Abu N. Chowdhury
Geco-Prakla/Schlumberger
Houston
Bangladesh is a part of the Bengal basin, bordered to the west and northwest by Jurassic-early Cretaceous volcanic trap rocks of the Rajmahal Hills, underlain by Precambrian shield and Gondwana sediments.
On the northeast, the basin is bordered by the Shillong plateau, where the shield reappears. The eastern border is formed by the Tertiary Sylhet and Chittagong Hills, up to 2,000 ft high, which constitute part of the Indo-Burman ranges (Fig. 1)(66527 bytes).
The Bengal basin is the largest delta basin (approximately 23,000 sq miles) in the world, at the confluence of the Ganges and Brahmaputra rivers. The deep sea fan complex that is being built outward into the Bay of Bengal has in excess of 12 km of sediments. Rate of sediment transportation within the basin, from the Himalayas and the mountains and hills to the north, east, and west, exceeds 1 billion tons/year.'
The tectonic and sedimentary history of Bangladesh is favorable for hydrocarbon accumulation. The basin is an underexplored region of 207,000 sq km where only 52 exploratory wells have been drilled with a success rate of more than 30%. In addition to the folded belt in the east, where gas and some oil have been found, the Garo-Rajmahal gap to the north and the deep sea fan to the south merit detailed exploration using state of the art technology.
TECTONICS, SEDIMENTATION
The Bangladesh basin, as part of the Bengal basin, was formed as a result of the late Mesozoic fragmentation of eastern Gondwana, on an Atlantic type margin along the northeastern boundary of the Indian plate (Fig. 2)(48063 bytes). The continental crust of the Bengal basin is separated from the oceanic crust along a northeast-southwest striking hinge zone marked by the Calcutta-Mymensingh gravity high (Fig. 1)(66527 bytes). The Rajmahal trap, associated with initial rifting of the eastern Gondwana (Figs. 1, 2) occurs in the vicinity of the hinge zone. Pre-rift sediments in the Bengal basin constitute intracontinental Permo-Carboniferous gondwanas deposited on a faulted Precambrian graben. Post-rift Eocene limestone (Sylhet nummulitic limestone) and prograded delta facies are well documented in the Bengal basin. The delta complex sediments are mainly transported by the Ganges and Brahmaputra rivers along possible failed rift systems in generally southerly courses from the slopes of the Himalayas (Fig 1). The sediment denudation from the Himalayas is directly related to the collision between India and Asia and the resulting Himalayan orogeny beginning in mid-Miocene time (Fig. 2)(48063 bytes).
Sediments were also received in the Bengal basin from the Indo-Burman flysch, formed by the subduction of the Indian plate and the underthrusting of sediment complexes at the possible northward extension of the Sunda subduction system to the east (Fig. 2F)(48063 bytes).
The progradation of the delta facies over the widely open ocean margin (Bay of Bengal) has continued to present time. Huge loads of sediments funneled through the submarine canyon, called "the swatch of no ground," initiated building of the largest deep sea fan complex in the Bay of Bengal. The progradation of the delta was very rapid during periods of low sea level in the Pleistocene, when all the sediments of the Ganges and Brahamaputra confluence bypassed the shelf and poured off the continental slope directly into the deep sea fan area. In contrast, during the high stands of sea level, such as present today, the sediments are trapped in deltas and much less sediment passes into the deep sea fan areas. The courses of the Ganges and the Brahamaputra rivers are probably tectonically controlled. They moved towards each other through time, finally joining at the head of the Bay of Bengal. To the north, they are located within the Garo-Rajmahal Gap, which is believed to have formed due to the eastward movement of the Shillong plateau (Fig. 1)(66527 bytes) along the Dauki fault.2 A reconstruction along the Dauki fault brings sediments of two provenances into close proximity (Fig. 3)(59652 bytes). The Dauki fault offsets the northeast striking hinge zone,3 running east between the Surma Valley and Upper Assam, connecting an area of thrusting between the eastern Himalayas and the Naga Hills (Fig. 1)(66527 bytes).
The strike-slip motion along the Dauki fault may be related to these thrusts, and the Garo-Rajmahal gap may represent a pull-apart basin. India's collision with Asia initiated strike-slip movement and extrusion of China towards the southeast, resulting in the opening of new basins (Fig. 2F)(48063 bytes) like Bohai in China.' Similar tectonism is suspected in the northern Bengal basin, along the Dauki strike-slip fault (Fig. 1)(66527 bytes); possibly, a tectonic escape towards the Bay of Bengal to the south is under way.
PETROLEUM PROSPECTS
Bangladesh has proven and probable gas reserves of approximately 21.5 tcf and recoverable gas reserves of 12.5 tcf., mostly concentrated in the Sylhet and Chittagong district in the eastern fold belt (Fig. 4)(49509 bytes). A recent oil discovery in the Sylhet district with initial one well production of 150 b/d drew attention from the international oil industry for further exploration.
Hydrocarbons discovered in Bangladesh are trapped in Mio-Pliocene delta sediments, associated with elongated, north-south faulted anticlines, in front of the Indo-Burman ranges, in Sylhet and Chittagong Hill districts, and the eastern bordering belt of the Bengal basin (Fig. 4)(49509 bytes).
Stratigraphic barriers are provided by shale sequences in these reservoirs. Most wells discovered in Bangladesh to date are located onshore. Nine welts have been drilled off Bangladesh, of which one, Kutubdia, found commercial hydrocarbons.
