Electro-facies analysis of the Late Cenozoic sediments of the Sylhet Trough, Bengal Basin (Bangladesh): Implications for interpretation of depositional sequence

Abstract

 The Sylhet Trough of the Bengal Basin is situated in northeast of the Indian plate and is comprised of Eocene to Recent sedimentary successions. The stratigraphy of the Cenozoic Sylhet Trough is poorly studied. The area is regarded as the potential province for hydrocarbon accumulation, targeting the Miocene Surma Group sandstones as the key reservoir rocks. The present study is concerned with sedimentation and depositional environment of the Late Cenozoic (Miocene to Recent) succession of the Sylhet Trough based on electro-facies analysis. Based on the overall GR log motifs, geometry and vertical relationships of electro-facies, four depositional sequences have been interpreted that are separated by unconformities. The lower three sequences (1 to 3) are grouped into Megasequence 1 and is equivalent to undifferentiated Miocene Surma Group sediments, representing tide-dominated marine to coastal (deltaic) depositional systems with evidence of cyclic marine regression and transgression. Within this megasequence, the lower most Sequence 1 is dominated by finer sediments and showing an overall coarsening upward succession, possibly representing an offshore to shelfal facies. The Sequence 2 broadly showing another coarsening upward trend with minor internal variations, which is interpreted as a deltaic succession. The overlying Sequence 3 is also showing coarsening upward succession, which is representing estuary deposits. The topmost Sequence 4 belongs to Megasequence 2 that corresponds to undifferentiated Tipam Group and Dupi Tila Group. This Sequence 4 represents stacked braided river sand bars and gradually changes into meandering river deposits in the upper sequence. Our stratigraphic framework of the subsurface Late Cenozoic sediments of the Sylhet Trough represents a basinward progradation of deltaic to continental-fluvial environments and correspond to retrogradation, progradation and aggradation stacking patterns (from the bottom).