Depositional facies of the subsurface Neogene Surma Group in the Sangu Gas Field, the Bay of Bengal: records for tidal sedimentation

The Neogene Surma Group sandstones are the proven hydrocarbon reservoirs in the Bengal Basin, Bangladesh. This research investigates the depositional facies of the Neogene sequences found in the Sangu-1 and Sangu-5 petroleum exploration wells in the Sangu Gas Field, located in the offshore area of the Bengal Basin, Bangladesh. The Sangu Gas Field, the first offshore gas field, is situated at Block SS 04 in the eastern part of the Hatiya Trough of the Bengal Fore deep and the western extremity of the Chittagong Tripura Fold Belt. This study, utilizing core samples, wireline logs, and seismic data, focused on the depositional characteristics of the Neogene sedimentary rocks. Wireline logs are employed to identify electrofacies patterns, whereas the seismic section is used to differentiate the boundaries of megasequences. The analysis of facies in core samples was conducted based on texture, lithologic association, and internal sedimentary structures. Three distinct facies assemblages were identified, including shale-dominated facies, alternating sandstone and shale assemblages, and sandstone-dominated facies. The presence of various sedimentary structures, such as lenticular, wavy, and flaser bedding, as well as cross-bedded sandstone with mud drapes, indicates evidence of tidal currents. The alternating beds of shales and sandstones observed in cores and well logs are attributed to repetitive transgressive and regressive phases. Overall, the depositional environment of the Neogene Surma Group sequences is interpreted as tide-dominated deltaic. This research will build understanding on the depositional architecture of the potential reservoirs in the offshore Bengal Basin.