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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Other significant milestones in oil exploration in Upper Assam were the discoveries of the Nahorkatiya, Moran and Rudrasagar oil fields inandrespectively.
Assam Arakan Basin | NDR – National Data Repository India
The metamorphic complex occurs mostly to the east of the Indo-Myanmar international border. It is expected that the source sequences within the Kopili and Barail formations in the subthrust would be at the peak oil generating state. This event was followed arxkan a pronounced south to southeastward tilt of the basin, mostly the geosynclinal part, which was, perhaps, caused by subduction related tectonic loading.
Cap Rock and Entrapment There are three well developed regional cap aseam 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.
During this time, sedimentation in aakan Surma basin including Sylhet trough and the Kohima 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. 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 Agakan shales.
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 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 part of the basin spreads over the Brahmaputra valley and the Dhansiri valley, the latter lying between the Mikir hills and the Naga foothills. The closing of the trench system was also initiated in the northeast and then gradually progressed southward.
Assam Arakan Basin
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. Geochemical analysis of exposed sediments from the Schuppen belt show a TOC range of 0. From the Digboi, the shelf runs westward to the southern slope of the Shillong plateau.
The basinal geosynclinal part is occupied by the Cachar, Tripura, Mizoram and Manipur fold belts. The limestones with negligible impurities were, perhaps, deposited on sea mounds.
Most of these hydrocarbon traps, particularly those developed in post- Barail sediments, orient parallel to the Naga thrust. In the North Bank of the Brahmaputra river, however, environmental conditions were deltaic with the deposition of sandstones with minor shales and siltstones. Disang shales, occurring in association with ophiolites, are somewhat metamorphosed here.
Home Assam Arakan Basin. Oil within the Kopili Formation composed predominantly of shales with subordinate sandstone occurs in strati-structural combination traps, as in the Geleki field.
Oil exploration in India commenced with the discovery of the Digboi oilfield in Upper Assam more than years ago, when, based on surface oil shows, a well was drilled on an exposed anticline, associated with the Naga thrust. During shallowing of the sea in the basinal area, the succeeding sediments of the Barail Group were deposited under environments ranging from moderately deep marine to deltaic. The average TOC ranges of different formations shale samples are as follows:.
These clastics, making the lower part of the Kopili Formation, were deposited in open marine conditions during Late Eocene, when marine transgression was waning out. The Andaman trenchwhich has been receiving mostly argillaceous sediments since, possibly, Upper Cretaceous-Paleocene, is the southward extension of the Indo — Burmese trench system.
The most prominent structural depression was formed in Kumchai — Manabhum area in front of the Mishmi uplift, where the Girujan Clay Formation attains a thickness of about m. 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. Continued indentation by the Indian plate caused westward propagation of tectonic forces, which in turn caused development of a decollement thrust at the base of the Upper Disang shales, and a number of synthetic thrust faults.
Further increase in the clastic influx in the stable shelf during Late Eocene to Early Oligocene resulted in marine regression with the deposition of the upper part of the Kopili Formation, consisting of shales, siltstone and subordinate sandstones, in shallow marine to pro-delta environments. 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.
This element contains most of the oil fields of the Upper Assam Shelf.
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. Oil reserves in Kopili sandstones Upper Eocene are found mainly in the Geleki field. But, bwsin Dhansiri Valley shelf, areas north of Lat.
Towards the close of Middle Eocene, limestone deposition ceased because of an increase in the influx of finer clastics in the shelf. Some of the prominent faults, particularly those near the Naga thrust, are reverse faults, e.
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. The generalized stratigraphic succession.