Hydrocarbon Reservoir make a place for rocks in which hydrocarbon, oil and gas are accumulated. The essential properties required for rocks to be a hydrocarbon reservoir are porosity and permeability. Sandstone and limestone are by far the most common hydrocarbon reservoirs in the world, followed by dolomite and conglomerate. Siltstone, shale or igneous rocks form hydrocarbon reservoirs only very rarely.
In Bangladesh, all the gas and oil reservoirs are sandstones. These sandstones occur interbedded with shale and silt stone layers in the Bhuban and Bokabil formations of Pliocene to Miocene age (2 to 24 million years before present). The depths at which the sandstone hydrocarbon reservoirs occur in Bangladesh vary from about 1,000m to 3,500m below the surface. The sandstone reservoirs mostly are medium to fine grained, well to moderately sorted with generally little clay matrix. These gas reservoirs generally have about 15% to 25% porosity, 100 millidarcy to 500 millidarcy permeability and 60% to 70% gas saturation. These reservoirs are considered well to excellent in quality. The sandstone reservoirs were deposited in fluvio-deltaic and shallow marine environments.
In the 22 gas fields discovered so far (mid 2000) in Bangladesh, the sandstone reservoirs contain an original in-place gas reserve of about 0.73 trillion cu m of, which about 0.45 trillion cu m is considered recoverable. However, the remaining recoverable gas reserves at present stand at about 0.36 trillion cu m. These reserves are placed in the proven plus probable category. The recoverable gas reserves in the individual gas fields range from as low as 0.015 TCF (Begumganj) to as high as 0.07 trillion cu m (Kailashtila). In most of the gas fields there are multilayer sandstone reservoirs with some fields having as much as 10 (Titas) gas reservoir horizons.
Geologists believe that chances of finding limestone hydrocarbon reservoirs are good in the Bogra shelf and the hinge zone area in the western to central part of the country. Evidence of oil and gas shows in the Sylhet limestone layer in exploratory drill holes testify to the above notion. However exploratory drilling in this horizon is far from adequate to comment on its potential on a firm basis. [Badrul Imam]