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may not yet have allowed the hydrocarbons to form and collect. A
young reservoir (e.g. 60 million years) often has heavy crude. In
some areas, strong uplift and erosion and cracking of rock above
have allowed the hydrocarbons to leak out, leaving heavy oil
reservoirs or tar pools.
Some of the world’s largest oil deposits are tar sands where
the volatile compounds have evaporated from shallow sandy
formations leaving huge volumes of bitumen soaked sands. These
are often exposed at the surface, and could be strip mined, but
must be separated from the sand with hot water, steam and diluents
and further processed with cracking and reforming in a refinery) to
improve its fuel yield.

Figure 3.2 – Scheme of the reservoirs

The oil and gas is pressurized in the pores of the porous
formation rock.
When a well is drilled into the reservoir structure, the
hydrostatic formation pressure drives the hydrocarbons out of the

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