The easternmost part of the Bengal foredeep or Surma basin is the most prospective area for finding additional gas because the degree of folding diminishes markedly in a westward direction. The foothills of the Tripura-Chittagong area and the Bengal basin (sometimes called Bengal foredeep or Surma basin) are locations of the gas fields in Bangladesh. These areas have sometimes been called the Outer Molasse basin. Folding occurred in four phases. Gas discoveries are in the Chittagong foothills. Similar structural features to those of the Chittagong foothills appear to be present in the extreme eastern part of the Bay of Bengal. Compressional folding did not affect the central and western part of the Bay of Bengal. However, by comparison with other areas of deltaic depositi n, rollover structures associated with growth faults may be significant. The Oligocene to Holocene rock sequences were deposited in environments that range from abyssal marine prodelta to subaerial delta plain. In productive areas onshore and offshore, hydrocarbon traps include asymmetric, elongate, faulted anticlines. Strategic traps and sedimentary growth structures are found in the Bengal basin. Miocene sandstones constitute the gas reservoirs; Eocene, Paleocene, and Oligocene carbonaceous shales and Miocene shales are the source rocks. In the central and western part of the Bay of Bengal area, the major uncertainties are the development and thickness of sandstones and the possible size of structures.

Thirteen gas fields have been discovered: (1) Kutubdia, (2) Chhatak (8 mmcf/day), (3) Kailashtilla, (4) Habiganj (14 mmcf/day), (5) Bakhrabad, (6) Murdi, (7) Begumgoni, (8) Beanibazar, (9) Sylhet, (10) Rashidpur, (11) Titas, (12) Semutang, and (13) Jaldi. Chhatak, Habiganj, Sylhet, and Titas fields were on production during 1979.

The gas in these fields occurs in multi-sandstone reservoirs in anticlines that probably developed in late Miocene to Pliocene time. The gas reservoirs are the lower Miocene Bhuban Formation and lower to lower-middle Miocene Boka Bil Formation. Both formations are included in the Surma Group.

Total recoverable gas reserves are 7 to 7.8 tcf. Total estimated gas reserves in place are 9.33 to 10.39 tcf and possibly 10 to 20 tcf of gas resources yet to be discovered.

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