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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In the Naga Schuppen belt, oil accumulations in the Assa, and the Champang oil fields occur in that zone of the shelf which is overridden by the Naga thrust.
During this time, sedimentation in the Surma basin including Sylhet trough and the Kohima synclinorium took place in intermontane arqkan, depositing the arenaceous Lower Dupitila sediments over a post—Girujan unconformity and the argillaceous Upper Dupitilas over a post-Lower Dupitila unconformity. Presence of pelagic fauna indicates that these sediments were deposited in shallow shelf to open marine conditions during Maestrichtian to Early Paleocene time.
The limestones with negligible impurities were, perhaps, deposited on sea mounds.
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. In the Dhansiri valley also, oil fields like the Borholla and Araakn and Nambar lie in the same belt.
Assam Arakan Basin | NDR – National Data Repository India
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. The Indo-Burmese trench system that developed during the oblique subduction of the Indian plate below the Burmese plate became the locus of deposition of Upper Disang shales under deep marine conditions. The shelf-to-basinal slope, i. The Assam-Arakan sedimentary Basin is a shelf—slope—basinal system.
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. 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.
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. Geochemical analysis of exposed sediments from the Schuppen belt show a TOC range of 0.
In the North Cachar area, exploration by deep drilling is yet to be initiated. 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. During the growth of these structures, the Girujan Clay Formation was deposited in the synclinal lows structural basins in Cachar area as indicated by seismic and well data from the Katakhal syncline of Cachar area where the Girujan Clay Formation is named as the Govindpur Formation.
Assam Arakan Basin
Towards the close of Middle Eocene, limestone deposition ceased because of an increase in the influx of finer clastics in the shelf. The Assam — Arakan Fold Belt This fold belt may be divided into two zones bounded by prominent thrusts, viz, i the Naga fold zone, lying in between the Disang and Tapu thrusts and having exposures of Disang shales and Barail sediments, and ii the central flysch zone, lying between the Tapu thrust and Changrang — Zunki thrust and having exposures of mainly Disang shales.
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.
In the Borholla field of the Dhansiri valley and Champang field of the neighbouring schuppen belt, oil reserves occur in structurally controlled subtle trap in fractured basement rocks. During this period, the Assam Shelf was being evolved in a zrakan margin tectonic setting and under shallow marine to brackish water sedimentation conditions.
Some of the prominent faults, particularly those near the Naga thrust, are reverse faults, e. 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.
This event was followed by a pronounced south to southeastward tilt of the basin, mostly the geosynclinal part, which was, perhaps, caused by subduction related tectonic loading.
During the last decade, oil and gas accumulations have been discovered within the Langpar and Lakadong formations of Paleocene to Lower Eocene age in several structures like Dikom, Kathaloni, Tengakhat, Tamulikhat, Shalmari, Baghjan, Panidihing, etc. In the North Bank of the Brahmaputra river, however, environmental conditions asszm deltaic with the deposition of sandstones araka minor shales and siltstones. This site is best viewed using current versions of Firefox v.
Organic matter richness of shales increases towards the Naga thrust. In the Borholla oil field, oil occurs in fractured granitic basement rock of Precambrian age. In this shelf—slope—basinal architecture, the hinge zone, at and across which the Upper Cretaceous-Eocene shelf facies changes over to basinal facies, is envisaged to lie below the Naga schuppen belt.
During Paleocene, there was a marine transgression on the southern edge of bazin Shillong Plateau, depositing sediments of the Therria Formation consisting of limestone, sandstone and shale.
East of the hinge zone, i. 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.