User:Rghose1/sandbox

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Surma basin is the major gas producing province of Bangladesh. It is located in the northeastern part of Bangladesh, comprised of around 1,00,000 sq km area. The basin is bounded by Chittagong-Tripura Fold Belt of Indo-Burma Ranges to the east and southeast, Dauki fault and Shillong Massif to the north, Indian shield platform to the west and Bengal foredeep to the southwest. The main hydrocarbon reservoir in the basin is the Mio-Pliocene Surma Group.

 Evolution of the basin 

The major uplift of Shillong massif and the westward movement of Indo-Burma Range on Burma plate are responsible for the formation of Surma Basin. The basin subsided between Oligocene and Pliocene times. It became an active depocenter after it was separated from the stable shelf due to the upthrust movement along the Dauki Fault. The shelf was located on the Surma basin during the Eocene time. The shelf shifted to the south in early Miocene and delta plain dominated the basin in Middle to Late Miocene. The basin became an active delta plain in Pliocene. Eastern part of the basin is occupied with series of anticlines and synclines. These anticlines were developed due to the collision between India Plate and Burma Plate in Early to Middle Miocene time.

 Lithology 

Surma basin has accumulated around 20,000 m sedimentary fill of deltaic, estuarine and shallow marine sandstone, siltstones and shale. The Surma Group of Miocene to early Pliocene sediments was deposited in a mud rich delta to open marine environments. The thickness of Surma Group varies from 2700 to 3900 m. Surma Group is divided into two units: lower Bhuban and upper Bokabil Formations. Alternating sandstone, sitstone and mudstone dominate these formations. Lower Bhuban formation is mostly arenaceous and upper Bokabil Formation is mostly argillaceous.

 Petroleum System 

Carbonaceous shales of Eocene, Oligocene and Miocene age are the potential source rocks. The total organic carbon content varies from 0.5 to 3 percent in theses strata. The Miocene shales of Bhuban and Bokabil formation are gas prone. The total organic carbon content is 0.9%. These rocks went into gas maturation level three to four million years ago. Thermal maturation at a depth of 6 to 7.5 km is sufficient to generate gas and condensates. Little oil is found in a few wells of Surma Basin.

The N-S trending anticlines in the eastern part of the basin act as the hydrocarbon trap. Along with structural traps, combination trap is also present in the basin; i.e. sandstone-filled channel deposits or sandstone beds laterally sealed by shale-filled channels. Some geologists believe that the Miocene Bhuban Formation has little organic content to be a good source rock. According to their studies, the underlying Oligocene Barail formation is more organic rich and generated the gas that actually migrated updip and accumulated in the Bhaban and Bokabil sandstone reservoirs. Hydrocarbons were migrated vertically through the fractures and pores in the strata.

The Tertiary sandstone of the Bokabil and Bhuban Formations are the reservoir rocks in Surma Basin. These sandstones are dominantly fluvial, deltaic and estuarine deposits. Porosity of these rocks ranges between 10 to 20 percent. The regional hydrocarbon seal is a widespread shale unit overlying the Surma Group, which is named as upper marine shale (ums). Along with the regional shale unit, locally distributed shale units in the middle to lower parts of Surma Group may also act as a seal for the hydrocarbons. The first discovered hydrocarbon gas field of Bangladesh was discovered in Surma basin named the Haripur gas field.

 References 

1.Parvin, Afroza; Woobaidullah, A. S. M.; Rahman, Md Jamilur (2019-06-24). "Sequence stratigraphic analysis of the Surma Group in X Gas Field, Surma Basin, Bengal Delta". Journal of Nepal Geological Society. 58: 39–52. doi:10.3126/jngs.v58i0.24572. ISSN 2676-1378.

2.Wandrey, Craig J.; (2001). "U.S. Geological Survey–PetroBangla Cooperative Assessment of Undiscovered Natural Gas Resources of Bangladesh." U.S. Geological Survey Bulletin 2208-A

3. Islam, Md Shofiqul; Hosain, Mosarraf ; Jolly, Yeasmin Nahar; Hossain, Mohammad Shahedul; Akter, Shirin; Kabir; Jamiul (2015). "Geochemical Analysis of the Reservoir Rocks of Surma Basin, Bangladesh." Geosciences 2015, 5(1): 1-7 DOI: 10.5923/j.geo.20150501.01

4. http://petrobangla.org.bd/sites/default/files/files/petrobangla.portal.gov.bd/annual_reports/3be221b4_b3e9_4fd7_9321_64704f848b1a/2020-01-15-19-21-66e6d0df9e8f14341d4126bb5551b060.pdf