Geology of the western shelf area of Bangladesh: A short note

Bangladesh is located within one of the greatest fluvial deltas system in the world. The three major river system (Ganges- Brahmaputra- and Jamuna) transport sediments, supplied by the Himalayan fold belt, over a wide flood plain in to the delta. The present BengalBasin is consist of  major part of Bangladesh,  part of the IndianState of West Bengal in the west and Tripura in the east. The Bengal Basin is bordered to the west by the Precambrian Indian shield, to the north by the Shillong massive and to the east by the frontal belt of the Indoburman Range. In the south it is open to the Bay of Bengal.

Tectonic Framework Of Bangladesh

The evaluation of BengalBasin is resulted from the collision of  Indian plate and  the Eurasian plate.The major tectonic elements of Bangladesh include (i) Indian / Stable platform in the  northwest (ii) Arakan Yoma in the east and (iii) Deep basin to the southeast. In between stable platform and  basinal part of Bangladesh there is another important flexure narrow northeast- south west trending zone called  ” Slope/ Hinge zone” which extends from Kolkata to Mymensingh through  Pabna and separate the platform and Basinal part. Here in short the Geology of Platform and so called Hinge zone are mentioned (Fig-1).

Stable Platform

Stable platform occupies Dinajpur- Rangpur- Bogra- Rajshahi area with thin to thick cover of sedimentary rocks above Precambrian basement; Stable platform is divided into (i) Rangpur saddle in the north with thin to limited sedimentary cover above the Precambrian basement and (ii) Bogra shelf with moderate sedimentary cover over the Precambrian basement. Basement is the shallowest in Rangpur saddle and the thickness of sedimentary deposits is about 150m at the shallowest depth in Madhyapara. From there the basement plunges gently towards the southeast upto the close to hinge zone.

Slope/ Hinge Zone

The Gravity, Magnetic and Seismic data indicate the presence of a structural zone, extending from Kolkata to Mymensingh. This zone has been referred to as the Kolkata -Mymensingh Hinge zone/ slope and is thought to represent the margin of the Indian platform continental crust. Analysis of the Seismic data from India and northern Bangladesh indicates that during early Tertiary time this Hinge / slope actually represented a contemporary edge to the continental shelf and was the location of Major shelf edge Carbonate buildups during Eocene time. This zone is about 25km wide and   is bounded by South slope of Rangpur Saddle and Pabna – Mymenshing Hinge line .Regionally the Hinge / Slope trends from East of Kolkata and Ranaghat (West Bengal) into Bangladesh over 250Km in a more Southwest / Northeast direction passing Pabna to the south and the  Hazipur  location and bends at Mymensingh east – northeast, finally adjoining the present  Dauki Fault System at an acute angle close to Chattak.

Strati graphy

On the Indian shield, over Precambrian basement predominantly clastic sediment have been deposited in Intracratonic basin during Upper Carboniferous to Upper Jurassic time. Especially in the northeastern part of Indian peninsular they are preserved in many isolated deposit  called GondwanaBasin, however, many of them are tectonically demarcated and are erosional remains of originally much larger intracratonic basins.

The Precambrian basement hosts the so called Gondwana basins. These, initially much bigger basins are remains of Intercontinental grabens which consist of Lower Gondwana formation dominated by terrestrial organic matter ( coal) of upper Carboniferous and Permian age, followed by the influx of Triassic to Lower Jurassic clastic sediments (Upper Gondwana) during increasing lateral crustal stretching                 ( Chakrabarti and Mukherjee, 1997). The Gondwana sediments are interpreted to have been deposited in low- sinuosity braided fluvial systems  flanked by vegetated overbank and swampy floodplain areas under cool climate conditions ( Uddin and Islam, 1992). Sediments of upper Jurassic and lower Cretaceous age are generally missing in the greater region indicating the time of hiatus/ erosion during the breakup of the Gondwana continent. This period was followed by deposition of Rajmahal ash layer, representing a history of volcanic activities in the greater region during upper Cretaceous.

