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The two main groups of sedimentary rocks are classified on the basis
of their origin.

1. Clastic Sedimentary Rocks-formed as a result of the
weathering or fragmentation of pre-existing rocks and minerals and
classified on the basis of their textures, primarily the sizes of the
grains. Sedimentary rocks are divided into coarse-grained:
conglomerates, medium-grained: sandstones, and fine-grained:
siltstones, mudstones, and shales. Within each textural category,
clastics are further subdivided by mineralogy, which reflects the
parent rock, for example, a quartz-rich sandstone or a feldspar-rich
sandstone.
2. Chemical or Biochemical Sedimentary Rocks-formed as a
result of chemical processes. Primary carbonate deposition results
from the precipitation and deposits formed by plants and animals that
utilize carbonates in their life processes. The most abundant mineral
chemically or biochemically precipitated in the oceans is calcite, most
of it the shelly remains of organisms and the main constituent of
limestone. Many limestones also contain dolomite, a calcium-
magnesium carbonate precipitated during lithification. Gypsum and
halite are formed by the chemical precipitation during the evaporation
of seawater.
There are five types of sedimentary rocks that are important in
the production of hydrocarbons:
Sandstones
Sandstones are clastic sedimentary rocks composed of mainly
sand size particles or grains set in a matrix of silt or clay and more or
less firmly united by a cementing material (commonly silica, iron
oxide, or calcium carbonate). The sand particles usually consist of
quartz, and the term “sandstone”, when used without qualification,
indicates a rock containing about 85-90% quartz.

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