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Bengal Basin

Background

The Bengal Basin is the largest fluvio-deltaic sedimentary system in the world, occupying over 800,000 square miles. It is located on the southern edge of Bangladesh and bordered on the West by India with a small portion bordering southeast Myanmar as can be seen in Figure 1. The majority of sediment deposited in the basin comes from the Ganges, Brahmaputra, and Meghna rivers; which is why the Bengal Basin is often referred to as the Gages, Brahmaputra, and Meghna Delta (GBM Delta). 80% of the sediment delivered to the basin comes in the four monsoon months: June, July, August, and September.

Figure 1

Formation of the Bengal Basin

The Bengal Basin is a result of continental collision. It formed as a result of the Indian plate sub-ducting below the Eurasian and Burmese plates. The Bengal Basin originated with the breakup of Gondwanaland in the late Mesozoic, roughly 126 Ma. The basin is the foreland basin of the Himalayas The north and northeast boundaries of the basin are trapped by the east–west extending Shillong Massif . The eastern side of the basin is bounded by the north–south extending the Tripura Fold Belt. The northwestern boundary of the basin is flanked by the basaltic traps of Rajmahal Hills. Finally, the south–southwest of the basic trap deposits lies the gneissic complex of the Archean Shield. During the middle Eocene, a large subsidence occurred which resulted in a marine transgression and deposition of limestone into the Basin. Roughly 49.5 Ma, the Basin started to experience a change in sedimentation being deposited. The carbonate-clastic platform sedimentary sequence was followed by a clastic deposit. The western part of the basin experienced deposition of lagoonal, argillaceous, and arenaceous sediments. The eastern and northeastern parts were occupied by an open arenaceous sediments. Approximately 10.5 Ma, intense tectonic activity began, which resulted in basin-wide regression. This initiated the modern Bengal delta as a shift from a marine-estuarine environment to a predominantly fluvial–tidal dominated environment. There are four main tectonic and structural zones identified in the basin: 1 The Basin Margin Fault Zone 2 The Shelf Zone 3 The Hinge Zone 4 The Deep Basin.

Figure 2

Figure 3

The Basin Margin Fault Zone

This NNE-SSW trending fault zone is comprised of crystalline/metamorphic rocks of Precambrian age. The zone is a result of distension and down-wariping of the shelf region during the Early Cretaceous in conjunction with the eruption of Rajmahal basaltic lavas.

The Shelf Zone

The western lying Shelf Zone is a more than 100km wide foreland shelf which exhibits narrowing from north to south. During the post-Paleocene period, the basin witnessed a gradual deepening. This created the environment necessary for the development of shelf carbonates during the Eocene. The sequences show a gradual change from a fluvial depositional environment to deltaic to marine from West to East.

The Hinge Zone

The Hinge Zone is a narrow zone which separates the thick Post-Eocene sediments in the East from the Shelf Zone of the West. This zone has given rise to a belt of flexures and faulting during the Oligocene and Miocene. This zone exhibits many truncations and wedgeouts.

The Deep Basin

The Deep Basin southeast of the Hinge Zone hosts a 10-15km, up to 22km thick prism of sediments. These sediments are largely made up of post-middle Miocene deltaic deposits. This unit has a Tertiary sedimentary succession which can be divided into the Jaintia Group(Paleocene to Eocene), the Barail Group (Oligocene), the Surma Group (Miocene to Early Pliocene), the Tipam Group (mid Pliocene), and overlying Dupi Tila and Madhupur Formations (Late Pliocene-Pleistocene). These different groups with their respective formations can be seen in Figure 3 with simplified lithology, thickness, age, and depositional environments. This zone is perceived by many as the most important zone due to the fact that it contains valuable Hydrocarbons. A cross section of the Deep Basin can be seen in Figure 4.

