Low-Temperature Geothermal Systems in Sedimentary Basin and Their Prospect in Bangladesh

Geothermal systems are distributed throughout the world in various ways on the basis of heat source, heat transfer, reservoir temperature, physical state, utilization and geological settings. Common classification of geothermal systems are: (a) volcanic systems with the heat source being hot intrusions or magma chambers in the crust, (b) convective systems with deep water circulation in tectonically active areas of high geothermal gradient, (c) conductive sedimentary systems with permeable layers at great depth (2-5 km), (d) geopressured systems often in conjunction with oil resources, (e) hot dry rock or EGS systems where abnormally hot masses of low permeability rocks are found at drillable depths, and (f) shallow resources in normal geothermal gradient areas utilized with ground-source heat pump applications. 50 years ago a classification for geothermal systems was proposed in Iceland which were divided into high- and low-temperature geothermal systems or fields based on (arbitrarily) inferred temperature, high temperature fields where a temperature of 200°C is reached at 1 km depth and low temperature fields where temperature is below 150°C in the uppermost km. The high-temperature geothermal fields are all related to volcanism whereas the low-temperature geothermal fields draw heat from the general heat flow of the crust. The geothermal systems are suitable for various applications based on subsurface temperature and fluid. Low-temperature geothermal system is directly related to the sedimentary basin which has thin conductive permeable sedimentary layers. Most of the low-temperature geothermal energy used as heating system in the winter dominant countries around the world. Bangladesh is one of the largest deltas which has a large sedimentary basin. Little prospect of geothermal resources in Bangladesh due to its geo-tectonic settings as well as thick sedimentary covers (130 m -20 km). Bangladesh has no volcanic regions, although in the east it lies close to the Eurasian-Indian plate boundary, which may be associated with high geothermal gradients. It has many abandoned gas wells which temperature is more than 100°C in the depth of 3 km to 4 km. The geothermal prospects, geological mapping of surface geothermal manifestations as well as low-temperature surveys are essential to evaluate the geothermal resources in Bangladesh.