Facies Analysis Of The Reservoir Rocks In The Sylhet Trough, Bangladesh

Abstract

This study examined the sedimentary facies and stratigraphic architecture of the Miocene aged Boka Bil formation of the Surma Basin, exposed along the Sari River section, Sylhet, Bangladesh. Three hundred meter thick succession was logged for lithology, sedimentary structures, bed geometry, grain size and trace fossils. In the lower part of Boka Bil, nodular shale with calcite cemented sandstone is interpreted as a periodic storm
influenced Inner Shelf deposit. Flaser-, lenticular- bedding, Bioturbated mudstone, ripple/trough laminated sandstone, herringbone/bidirectional laminated sandstone, wave rippled parallel laminated sandstone are interpreted as wave influenced Tidal Flat deposit. Above this association, the wave influenced Mouth Bar and Abandon Channel facies is reported. These regressive facies association are comprised of massive sandstone, parallel laminated sandstone with wave ripples, trough bedded sandstone, conglomerate and shale. As the basin ward progradation continued, Shoreface Sandstone was deposited. The Upper Marine Shale above this sandstone facies remarks, the last transgressive phase in the Boka Bil Formation. The overall facies association gets shallower in the
stratigraphicaly up direction and the whole section is interpreted as Wave-Tide Dominated Prograding Deltaic system. Three types of reservoir sand bodies are interpreted from the exposed outcrops. The Shoreface sand and the Mouth Bar sands have good reservoir characteristics. They have poorly cemented, moderate to well sorted- medium to coarse grain sand. The Tidal Flat sand can act as secondary reservoir, but the porosity-permeability distribution and the sand body geometry will not be as good as the mouth bar and shoreface sand bodies.