Figure 4

Hydrocarbons in the Bengal Basin

The Surma Group, mentioned above, is up to 5km thick and includes the Bhuban and Boka Bil Formations. This group is exposed in the Sylhet and Chittagong hills. Both formations are made up of alternating sandstones and shales which were deposited in a deltaic to shallow-marine environment. These two formations are arguably the most important in the entire basin as all of the hydrocarbons that have been found in Bangladesh have been found here. Bangladesh has proven and probably gas reserves of roughly 21.5 trillion cubic feet with 12.5 trillion cubic feet thought to be recoverable. The kerogen that lies within the basin is mostly Type III Kerogen, which is the gas prone kerogen and explains why a majority of wells drilled here are gas wells. The more oil prone kerogen, or Type II, is also found here; in much smaller quantities than the Type III. Type I kerogen has also been found, but in extremely small quantities and is considered very rare in the basin. There are 4 main necessities in order to constitute something a a Petroleum System: 1 Source Rock/Rocks 2 Migration Pathway 3 Reservoir Rock 4 Traps and Seals.

Source Rocks

The source rocks of the Bengal Basin are not very well understood; however, Gondwana sediments of Permian age have been suggested as the source rocks. Specifically, Permian coals whose kerogen is thought to be of continental origin created the gas as the basin was so hot that the kerogen was overcooked from the liquid to gas phase. Although most of the hydrocarbons of the Bengal Basin are gaseous, there is some oil that has been discovered as well. Alternate source rocks which created the oil are thought to be younger rocks of Neogene age and are comprised of Eocene limestones and Oligocene shales. These oil-bearing source rocks preserved the liptinite kerogen which generated the oil.

Migration Pathway

The Migration process was triggered after the convergence of the Indian plate with the Burma plate. The Bengal Mio-Pliocene flysch buckled when the plates converged which caused the formation of the Sylhet and Chittagong Hill tracts, where hydrocarbons migrated from. Surface oil and gas seeps that can be seen today prove that the migration process is still ongoing.

Reservoir Rocks

The Miocene sandstones have made for excellent reservoir rocks, with a permeability varying anywhere from 100 to 500 milidarcies with a porosity ranging from 10 to 20%, in some places reaching 30%. These fine to medium grained sands were once deposited in fluvial, deltaic, and estuarine environments.

Traps and Seals

The traps of the Bengal Basin are mainly structural in nature with almost all of them located in the eastern fold belt, known as the Chittagong-Tripura fold belt. Furthermore, the volcanic Rajmahal Hills, which are associated with the initial rifting of the eastern Gondwana, act as good trap rocks. There are also traps that are stratigraphic in nature(mostly shales) and even a few combination traps. A majority of the traps are anticlines that were formed from the collision of plates. An overlying marine shale acts as a basin-wide seal.

Demands

Future of the Bengal Basin

The first gas field was discovered in 1956 with production starting in 1962. Since then, 69 wells with 22 gas field discoveries and 1 oil discovery have been drilled. The Bengal Basin has been at the heart of a major dispute between India, Bangladesh, and Myanmar over who can drill and lay claim to the hydrocarbons located within the basin. A United Nations court ruling in 2014 awarded more than three quarters of the disputed area to Bangladesh. Being that the dispute was only resolved fairly recently, not much exploration and production has occurred in the basin. With the ruling, Bangladesh is eager to utilize resources to capture the hydrocarbons, especially since natural gas consumption is growing worldwide, but particularly in developing countries in South Asia. The key domestic player in the energy sector is the state-owned Petrobangla. The country; however, is seeking greater foreign collaboration in its offshore area, and launched a licensing round in 2016 for several shallow and deepwater fields. The Indian companies, although they were not awarded as much area as Bangladesh, are keen to participate in the exploration and production as well. In 2012, Bangladesh struck its first commercially viable oil field, with the oil resources estimated to be about 153 million barrels. According to Petrobangla officials, the reserve is enough to power Bangladesh for two years. Figure 5 depicts Petrobangla's overview of future gas demand. As can be seen by the figure, roughly 58 trillion cubic feet of gas is in demand. the Bay of Bengal will play a huge roll in bolstering the economy of Bangladesh and could possibly allow them to be energy independent.