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The folded belt of the northeast Indian states of Tripura and Mizoram and the adjoining parts of Assam (Cachar district) and Manipur constitute a part of the Assam-Arakan geosynclinal basin and lie between the present-day foredeep of Bangladesh and the hinterland of Burma. This region, with its characteristic succession of synorogenic ridges and valleys, can be subdivided into a frontal subbelt of Tripura and south Cachar comprising narrow, boxlike, and cuspate anticlines separated by wide, flat synclines, and an inner mobile subbelt of Mizoram and west Manipur consisting of tight, linear, commonly isoclinal anticlines and synclines, festooned into salients and reentrants. Passing from east to west, deformation in this belt becomes progressively younger and less intense. he anticlines are commonly bounded on one or both flanks by longitudinal listric reverse faults. The individual structures are internally segmented by cross faults and oblique faults of multiple alignments, some of which have strike-slip components and have offset anticlinal axes and flank faults. These multidirectional trends combine at places to form doglegs and trap doors, disrupting a more general north-south relay pattern, and indicate polyphase deformation with structural styles grading from those associated with basement-involved compressive block-faulting to detached thrust-fold assemblages and further modified by shale flowage. Such compressional tectonic styles are characteristics of areas close to convergent plate boundaries.
The Tripura-Mizoram region exposes mainly Neogene-age clastics of molassic facies, comprising about 6 km (20,000 ft) thick, poorly fossiliferous succession of alternating shales, mudstones, siltstones, and sandstones in varying proportions, which have been lithostratigraphically subdivided from the bottom upward into Surma (Miocene-Pliocene), Tipam (upper Pliocene), and Dupi Tila (Pleistocene) Groups, with conditions of deposition ranging from shallow marine/deltaic at the bottom to fluviatile/lacustrine at the top.
Numerous surface and subsurface manifestations of oil and natural gas in Tripura, Cachar, and Mizoram areas, occurrence of gas fields in the neighboring areas of Bangladesh, and a favorable geologic history indicate the existence of conditions conducive to petroleum generation in this region. On the basis of geochemical studies, it has been concluded that there are good prospects of striking gas in the relatively shallower zones and both oil and gas in the deeper zones in several structures of Tripura and Cachar. The expected hydrocarbon traps include discrete culminations and trap doors in individual anticlines, cross-faulted noses, subsidiary flexures in synclinal areas, sub-thrust warps against block-bounding faults, and various associated stratigraphic traps, as well as combinatio traps resulting from clay diapirism. However, on account of logistic constraints, unfavorable subsurface conditions (including steep dips and high pressures), inherent stratigraphic and structural complexities, and a paucity of seismic data, exploratory operations conducted in this region have had limited success. An accelerated exploration program with the help of improved techniques and additional resources is being initiated.
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