability (0.6 to 4.5 mD). These sandstones are typically poorly sorted, strongly compacted and contain significant higher proportions of cements.