363 Journal of Petroleum Geology,Vol. 43(4), October 2020, pp 363-382 GEOPHYSICAL CHARACTERIZATION OF THE SANGU GAS FIELD, OFFSHORE, BANGLADESH: CONSTRAINTS ON RESERVOIRS Md. Upal Shahriar*, Delwar Hossain*, Md. Sakawat Hossain*+, M. Julleh Jalalur Rahman* and Kamruzzaman* The only produced offshore gas feld in Bangladesh, known as the Sangu feld, is located in the Hatiya Trough in the east of the Bay of Bengal, and has estimated total reserves of about 1055 BCF GIIP. The early shut-down of the feld in October 2013 may have resulted in signifcant volumes of recoverable gas being left in the subsurface over a depth range of 1893 m to 3640 m. In this paper, seismic and well log data were analyzed and interpreted in order to investigate the structure and stratigraphy of the Sangu feld, together with the lithology, extent and petrophysical properties of the reservoir. The general lithostratigraphy at Sangu has some similarity to that of the Surma Basin of the Bengal Foredeep. Reservoir rocks consist of Miocene and Pliocene deltaic sandstones and deep-water clastics. The source rock is the Miocene Bhuban Shale which is mature for gas generation in the Hatiya Trough. Three Neogene seismic stratigraphic megasequences were recognised at Sangu and are interpreted to have been deposited respectively in fuvial, delta front and shelf slope or marginal marine settings. Based on an analysis of wireline logs from wells Sangu-1 and Sangu-5 and on seismic-to-well ties, a series of reservoir units referred to (from the base up) as the T1 (E, D, C, A&B), Supra-T1, T2 and T3 have been identifed. Petrophysical analyses showed that the average total porosity of these reservoir units is >13%, the permeability is in general less than10 mD, and the gas saturation ranges from 24% to 80%. Mapping of the reservoirs shows that the structure at Sangu is an asymmetric anticline with a NNW-SSE axial trend. Amplitude data have allowed the delineation of two other potential reservoir zones in the feld at depths of 2900-3000 m and 3550-3750 m.The study will contribute to future offshore gas exploration and development in the Bay of Bengal region based on the geological and geophysical characteristics of the reservoirs delineated. *Department of Geological Sciences, Jahangirnagar University, Dhaka 1342, Bangladesh. +Corresponding author: sakawat@juniv.edu Key words: Sangu feld, Bangladesh, Bay of Bengal, Hatiya Trough, Neogene, reservoir characterisation, reservoir mapping, amplitude extraction, gas reservoir. INTRODUCTION The Sangu feld was the frst ofshore gas feld to be developed in Bangladesh and is the only ofshore asset in the country which has been produced. The feld was discovered in 1996 and had a calculated initial confrmed GIIP of 1612 BCF (Cairn Energy, 1998). The Sangu feld is located in Block-16 (Petrobangla, 2003), later renamed Block SS 04 in 2016 (Petrobangla, 2016) in the east of the Bay of Bengal, ~ 45 km SW © 2020 The Authors. Journal of Petroleum Geology © 2020Scientifc Press Ltd www.jpg.co.uk