Assam Basin

Assam Basin This is one of the two principal petroliferous basins of India. The shape of the relatively narrow NE-SW trending Assam Basin is controlled by mountains, hills and plateaus. The Main Boundary Thrust (MBT) of the eastern himalayas forms its northern boundary while the shillong plateau and the Mishmi Hills are on the south. The Mikir Hills and Mijo Hills bound it on the east and it is open to the Sub-Himalayan foredeep occupied by the Indo-Gangetic plain in the west.

The basin is filled up with the sediments deposited by the Brahmaputra river system which forms a huge alluvial tract. In Upper Assam, the brahmaputra river flows in a south-westerly direction before taking a westerly turn north of the Shillong Plateau, then flows to the south near Dhubri and finally joins the ganges river near Goalandaghat. Morphological features developed within and around the basin reveal that the surrounding landmasses control the morphology and the river courses. The two great earthquakes of 1897 and 1950 have affected the basin morphology to a great extent.

An earthquake of big magnitude (1950) changed the course of the Brahmaputra river and its surface relief, and widened the river from 2.5 km to 10 km, causing frequent flooding. It is evident from the morphological study that the Assam Basin is being raised up and is getting narrower due to compression from the Naga Hills and Arunachal Himalayas. Migration of the Brahmaputra and the Buri Dihing rivers has been caused due to upliftment of the Naga Hills. Narrowing of the river valley is distinctly displayed in the area north of the Shillong Plateau, especially between the Mikir Hills and Itanagar Mountain Ranges where the Siwalik foothills clearly show migration of the river valley. The exposed Pre-Cambrian rocks across the river are only 35 km south of the Bhutan foothills.

The Assam Basin is filled with about 5 km of terrigenous Tertiary sediments above the Pre-Cambrian igneous and metamorphic basement. The total sedimentary thickness may exceed 7 km in the north near the Himalayan foothills.

In the Assam Basin the Tertiary sequence has been classified into several rock-stratigraphic units on the basis of gross lithological characteristics, heavy mineral assemblages, wire line log signatures and elastic properties. A major unconformity separates the Tertiary sequence into two Super Groups - Naga and Brahmaputra. Shallow marine to parallic sediments of the Palaeogene time consisting of the Jaintia Group and Barail Group belong to Naga Super Group which includes the Teok Formation, Sylhet limestone (Middle Eocene) and Kopili Formation (Upper Eocene) and the Barail Group (Oligocene) comprising of Naogaon Stage and Rudrasagar Formations. Fluvial and deltaic Neogene sediments of the Brahmaputra Super Group consist of the Surma Group (Lower Miocene), tipam group (Middle Miocene) and Moran Group. The surma group includes the Geleki Sandstone while the Tipam Group comprises 3 formations - Lakwa Sandstone, Girujan Clay and Nazira Sandstone. The Moram Group has two formations - Namsang and Dekiajuli. The Geleki Sandstone is considered equivalent to the Surma Series of the type area in the Surma valley.

Due to basin wide subsidence, the Assam Basin was slowly invaded by the Palaeocene sea with development of Tekok Formation followed by carbonate rock of Sylhet limestone (90-270m) and Kopili shale (350-460m), successively of the Middle and Upper Eocene age. The Assam Basin was separated from the Burmese Basin by upliftment of the Arakan-Chin Hills due to basin wide positive movement at the onset of the Oligocene age. Naogaon Sandstone (180-670m) was deposited in brackish water environment while Rudrasagar Formation (30-520m) consisting of shale and coal was laid in a brackish lagoonal condition of deposition

At the end of the Oligocene, a complete withdrawal of the sea commenced, followed by widespread erosion giving rise to a pronounced unconformity. Geleki Sandstone (200-780m) was deposited in brackish water condition followed by Lakwa Sandstone (160-550m). In the southern part of the shelf area, a thick sequence of fresh water mottled clays - Girujan Clay (0-850m) was deposited. During the middle Miocene, Nazira Sandstone (0-580m) was deposited in a fluviatile environment. Also, the Namsang Formation (250-520m) consisting of sandstone and clay was overlain by Dekiajuli Sandstone deposited in a fluviatile condition. Here the thickness of Holocene alluvial deposits varies from 300-650m.

Occurrence of oil in the Assam Basin was known as early as 1825 where Digboi is the largest oilfield. The current production from the Digboi field is about 240 barrels/day where more than 1,000 wells have been drilled so far. Naharkatiya, Morane, Rudrasagar are the other major oilfields of the Assam basin. [DK Guha]