Geology and coal bed methane resource potential of the Gondwana Barapukuria Coal Basin, Dinajpur, Bangladesh

摘要

With an area of 5.16 km~2, the Barapukuria coal deposit is one of the five largest Gondwana coal basins in Bangladesh, and is located in the north west of the country close to the towns of Dinajpur and Saidpur. The existence of the basin was initially indicated by a negative gravity anomaly in oil and gas exploration. Exploration for the deposit was commenced by the Geological Survey of Bangladesh (GSB), with seven surface boreholes that confirmed the existence of a significant coal deposit The deposit occurs as an asymmetrical synclinal structure with an axis striking approximately N-S. The deposit is limited to the east by a large normal fault which has displaced Archaean metamorphics against the Gondwana sediments. The coal-bearing sediments are comprised of Gondwana Permian-age sandstones, siltstones, subordinate carbonaceous shales, and six correlated coal seams. The Gondwana sediments are unconformably overlain by Tertiary and Quaternary deposits, against which the coal seams are successively subcropped to the west Within the structural limits of the basin, approximately 377 Mt coal in-situ has been quantified in the six coal seams that range in depth from 118 to 518 m below surface. Due to the synclinal nature of the deposit, the upper coal seams, designated I to V, occur over diminishing areal extent with decreasing depth. The principal seam of interest is the lowermost Seam VI, with a variable thickness across the deposit from 22 m in the northern part of the deposit to more than 42 m in the southern and eastern areas. Development of the Barapukuria Mine, the country's first coal mine, commenced in 1996 with the construction of two vertical shafts. Coal production from Seam VI began in 2005 and continues at the present time. Seam VI coal is high volatile B bituminous rank. About 34 Mt of coal has been estimated as recoverable resources, utilising descensional multi-slice longwall mining. The mine design and development have been severely constrained by adverse seam gradients and the presence of the overlying water-bearing Tertiary Dupi Tila sediments. The potential of coal bed methane extraction has been investigated as an alternative to underground mining. The study considers the Barapukuria deposit in terms of its geological structure, geothermal gradient, and the rank, porosity and permeability of the coal seams as determined by several phases of exploration of the area. The methane content of the bituminous coal at Barapukuria varies within the range 6.51-12.68 m~3/t, representing a potential resource of more than 5 Gm~3 of gas.