The Cenozoic period is characterised by the deposition of fluvial to shallow marine slope and deltaic sediments. This formation consists of shale to silt-rich formation with intercalated sandstones (Fig-2).

One of the master basin with regional extent might have stretched from the Raniganj and Rajmahal basin in Indian west Bengal across northwest Bangladesh as far as the Garo Hills in west Meghalaya . In Bangladesh its erosional remains are preserved in grabens and half grabens under Cretaceous and / or Tertiary cover. This have been encountered in several wells and can be traced along seismic line which allow to draw at least at rough picture on the occurrence and extent  of these structures.

Hydrocarbon Prospect

On the western Shelf pre- rift Lower Gondwana rocks of Carboniferous to Permian age comprise fluvio- deltaic sediments that infill a series of N-S and NNE- SSW trending half grabens. This western area did not take part in any significant tectonic movements, capable of giving rise to large potential hydrocarbon traps similar to those seen in the eastern part of the country. On the western shelf area the trapping potential is Structural-Stratigraphic and includes updip wedge out, pinchout traps- against top basement surface and truncation traps below the break-up unconformity.

In the lower shelf area, i,e, a few kilometer updip the hinge ,presumably upper Cretaceous clastic sequence subcrop in basinward strongly (Several 100m) thickening series of wedges. These wedges have basin ward a very favourable and mature hydrocarbon catchments area (For Oil presently even overcooked), provided, source rocks are available. However, this Cretaceous Subcrop  play has an updip seal risk of reservoirs (Hiller,1988).

In Bangladesh the maximum thickness of Gondwana sediments proven by drilling is 1200m in Singra well. The maximum measured maturity of the coal seam of Gondwana sediments in Singra well reached 1.58% Rm, which means over mature with respect to Oil generation but still within the gas generating phase.

The burial history gives information about the development of the maturity and the hydrocarbon generation during time. In the Singra / Kuchma area, it shows  further that at least 1,500m of sediments must have been eroded prior to the transgression and deposition of Cretaceous sediments (SK. A. Islam and G. Eichoff.  2001).

The previous thick cover of sediments and probably also Volcanic as well as a most likely higher geothermal gradient and heat flow, respectively, during Mesogoic time caused a significantly higher paleomaturity of the Gondwana coals in the Shelf area then one can calculated on the basis of the recent geological situation and by using the recent maturity trend.

Hydrocarbon generated from Gondwana sediments in the shelf area  and it escaped in pre- Creataceous time- except the coal seam gas and because no postcoalification took place, gas from Gondwana source rocks could be trapped in possible structure of the present sedimentory cover, which needs extensive investigation.

However, at present in the Sherpur/ Pabna/ Kushtia area the situation is rather different. Here the Gondwana sediments indicated by interpreting the seismic line and by extrapolating the geological situation from Indian West Bengal- must have reached their paleomaturity again in Miocen time. Thus, since then here a postcoalification and second phase of methen generation has been taking place. However, because most of the methen from coals has been generated up to their paleomaturity of about 1.6 Rm, there was/ is only an amount of gas left to be generated and subsequently captured in possible traps of Cretaceous/ or Tertiary age (SK. A. Islam & G. Eickhoff,2001).

The upper stable Shelf ie updip of the Bogra – Fault system, must be classified as nonprospective for the Cenozoic sediments; an average temperature gradient of around 30C / 100m and sedimentary thickness not more then 2 – 2.5Km leave the Upper Stable Shelf immature for Hydrocarbon generation, this part is considered to be situated in the migration shadow of the Lower Stable Shelf and its traps respectively. The Lower Stable Shelf, however, consider thickness of Cenozoic sediments close to the Hinge/ slope  has to be Judge as hydrocarbon prospective.

Adjacent to Slope/Hinge area Hazipur well was drilled where encountered residual oil shows which has given proof of migrating hydrocarbons. Mature depths a thermogradient of around 2.3 – 2.50C/100m and deep catchment area classify the seal risk stratigraphic play along and above the Sylhet limestone Slope/ Hinge zone not only as oil but also as gas / condensate prone (Hiller, 1988).