Page 27 - 4822

The oil and gas bearing structure is typically a porous rock
such as sandstone or washed out limestone. The sand might have
been laid down as desert sand dunes or seafloor. Oil and gas
deposits form as organic material (tiny plants and animals)
deposited in earlier geological periods, typically 100 to 200
million years ago, under ,over or with the sand or silt, is
transformed by high temperature and pressure into hydrocarbons.
For an oil reservoir to form, porous rock needs to be
covered by a non-porous layer such as salt, shale, chalk or mud
rock that can prevent the hydrocarbons from leaking out of the
structure. As rock structures become folded and uplifted as a result
of tectonic movements, the hydrocarbons migrates out of the
deposits and upward in porous rocks and collects in crests under
the non permeable rock, with gas at the top, then oil and fossil
water at the bottom.

Figure 3.1 – Scheme of the reservoir

This process goes on continuously, even today. However,
an oil reservoir matures in the sense that a too young formation

27