| 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.
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Sandstone concretion (Teknaf) |
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.
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Sandstone concretion (Cox's
Bazar) |
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]
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