The Bangladesh basin contains enormous thicknesses of sediments (in excess of 10 km), the bulk of which are the Ganges-Brahamaputra delta sediments. Very few of the wells drilled in Bangladesh penetrated below Miocene, the maximum TD being 13,000 ft.
The source rocks for hydrocarbons found in Bangladesh are not well understood. Gondwana sediments of Permian age have been suggested as source rocks according to one school of thought. This would be a likely candidate for the source of methane gas found in Bangladesh, particularly in the northern portion of the basin where coals are preserved on continental crust. The gas associated with these early rift coal deposits was warmed up due to the convecting currents below the basement and then migrated along basinal faults formed during thermal subsidence of the basin. The migration process was probably triggered after the convergence of the Indian plate with the Burma plate to the east, a possible northward extension of the Sunda subduction system (Fig, 2F). The Bengal Mio-Pliocene flysch buckled at this convergence and subsequently rose to form the Sylhet and Chittagong Hill tracts (Figs. 1, 4) of the Indo-Burman ranges, where hydrocarbons migrated from the subsiding basin located west and south.
Surface oil and gas seeps (Fig. 4)(49509 bytes) in Bangladesh indicate that the migration process is still going on.
With Permian coals as the possible source rocks in Bangladesh, it is expected that most kerogen available was of continental origin, and also the basin was probably so hot that the kerogen was overcooked to the gas phase.
Younger rocks of Neogene age (Eocene limestones and Oligocene shale), formed on the young ocean margin, may provide the alternate source rocks for preserving and maturing liptinitic kerogen generating oil, under favorable tectonic and climatic environments. Some studies in Bangladesh supported by vitrinite reflectance indicate that source rocks well within the oil window are present in the basin.
Bangladesh remains an underexplored sedimentary basin. Future exploration targets should include the Garo-Rajmahal gap along with the Dauki-strike slip fault to the north, where a pull-apart or early failed-rift (Ganges-Brahamaputra trough) basin may exist (Fig. 1)(66527 bytes).
Additional potential lies in the post rift delta complex and the deep sea fan to the south (Fig. 1)(66527 bytes). Reservoir quality sands are known to be present in deep sea fans formed on oceanic crust. These sands are transported from the shelf areas to the slope basins by major rivers like the Ganges (Fig. 2)(48063 bytes) during the lowstands of sea level. Natural storms which are common in Bangladesh also act as transporting agents for the coarse grained materials, deposited as slope fans or basin floor fans. A systematic sequence stratigraphic study combined with geophysical analysis would aid in unraveling the depositional fair-ways and chrono-stratigraphy of the reservoir rocks in the Bangladesh basin.
SEISMIC TECHNOLOGY
Sequence stratigraphy: Seismic and well log data are divided into several sequences and depositional systems tracts and age dated with reference to biostratigraphic data and sea level cycles. Stratigraphic dating is a major problem in Bangladesh. A chronostratigraphic framework of Bangladesh can be built using an integrated method of sequence stratigraphic interpretations.5 Additionally, the method enhances predictability of lithrifacies and helps isolate source and reservoir rocks. Tectonic events and migration of hydrocarbons can also be reconstructed using this technique.
Amplitude vs. offset: Seismic source and receiver distances are varied to study changes in the seismic signature of hydrocarbon fluids and non-hydrocarbon fluids. Hydrocarbon prediction is done with this method.
Time-amplitude study: Net pay thicknesses are measured from this study. Thin beds not resolved by seismic time result in high amplitude curves at tuning. The amplitude decreases from this peak amplitude as the thickness decreases. Calibration of seismic amplitudes with known pays enables one to estimate pays in unknown areas.
CONCLUSIONS
- Bangladesh has evolved, as a result of the fragmentation of eastern Gondwana, with the development of the largest delta and deep sea fan in the world.
- Pre-rift Permian Gondwana coals probably represent source rocks for the methane gas found in Bangladesh.
- Post-rift Eocene limestone, Oligocene shale, and Mio-Pliocene deltas are potential source (and reservoir) rocks in Bangladesh.
- The Garo-Rajmahal gap, which may represent a pull-apart, or a rift basin, warrants further investigation from both scientific and economic points of view.
- Bangladesh is an underexplored basin. A systematic basin analysis using the latest geological and geophysical tools should be undertaken before exploration drilling in Bangladesh.
ACKNOWLEDGMENTS
The author thanks Geco-Prakla for supporting this article. I thank Jerry Watson for his encouragements and review of this article. Thanks are due to David Risch and George Jamieson for their constructive comments.
REFERENCES
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- Evans, P., The tectonic framework of Assam, Geol. Sec. India Jour., Vol. 5, 1964, pp. 80-96.
- Curray, J.R., Emmel, F.J., Moore, D.G., and Raitt, R.W., History of the Northeastern Indian Ocean, in The Ocean Basins and Margins, Narain, A.M., and Stehli, Francis C., eds., Vol. 6, 1982, New York.
- Tapponier, R., Petlzer, G., LeDain, A.Y., Armijo, R., and Cobbold, P., Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine, Geology, Vol. 10, 1982, pp. 611-616.
- Chowdhury, A.N., Risch, D.L., and Hannan, A.E., Use of sequence stratigraphy in hydrocarbon prospecting: an example from the Green Canyon area, offshore Louisiana, Transactions of the Gulf Coast Association of Geological Societies, Vol. 54,1994.
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