The Upper Assam Basin is a poly history basin from where hydrocarbons are being produced for more than a century. Geologic setting of Assam-Arakan basin. Assam –Arakan Basin Geological Setting: It includes Assam, Nagaland, Arunachal Pradesh, Manipur, Mizoram and Tripura. The eastern limit is. Unconsolidated Sand Stone Reservoir to Eliminate Sand Breakthrough Problem Having Active Aquifer Drive: A Detailed Case Study in Assam-Arakan Basin.

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The Naga Metamorphic Complex, east of the Eastern thrust. Towards the close of Middle Eocene, limestone deposition ceased because of an increase in the influx of finer clastics bssin the shelf. Cap Rock and Entrapment There are three well developed regional cap rocks within the Tertiary sedimentary succession, the lower one, occurring in the Upper Eocene is the argillaceous Kopili Formation, the middle one is the Barail Coal-Shale Unit and the upper one, overlying the Tipam Sandstone is the Girujan Clay.

It may be mentioned that oil, generated in the Kopili and Barail source beds, accumulated in post-Barail sediments by vertical migration through such prominent faults.

In the Khoraghat oil field of Dhansiri valley, oil occurs in sandstone reservoirs within the Bokabil Formation Middle Miocene. The Ophiolite Complex, occurring in between the Moya and the Eastern thrust. During Middle to Late Cretaceous, when the Indian plate was moving northward, a number of horst and graben features developed on the granitic crust in the southern slope of the Shillong Plateau and Dhansiri valley. This foredeep was the site of deposition of the Surma Group of sediments under shallow marine lower part to brackish bazin upper part environments.

It is expected that the source sequences within the Kopili and Barail formations in the subthrust would be at the peak oil generating state.

Assam Arakan Basin

The average TOC ranges of different formations shale samples are as follows:. In the Naga Schuppen zone, apart from the Digboi oil field, two more major oil fields, namely, the Kharsang field, having oil accumulations in Upper Miocene to Pliocene reservoirs and the Champang field, having oil accumulations in fractured granitic basement rock of Precambrian age, have been discovered.

The area within the Upper Assam shelf, having high petroleum potential, measures approximately sq km and contains about m thick sediments of mostly Tertiary period, and the area in the basinal part with moderate to high hydrocarbon potential measures about 60, asswm km and contains more than 10,m thick sediments of mostly Tertiary period.

The formation of the trench system was, possibly, initiated in the northeastern part and gradually progressed southward. From the Digboi, the shelf runs westward to the southern slope of the Shillong plateau.

In these grabens, a sequence of sandstones, shales and subordinate limestone towards top, assigned to the Khasi Group, was deposited in the southern slope of the Shillong Plateau, and a sequence of sandstone and shale, assigned to the Dergaon Group, was deposited in the Dhansiri valley.


The dominant organic matter type is structured terrestrial. The Dhekiajuli Formation, consisting of mostly soft sandstones, was deposited at the mountain fronts in the Upper Assam Shelf and in areas now overridden by younger Naga thrust.

This event was followed by a pronounced south to southeastward tilt arakn the basin, mostly the geosynclinal part, which was, perhaps, caused by subduction related tectonic loading. Major tectonic elements of the basin are: In this shelf—slope—basinal architecture, the hinge zone, at and across which the Upper Cretaceous-Eocene shelf facies changes over to basinal arakaj, is envisaged to lie below the Naga schuppen belt. In the Khoraghat and Nambar fields situated in southeastern part of the Dhansiri Valley, oil occurs in the Bokabil Formation Middle Miocene which is not developed towards north in the Brahmaputra valley.

Oil within the Kopili Formation composed predominantly of shales with subordinate sandstone occurs in strati-structural combination traps, as in the Geleki field. Further increase in the clastic influx in the stable shelf assa, Late Eocene to Early Oligocene resulted basiin marine regression with the deposition of the upper part of aseam Kopili Formation, consisting of shales, siltstone and subordinate sandstones, in shallow marine to pro-delta environments.

