Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010 Geothermal Potential in Bangladesh - Results from Investigations of Abandoned Deep Wells Debendra K. Guha 1, Herbert Henkel 2, Badrul Imam 3 1 Gono University, Dhaka, Bangladesh, email: [email protected] 2 Department of Land and Water Engineering, Royal Institute of Technology, Stockholm, Sweden, email: [email protected] 3 Department of Geology, University of Dhaka, Dhaka, Bangladesh, email: [email protected] Keywords: Tectonic frame, litho-stratigraphy, temperature, subsidence in the west, and rapid subsidence in the central thermal gradients part with active folding in the east. The volume of accumulated sediments is over 4 million cubic kilometres ABSTRACT with a present estimated rate of deposition of about 2.5 billion metric tons per annum (Khan 1991). Temperature data for a large number of abandoned deep wells, once drilled for oil and gas exploration, have been The foredeep formed south of the Main Boundary Thrust collected, together with results from other sources for (MBT) of the Eastern Himalayan Ranges due to the geothermal conditions. These depend on the ongoing subsidence of the Indian Shield, occupies the north-western tectonic activity in parts of Bangladesh. Rapidly filling part of Bangladesh, where basement is reached at 2500 m deep sedimentary basins represent volumes with low -1 depth at Salbanhat well in Tetulia. The Shillong Plateau thermal gradients (ca 20 K km ), while rising regions with consisting of Garo-Khasi-Janitia-Mikir Hills of Meghalay shallow crystalline rocks represent volumes with (India) with about 2000 m elevation, form the north-eastern considerably higher thermal gradients. Several deep drill boundary of Bangladesh. In contrast the down faulted holes in the SE slope of the Rangpur Saddle have gradients -1 Surma Basin / Sylhet Trough contains up to 20 km of of ca 35 K km . This region is currently under further sediments. The major tectonic elements of Bangladesh thus investigation with respect to the hydraulic conductivity of include the following units – see Figure 1. their deep stratigraphic formations. In the central part of the Rangpur Saddle local high groundwater temperatures are observed, especially in connection with graben structures in the crystalline basement where Gondwana age coal containing sedimentary units make up the graben fill. At the base of the coal beds the groundwater temperature is in excess of 50 oC. Further studies are planned in order to determine the thermal gradient, the stress and the fracture status of the rising crystalline basement. A third geothermal target region is the fold belt in the SE part of Bangladesh, where locally warm springs occur. A geothermal map of Bangladesh is presented here for the first time. The geothermal investigations are a joint project between the Royal Institute of Technology, Sweden and the Department of Geology at the Dhaka University in Bangladesh. The project is sponsored by the Futura Foundation in Stockholm. 1. GEOLOGICAL BACKGROUND Bangladesh located at the head of the Bay of Bengal, is formed by the successive delta systems developed by the two great rivers of the Ganges (Padma) and the Brahmaputra (Jamuna). The Himalayan mountain ranges were uplifted as a result of the collision with the northward drifting Indian Plate after the break-up of the Gondwana Continent starting in Upper Jurassic / Lower Cretaceous time. The pre-collision geology of Bangladesh can only be studied in north-western Bangladesh, where the continental Gondwana sequence with alternating carbonaceous sandstone and coal seams were preserved in graben structures of which the shallowest deposits can be seen in Fig.1. Tectonic map of Bangladesh, updated from Guha the Barapukuria graben at depths of only 117 m. Here the (1978), showing the major tectonic units Gondwana (Permian) sediments overlie the Achaean discussed in the text. basement which and in turn are unconformably overlain by The Indian Shield/Platform contains Achaean rocks and the Plio-Pleistocene deposits of Dupi Tila Sandstones and forms the western margin of Bengal Basin. Vast areas of Madhupur Clay under a thin alluvial cover. The post- West Bengal (Midnapur), Bihar (Singbhum) and Orissa collision geology is a record of uplift in the north, slow 1 Guha et al have basement outcrops containing gneisses, schists and of Plio-Pleistocene age. It is evident from seismic data granites. acquired by the Oil and Gas Development Corporation (OGDC) during 1963-64 in the Rangpur - Dinajpur The Bengal Foredeep occupies the vast area between the districts, that both the northern and the southern slopes of Hinge Line and the Arakan Yoma Folded System and plays Rangpur Saddle are quite gentle. In the southern slope, the the most important role in the tectonic history of Bengal basement plunges gently from Madhyapara towards the Basin. The Bengal Foredeep can be divided into two major south east up to the Hinge Zone, an area known as the regions: