Sandstone a sedimentary rock composed of sandsize particles or fragments. The sands are produced by weathering and erosion of pre-existing rocks into sand size particles that are brought to the depositional basin by water or wind media. After deposition the sand is eventually buried by geologic processes of subsidence whereby sand is physically compacted and chemically cemented and indurated to be transformed into sandstone. The degree of hardness of sandstone depends on the level of compaction and cementation of sand in the subsurface, a process known as diagnosis.
Sandstone is the second most abundant sedimentary rock after shale. It is one of the most important rocks to form petroleum reservoirs. In fact, almost half of the petroleum reservoirs throughout the world is composed of sandstone. All the gas reservoirs of Bangladesh gasfields are formed of sandstone.
These are good to excellent quality reservoirs because of the high porosity and permeability of the rock. Apart from forming the gas reservoirs, sandstone is a very common rock in the geological group-formations of Bangladesh. The younger geologic units known as Dupitila Formation and Tipam Formation are predominantly composed of sandstone, and formed the low hills in the eastern hilly areas of Bangladesh. The underlying Surma Group unit is composed of subequal sandstone (gas reservoirs) and shale interbeds. Sandstone is often used as building material as blocks in the construction of large buildings.
Sandstone form from sands deposited in river, in delta, in shallow seas, in deep seas, in desert or other sedimentary environments. In each case, the sandstone generally has textural, structural and compositional characteristic diagnostics. The desert sands are clean, well sorted whereas the river sands are dirty with some clay.
The colour of the sandstone generally depends on the composition especially the cementing materials or the component grains; for example, the red sandstone is red due to red hematite cement, grey sandstone is grey due to the clay-rich materials, green sandstone is green due to a mineral called glauconite and white sandstone is white due to pure quartz nature of its composition. [Badrul Imam]