Gas Seepages from Atharamura Anticline, Tripura, NE India
Omnath Saha, Watitemsu, J.M. Umlong, P. Panja, S. Kumar, Geologists;
J. Pramanik, Sr. Geologist; N. Padmaiah, Director
Geological Survey of India, State Unit: Tripura & Mizoram, North Eastern Region, Agartala- 799 006
---------------------------------------------------------------------------------------------------------------Tripura state covering an area of 10,477 sq. km and located at the SW margin of the
northeastern India is well known for its hydrocarbon potential. Tripura Fold Belt (TFB) is
represented by immature first order topography; the topographic highs are longitudinal ridges
formed by regional scale N-S trending anticlines, while the intervening low lying areas
represent the corresponding synclinal valleys. Five distinct anticlinal hill ranges with broad
synclinal troughs constitute TFB. Major anticlinal hill ranges include Baramura, Atharamura,
Langtarai, Sakhan and Jampui from west to east. Atharamura anticline is the longest i.e. ~
100 km and well exposed in TFB (Brahma et al., 2013).
TFB forms a part of the frontal folded belt of Assam-Arakan region and exposes mainly thick
pile of molassic sediments of poorly fossiliferous successions of Surma, Tipam and Dupitila
rocks of Neogene age. A series of north-south trending, sub-parallel, doubly plunging tight
anticlines are disposed in an en-echelon pattern with slight convexity towards west; these
anticlines are delimited by longitudinal reverse faults on one or either limbs. Numerous
younger transverse or oblique faults of multiple alignments with strike slip component offset
the longitudinal faults and fold axes (Khar and Ganju, 1984; Kunte, 1989).
In course of geological mapping during field season 2013-14 & 2014-15 in the southern
extension of Atharamura Anticline in Gomati and Dhalai districts, Tripura, two gas seepages,
hitherto unreported, have been noted i.e. GS 1 and GS 2 (Fig. 1). GS 1 is noted on the eastern
tributary of Ampi Chhara, about 1.5 km NW of Kushirampara, Gomati district, Tripura
(23°40´24´´/91°44´12´´). Gas bubbles emanate through N-S trending grey splintery shalesiltstone interbeds of the Upper Bhuban Member of Bhuban Formation of Surma Group; this
is the oldest rock unit exposed and occupies the core of Atharamura Anticline. Profuse
effusions of gas bubbles along the river bed are the surface manifestations (Plate 1, a-b).
GS 2 is located near the headwaters of Ekjan Chhara (23°37´42´´/91°44´14´´) about 2 km
northwest of Chandrakumarpara, Gandacherra, Dhalai district, Tripura. It can be approached
from Gandacherra by a jeepable road upto Chandrakumarpara, Dhalai district. Beyond that, it
can only be approached through dissected terrain along a foot track for about 4 km towards
northwest from Chandrakumarpara. Gas emanates through N-S trending fractures along the
anticlinal axial trace within dark grey splintery shale and siltstone interbeds of Upper Bhuban
Member (Plate 1, c-d). Beds are subvertical and highly jointed. In both the gas seepages (GS
1 & 2), discharge of a faint organic foul smell akin to H2S is common. The rate of gas
discharge of the latter is relatively higher than that of the former.
Geological Survey of India, North Eastern Region, Agartala
Besides the above, another gas seepage reported by S.N. Sen (1958) is also located in the area
covered by specialized thematic mapping. This lies to the south of the former gas seepages at
Laban Chhara, a tributary of Mailak Chhara (GS 3, 23°33´35´´/91°44´03´´). It is about 8 km
ENE of Amarpur, a subdivision of Gomati district. The gas emanates here through E-W
trending joints / fractures in a moderately westerly dipping (25°) sandstone of Upper Bhuban
Member on the western limb of the Atharamura Anticline (Plate 1, e-f). Sandstones are bluish
grey, well indurated and medium bedded with subordinate siltstone and shale. Minor
discharge of hot water is also noted in the vicinity emitting foul smell of sulphur. This hot
water is slightly saline and so the name ‘Laban Chhara’.
These gas seepages are solely confined to all along the hinge zone of the Atharamura
Anticline. The axial part of the Atharamura is structurally highly disturbed due to N-S strike
parallel longitudinal faults; this fault system coupled with the weak and fractured nature of
the axial part probably acts as a conduit for the natural gases to escape to the surface. It is
imperative to undertake further detailed investigation to exploit these untapped natural
resources.
Geological Survey of India, North Eastern Region, Agartala
Fig. 1: Gas seepages from Atharamura Anticline, Gomati district, Tripura.
Geological Survey of India, North Eastern Region, Agartala
Plate 1: Emission of gas bubbles and gas flames i) Ampi Chhara (GS 1, a-b), ii) Ekjan Chhara
(GS 2, c-d), and iii) Laban Chhara (GS 3, e-f).
References
Brahma, J., Sircar, A., Karmakar, G.P., 2013. Hydrocarbon Prospectivity in Central part of
Tripura, India, using an integrated approach. Jour. Geography and Geology, 5 (3), pp.
116-134.
Khar, B.M., and Ganju, J.L., 1984. Tectonic of Tripura folds - probable mechanics of folding
and faulting. Petroleum Asian Jour., 1, pp. 66-77.
Kunte, S.V., 1989. Tripura-Cachar fold belt genesis of structural style. ONGC Bulletin, 26
(2), pp. 1-15.
Sen, S.N., 1958. A note on the occurrence of natural gas in Tripura state. Geol. Surv. Ind.
Unpubl. Rep. Field Season 1956-57.
Geological Survey of India, North Eastern Region, Agartala