a Western Platform Flank and an Eastern Folded Bogra shelf. Figure 2 shows an extention of the Tectonic Flank. The Western Platform flank is further subdivided map of Bangladesh covering NW Bangladesh with some into the Faridpur Trough, the Barisal-Chandpur High, the further structural features added. Figure 3 shows a Hatiya Trough, the Madhupur High and Sylhet Trough. generalised thermal section across the Rangpur Saddle. The Arakan-Yoma Folded System: Only the western marginal part of the Arakan-Yoma Folded System has been shown in the tectonic map (Fig.1). The oldest rocks exposed in the Araka-Yoma belong to the axial system of Triassic age which are highly folded and thrusted and consist of dark shales, sandstones and limestones. A wide belt of Cretaceous and Eocene rocks stretches from the Assam-Myanmar border west of the upper reaches of the Chindwin River along the inaccessible mountainous tracts of the Arakan-Yoma down to Cape Negaris. This vast area of Tertiary folding represents a sub-meridional trending belt of uplifts composed of Pre-Cambrian, Paleozoic and Mesozoic rocks. It is essential to have clear conception about the structural tectonic framework of Bangladesh in order to evaluate the prospect of mineral resources including the geothermal energy potential. The first tectonic map of the then East Pakistan was prepared by in 1964. Continued improvements were made in connection with hydrocarbon prospecting. Concise characteristic of basic tectonic elements of the then East Pakistan and its adjacent areas prepared in 1965 (Guha and Kitovani 1965). The tectonic framework and oil and gas prospects of Bangladesh were published in Guha (1978 ).In the next chapters, the main geological regions are Fig. 2. Geotectonic map of NW Bangladesh. (the shortly described from the NW (Sub-Himalayan Foredeep) locations of T Thakurgaon, B Barakupuria, J to the SE (Arakan-Yoma Folded System), with comments Jairpurhat, M Madhyapara, that are mentioned on their geothermal prospects. in the text). The Garo-Rajmahal Gap is between the western and eastern S-N striking fault zones 2. THE TECTONIC UNITS AND THEIR (blue), which seen in the NW-SE direction (Fig. GEOTHERMAL PROSPECTS 3) as the Rangpur Saddle. Areas of recent uplift 2.1 Sub-Himalayan Foredeep. are light gray and outlined green. The region of Gondwana graben structures is dark gray and The Himalayan Foredeep lies south of the Main Boundary outlined brown. Thrust (MBT) all along the foothills of the Himalayas. At Salbanhat, at the NW tip of Bangladesh the basement An underground mine for extraction of hard rock is now in occurs at 2500 m depth. The temperature at this depth is 79 operation at Madhyapara. There are a number of graben oC. The Northern Slope of Rangpur Saddle has been structures in the south-western part of the Rangpur Saddle, included in the Himalayan Foredeep by Reiman (1986) in some of which have high grade sub-bituminous Gondwana his tectonic scheme of Bangladesh. The Neocene of the coal of Permian age. At Barapukuria, an underground mine Siwaliks are well developed in the Himalayan Foredeep and is in operation now. A feasibility study of Gondwana coal attains thickness of 3 to 4.5 km with predominantly deposits found in Khalaspir is yet to be completed, while sandstones, subordinate shales and clay and gravel beds. the feasibility study of Fulbari coal deposit is completed. Although the sedimentary units have high porosity, the thermal gradient is too low to make a geothermal prospect The Garo-Rajmahal Gap between the Indian Shield of in association with the only deep well at Salbanhat. Rajmahal and the Pre-Cambrian basement of the Garo Hills (Shillong Massif is the result of the Himalayan orogenic 2.2 Rangpur Saddle and the Garo – Rajmahal Gap movements during the Pleistocene along a NW-SE zone of fractures now occupied by the courses of the Ganges and The Rangpur Saddle represents the Indian Platform and Brahmaputra rivers. In the western extremity of the Garo connects the Indian Shield with the Shillong Massif and the Hills, there are a number of basement faults and monoclines Mikir Hills. The Shillong Massif is a large thrust block of trending N-S to NW-SE with persistent dips towards west. the Indian Shield. In the Rangpur Saddle the basement is The recent Dhubri earthquake epicentre was located along a most uplifted and covered with thin sedimentary deposits. fault plane in the western margin of the Garo Hills. This The tentative boundary of the Rangpur Saddle at the fault passing through Dhubri and trending approximately northern and the southern slopes has been seismically N- S has been traced by geophysical methods for 120 km. defined by the approximately 700 m depth contour. In the On the western part of Garo-Rajmahal gap a similar N-S Madhyapara area the basement occurs at only 130 m depth trending fault has been traced for 160 km from Kishengonj and is overlain by Dupi Tila Sandstone and Madhupur Clay 2 Guha et al to English Bazar in Malda.