Bengal delta is one of the active deltas in the world and Bangladesh is located within this deltaic region. Every year, numerous rivers flowing through Bangladesh carry a huge load of sediment and deposit them into the Bay of Bengal. Since plants and animals dying millions of years ago are the source of petroleum, the sediment of Bengal Delta carries huge mass of organic substance that lead to the generation of hydrocarbon through a process of sediment compaction, maturation and migration under certain pressure and temperature.
In Bangladesh, natural gas was discovered in a belt stretching from the country's northeast and east to the southeast and southern parts. Sangu is Bangladesh's first and only offshore gas-field, although shut down recently. Besides, mineral oil was discovered at Haripur in Sylhet - the oil field's present output is insufficient. Evidently, Bangladesh is lagging far behind other countries in exploration of oil and gas since the petroleum sector failed to reach an expected level of success in the country's overall development. Recently, International Court of Justice (ICJ) arbitrated Bangladesh's sovereignty over its exclusive economic zone (EEZ) in the Bay of Bengal - opening up a vast area for offshore petroleum exploration.
Geologically, Mio-Pliocene formation (time-span: 1.65 to 23 million years) is the formation of oil and gas coinciding the formation of Bengal Basin where depth ranges from 3.0 to 5.0 kilometres from surface and can be less in the deep sea - 2 to 3 kilometres from seabed. Seabed stands below the water depth (time-span: 1.65 to 23 million years) which ranges 1 to 1.5 kilometre at deep sea. The formation of seabed is hardly affected by the recently deposited sediments since the deep sea is far away from depositional arch of shallow sea. Evidently, hydrocarbon formation will not be deeper. However, presence of high pressure in the subsurface geology acts as the main barrier for drilling, an expensive activity. Tectonically, Bangladesh is located in the subduction zone between Indian Plate and Burmese Plate. Tectonic features in natural formations such as major and minor fault, fracture and unconformity change the reservoir of oil and gas as well as represent abnormal pressure indicating difficulties in drilling. Tectonic force is one of the important reasons leading to a discontinuity of rock formation and decomposition of the reservoir potential of hydrocarbon formation.
Moreover, earthquake is an element of plate movement that not only damages the surface but also smashes up the rock formations by creating faults and fractures at the deep underground - affecting the oil and gas reservoir, migration of hydrocarbon resources and the generation of new abnormal pressure zones in the natural formation. Near the south, southwest and west of Bengal Basin, the deep sea is like a trough that consists of organically rich and thick sedimentary rock covered by huge water body above the seabed; this may reduce the consequence of tectonic force to rock body of this area. Hazards in underground drilling are less likely in rock formation at deep sea. Less hazards may speed up the drilling operation and reduce the cost as well until the safe completion of drilling a well. Geologically, the south, southwest and western parts of Bangladeshi EEZ is more favourable for drilling as the high pressure zone gradually becomes deeper from northeast to southwest in its partial formation.
Furthermore, the presence of hydrocarbon at the shallow depth of Pleistocene (time-span: 1.65million years) formation is prominent in the Gulf of Thailand. In Bangladesh, the possibility of an offshore oil discovery lies in the Pleistocene formation. During the recent drilling at Sangu (7) B in the Sangu Platform, engineers observed traces of oil at the shallow depth around 950 metres to 1,100 metres from the surface - suggesting the prospect of oil reserve in the Pleistocene formation at deep sea. Besides, it is necessary to justify reserve quantity and measure expenses relating to the production stage before any discovery of oil and gas. Therefore, reserve quantity of hydrocarbon is also important for the declaration of any oilfield or gasfield.
Offshore drilling in Bangladesh is almost nonexistent and the exploration data is not sufficient to analyse the country's oil and gas reserve. However, from the deltaic nature, depositional history and sediment criterion, it seems that deep offshore and adjacent areas might be rich in oil and gas. Bangladesh is spending huge foreign currency on importing crude oil and hence drawing back in terms of a sustainable development. The time has come to focus on petroleum exploration in the vast offshore areas in pursuit of 'black gold'.
The writer is a geologist and an international petroleum exploration specialist.
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