Facies and architectural element analysis of the Upper Bokabil Sandstone in the Bengal Basin

Abstract

The Upper Bokabil Sandstone is a geologically significant unit in the Bengal basin, hosting the most prolific hydrocarbon reservoirs and serving as a source of groundwater. Despite its economic importance, the depositional setting of this unit has not yet been thoroughly studied, largely due to the limited accessibility to the outcrops. This research aims to provide the first modern sedimentological interpretation of the Upper Bokabil Sandstone in the Bengal basin utilizing data from outcrops and well logs. A generic model of the depositional environment is proposed based on sedimentary facies and architectural elements. The sedimentary facies of the Upper Bokabil Sandstone were examined at outcrops in the Sari River section of Jaintiapur (northern Bengal basin) and Paglir Bill in Ukhiya (southern Bengal basin). A total of 12 fluvial lithofacies and multiple high and low order erosional surfaces were identified. Facies associations and their relationship with their bounding erosional surfaces have been utilized to identify the architectural elements. The unit is characterized by laterally and vertically amalgamated channels and bar complexes, with downstream and lateral accreted barforms being the dominant architectural elements. Lateral Accretion (LA) is more prevalent in the southern part of the basin. The presence of the fluvial facies, distinct architectural elements associated with typical braided river and lack of aeolian facies lead to the interpretation of a low sinuosity braided river with seasonal fluctuation in discharge as the depositional environment of the Upper Bokabil Sandstone.