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The Bengal Basin is divided up into a series of sub-basins/troughs and regional highs. The onshore part of the Bangladeshi sector of the basin is made up of the Surma Sub-basin, which is bounded to the south by the Tangail-Tripura High. This separates the Surma Sub-basin from the Faridpur Trough, which extends offshore into the Indian sector of the basin. The Barisal-Chandpur High separates the Faridpur Trough from the Hatia Trough, which is the principal depocentre of the Bangladeshi offshore sector.
Geohistory
After Precambrian peneplanation of the Indian Shield, sedimentation in the Bengal Basin started within a series of isolated basins on the basement. These half-graben "Gondwanan" basins contained alluvial to fluvio-deltaic depositional systems. Break-up of Gondwanaland took place in the Late Jurassic to Early Cretaceous, separation of India from Australia/Antarctica occurring along a northeast - southwest line, with concomitant extrusion of a thick series of flood basalts. Following separation, India moved rapidly northwards, with oceanic crust emplacement occurring in the south and east resulting in the development of shelf to deep basinal passive margin profile. Upper Cretaceous to Lower Paleocene fluvio-deltaic systems developed on the thermally subsiding shelf, with increasingly deeper water systems developing in the basinal area. By the Eocene, because of a major marine transgression, the stable shelf came under a carbonate regime, whereas the deep basinal area continued to be dominated by deep water sedimentation. Highstand conditions prevailed after this, depositing open marine shales over much of the basin by the Late Eocene.
The rapid northward movement of the Indian Plate resulted in the beginnings of collision with the Asian Plate to the north during the Middle Paleocene. By the Early to Middle Eocene "hard" collision of the plates began, resulting in the onset of the Himalayan Orogeny. In the eastern part of the basin, collision occurred between oceanic crustal units and the Burma Block of the Asian Plate. Oceanic units began to be subducted below the Asian Plate and associated uplift and deformation of sedimentary cover formed an accretionary wedge (Indo-Burman Ranges). A consequence of this upbuilding and tectonic loading was the rapid subsidence of the Bengal Basin and the generation of a foredeep style profile to the basin, as well as increased sediment input from the growing orogens. Sea-level fall and increased sediment input resulted in a major regressive phase beginning in the Oligocene, with deltaic and shallow marine systems becoming re-established on the shelf feeding deep water systems in basinal areas.
Continued collision, westward translation of the accretionary wedge and associated uplift caused further regression in the Miocene. Extensive deltaic systems continued to develop on the shelf, but also became more important in the northeast of the basin (Surma Sub-basin) with sediment supplied from the orogenic belts to the east. These tide-dominated deltaics to shallow marine muddy shelf systems passed out into deeper shelf to deep marine turbidite systems. By the Pliocene, the eastern deformation front/accretionary wedge had developed sufficiently to begin to affect the sedimentary succession of the Bengal Basin. Extensive folding, thrusting and strike-slip movements took place in response to this. A significant, tectonically induced sea-level fall at the Miocene - Pliocene boundary resulted in the depositional system changing from a predominantly deltaic system to a regressive fluvial system that fed out into deltaics and deeper marine systems further south. By the mid- to Late Pliocene, the Present Day configuration of the Bengal Basin had largely taken shape. The tectonism and gradual regression of the sea during the Pliocene had developed the Ganges - Brahmaputra Delta system in the onshore and shallow offshore part of the basin, which fed out into the Bengal Deep Sea Fan system in the deeper offshore area.
Source Rocks
There are a number of proven and potential source rock units in the Bangladeshi part of the Bengal Basin. The principal source rocks are the Middle Oligocene Jenam Formation (Barail Group) and the Lower to Middle Micocene Bhuban Formation (Surma Group). The Jenam Formation is considered the main source in the northern part of the basin (e.g. Surma Sub-basin), but in the Hatia Trough area, burial depths are such, that it is likely to be overmature in this area. Resultantly, only the younger Bhuban Formation is considered a viable source in the area of interest, as it has not been buried sufficiently to reach over maturity.
The Bhuban Formation contains shales that were deposited within a variety of depositional environments, from tidally influenced deltaic/estuarine, southwards through shallow marine and out to into deep marine. In this southward direction, the formation thickens and the levels of shale increases with the development of more distal facies. In general, studies have revealed that the source rock quality of the Bhuban Formation in the Bengal Basin is not particularly good, with TOC values of 0.2 - 0.7 % and S2 yields of 0.2 - 0.4 mgHC/g Rock. H/C ratios for Bhuban Formation kerogens are 0.4 - 0.7 suggesting that they have potential for gas generation, although evidence from the adjacent Tripura-Cachar Basin suggest that the potential for liquids generation should not be discounted. Although the source quality of the Bhuban Formation is not particularly good, the thick, widespread nature of the shales within the Bengal Basin suggest that it has the capacity to contribute a significant proportion of hydrocarbons. Modelling suggests that the Bhuban Formation begun gas (and small amounts of condensate) generation and expulsion at about 5 Ma (Pliocene), where it is buried below 7 km.
Reservoirs
The principle reservoirs in the Bangladeshi part of the Bengal Basin are the Miocene age Bhuban and Boka Bil formations (Surma Group) which occur in the Surma Sub-basin, Tangail - Tripura High and Faridpur/Hatia troughs. The Lower to Middle Miocene Bhuban Formation, which forms the lowermost of these formations, consists of pro-delta to delta front sedimentary facies, that were deposited in a mud-rich, tidally influenced deltaic system that prograded towards the south and east. The overlying Middle to Miocene Boka Bil Formation consists of a complex interplay of tidally influenced deltaic and shallow marine sediments. The formation is shale/mudstone rich, but sandstone units are common. Thinly-bedded sandstones deposited in tidal/deltaic environments, and interbedded with shales, also have good porosity and permeablility and make good reservoirs. Abundant channelling within the formation results in reservoir heterogeneity and compartmentalization.
Distal equivalents to the Miocene Surma Group as well as the overlying Plio-Pleistocene Tipam and Dupi Tila groups in Bangladesh (there is no formal stratigraphy for deep water of Bangladesh, so equivalent terms are used) are considered to have potential in the southerly and eastern parts of the offshore Hatia Trough, in the areas covered by the blocks within this licensing round. These consist of turbidite systems that were sourced from the West Bengal Shelf (through northwest - southeast trending canyons), Surma Sub-basin and latterly during the Pliocene from the Eastern Foldbelt/Indo Burman Ranges to the east. Data on detailed reservoir characteristics and quality are not available, but these turbidites are considered to have suitable reservoir quality.
Traps
Trapping within the reservoir units in the area of interest are likely to be structural, stratigraphic or a combination of both. Depositional pinchouts/wedgeouts of reservoir sandstone, either as submarine fan lobes or as channel features would be expected to provide the stratigraphic trapping components. Structural traps would be supplied primarily by fault related structures, produced either by extensional or by compressional forces. Along the eastern margin of the Hatia Trough, compressional tectonics related to the development of the Eastern Foldbelt/Indo Burman Range, and associated wrench tectonics has produced a series of reverse faults and compressional flower structures, trending north-northwest - south-southeast. Anticlinal features may also feature in these areas. The development of a thick Miocene to Pleistocene sedimentary wedge in this area may also lead to instability and the development of gravity related movements. These could produce such structural features as rollover anticlines and compressional toe thrust features. Combinations of theses structural trapping features with stratigraphic features such as the depositional pinchouts may also produce viable traps. |