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Sedimentary rocks are formed by the laying down of
sediment in seas, rivers or lakes. The particles of sediment that
accumulate are eventually cemented together to form sedimentary
rock by the percolation of mineral rich waters through the spaces
between the particles. It is these sedimentary rocks in which
hydrocarbons are normally found.
Metamorphic rock is formed by the metamorphosis of
existing rock, be it igneous or sedimentary, by extreme heat and
pressure. These factors cause recrystallization of the minerals in
the rock. Metamorphic rocks do not normally contain
hydrocarbons.

Porosity
Petroleum is not found in underground rivers or caverns,
but in pore spaces between the grains of porous sedimentary rocks
(Figure 3.4). But what does the word porous mean? This is most
easily explained by illustration.

Figure 3.4 - A piece of porous sedimentary rock. The
pore spaces are the white areas between the dark grains. It is
within such pore spaces that fluids such as oil, natural gas or
water can be found in the subsurface

A sponge is a good example of a household object that is
porous or in other words possesses porosity. If we take a dry

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