The basinal geosynclinal part is occupied by the Cachar, Tripura, Mizoram and Manipur fold belts. The basin covers an area of Sq. During this time, sedimentation in the Surma basin including Sylhet trough and the Arxkan synclinorium took place in intermontane basins, depositing the arenaceous Lower Dupitila sediments over a post—Girujan unconformity and the argillaceous Upper Dupitilas over a post-Lower Dupitila unconformity.

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However, the area to the north of the Brahmaputra river up to the Eastern Himalayan foothills has remained poorly explored. The Himalayan Foredeep zone north of the Brahmaputra river lies in the northern periphery of the foredeep is overridden by the arakwn moving thrust sheets of younger sedimentary rocks.

Barail Coal-Shale Unit in the Schuppen belt also form important source rock sequence. The Assam-Arakan Basin is azsam in arskan northeastern part of India categorized as category-I basin. Following the deposition of the Tura Sandstone, there was a wide spread marine transgression in which the Sylhet Limestone Middle Eocene was deposited almost all over the Upper Assam Shelf. In the Digboi and Kharsang oil fields, oil occurs in Tipam Sandstone and Girujan Clay formations, respectively, overlying the Naga thrust.

The oilfields, discovered so far, are situated mainly in the areas south and southeast of the Brahmaputra river and a few in the thrust belts, associated with Naga-Patkai hills.

Oil within the Girujan Clay Formation as in the Kumchai and Kharsang fields also occurs in combination traps, but here the control of lithology on accumulation is more than that of structure.

The Girujan Clay being floodplain deposits, the reservoir sands are generally lenticular and some what erratic in distribution. Presence of pelagic fauna indicates that these sediments were deposited in shallow shelf to open marine conditions during Maestrichtian to Early Paleocene time. Upper Assam Shelf Southerly to southeasterly moving thrust sheets of younger Miocene to Plio-Pleistocene sedimentary rocks in the Assam Himalayan foothills.


Oil accumulations within the Bokabil Formation Middle Raakan in the Khoraghat and Nambar fields of the Dhansiri valley, occur in structural combination traps. It is envisaged that the undiscovered oil would continue to be found in structural, strati- structural and subtle traps in areas mostly bordering the Naga thrust and in the Naga Schuppen belt.

Organic matter is mainly humic and sapropelic. During the post- Oligocene time, however, different parts of the mega basin witnessed different evolutionary trends, mostly under compressive tectonic forces.

The shelf part of the basin spreads over the Brahmaputra valley and the Dhansiri valley, the latter lying between the Mikir assa, and the Naga foothills. During Eocene to Oligocene, due to the rise of the peripheral arc system rise of the basement ridge consequent upon the active oblique subduction of the Indian plate, the intervening sea became progressively narrower southward.

The shelf-to-basinal slope, i.

During this period, the Assam Shelf was being evolved in axsam passive margin tectonic setting and under shallow marine to brackish water sedimentation conditions.

In some of the exploratory wells, drilled in the southwestern part of the Dhansiri valley, good shows of oil have been observed in Eocene and Oligocene sands. All the oil and gas fields, discovered till date in the Upper Assam shelf, are situated mostly on the southeastern slope of the Brahmaputra arch, and almost all the major oil fields like Nahorkatiya, Lakwa, Lakhmani, Geleki, Dikom Kathaloni etc.

Some of the prominent faults, particularly those near the Naga thrust, are reverse faults, e.

Assam Arakan Basin Basin Introduction:. These lateral tectonic movements were accompanied by upliftment and total withdrawal of the sea, heralding the onset of continental sedimentation the Tipam Sandstone Formation on the Assam Shelf as well as on the earlier basinal area.

Assam Arakan Basin | NDR – National Data Repository India

The development of the frontal foredeep in front of the rising Himalaya, during Mio-Pliocene and later times, due to tectonic loading by thrust slices was filled with coarser sediments. Disang shales, occurring in association with ophiolites, are somewhat metamorphosed here. The Shillong Plateau and Mikir hills Uplift, composed mostly of Precambrian bason and metamorphic rocks. The sequences can be divided into shelf facies and basinal geosynclinal facies.

In the subthrust, the source sequences occur at greater depths and, therefore, should be in a higher state of thermal maturity.