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 shelf-to-basinal slope, i. Organic matter is mainly humic and sapropelic. 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.
Upper Assam Shelf Southerly to southeasterly moving thrust sheets of younger Miocene to Plio-Pleistocene sedimentary rocks in the Assam Himalayan foothills. Geochemical analysis of exposed sediments from the Schuppen belt assamm a TOC range of 0. 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.
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. The Girujan Clay being floodplain nasin, the reservoir sands are generally lenticular and some what erratic in distribution. The Assam — Arakan Fold Belt This fold belt may basij divided into two zones bounded by prominent thrusts, viz, i the Naga fold zone, lying in between the Disang and Tapu thrusts and araakan exposures of Disang shales and Barail sediments, and ii the central baisn zone, lying between the Tapu thrust and Changrang — Zunki thrust and having exposures of mainly Disang shales.
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.
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.
Recently gas has been discovered in the sandstone reservoir of Dirok structure within Girujan Formation. Disang shales, occurring in association with ophiolites, are somewhat metamorphosed here. The southeastern slope of the Upper Assam Shelf, southeast of the Brahmaputra arch, having local structural highs and lows, upto the Naga thrust, and extending 8 to 10 km beneath the Naga schuppen belt.
Home Assam Arakan Basin. In the Khoraghat oil field of Dhansiri valley, oil occurs in sandstone reservoirs within the Bokabil Formation Middle Miocene. 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.
However, the area to the north of the Brahmaputra river up to the Eastern Himalayan foothills has remained poorly explored. 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.
In the Digboi and Kharsang oil fields, oil occurs in Tipam Sandstone and Girujan Clay formations, respectively, overlying the Naga thrust. The Ophiolite Complex, occurring in between the Moya and the Eastern thrust. This site is best viewed using current versions of Firefox v. The Assam-Arakan Basin is situated in the northeastern part of India categorized as category-I basin.
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.
Towards the close of Middle Eocene, limestone deposition ceased because of an increase in the influx of finer clastics in the shelf. East of the hinge zone, i. In the subthrust, the source sequences occur at greater depths and, therefore, should be in a higher state of thermal maturity.
Oil accumulations within the Bokabil Formation Middle Miocene in the Khoraghat and Nambar fields of the Dhansiri valley, occur in structural combination traps. 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 North Bank of the Brahmaputra river, however, environmental conditions were deltaic with the deposition of sandstones with minor shales and siltstones. 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.
The generalized stratigraphic succession.
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. 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.
Oil reserves in Kopili sandstones Upper Eocene are found mainly in the Geleki field.
Most of the prominent faults continue upward into post-Tipam sediments, and the rest die out in the lower part of the Tipam Formation. In the Dhansiri valley also, oil fields like the Borholla and Khoraghat and Nambar lie in the same belt. 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 formation of the trench system was, possibly, initiated in the northeastern part and gradually progressed southward.
The basin covers an area of Sq.
Assam Arakan Basin
Major tectonic elements of the basin are: It is expected that the source sequences within the Kopili and Barail formations in the subthrust would be at the peak oil generating state.
The average TOC ranges of different formations shale samples are as follows:. Assam Arakan Basin Basin Introduction:. 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. 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.
Assam Arakan Basin | NDR – National Data Repository India
Organic matter richness of shales increases towards the Naga thrust. This element contains most of the oil fields of the Upper Assam Awsam. The Andaman trenchwhich has been receiving mostly argillaceous sediments since, possibly, Upper Cretaceous-Paleocene, is the southward extension of the Indo — Burmese trench system. 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.