Page 19 - Petroleum and Gas Field Processing

1.2 FORMATION AND ACCUMULATION OF OIL AND GAS
1.2.1 Formation of Oil and Gas
Several theories have been proposed to explain the formation and origin of
oil and gas (petroleum); these can be classified as the organic theory of
petroleum origin and the inorganic theory of origin. The organic theory
provides the explanation most accepted by scientists and geologists.
It is believed, and there is evidence, that ancient seas covered much
of the present land area millions of years ago. The Arabian Gulf and the
Gulf of Mexico, for example, are parts of such ancient seas. Over the
years, rivers flowing down to these seas carried large volumes of mud and
sedimentary materials into the sea. The mud and sedimentary materials
were distributed and deposited layer upon layer over the sea floor. The
buildup of thousands of feet of mud and sediment layers caused the sea
floors to slowly sink and be squeezed. This eventually became the
sedimentary rocks (the sandstones and shales, and the carbonates) where
petroleum is found today.
The very large amount of small plant and animal life, which came
into the sea with river mud and sedimentary materials, and the much
larger amount of small marine life remains already on the sea floors
constituted the source of petroleum. These small organisms died and were
buried by the depositing silt and, thus, were protected from ordinary
decay. Over many years, pressure, temperature, bacteria, and other
reactions caused these dead organisms to change into oil and gas. The gas
was formed under the higher-temperature conditions, whereas the oil was
formed under the lower-temperature conditions. The rocks where oil and
gas were formed are known as the source rock.

1.2.2 Accumulation of Oil and Gas
The oil, gas, and salt water occupied the pore spaces between the grains of
the sandstones, or the pore spaces, cracks, and vugs of the limestones and
dolomites. Whenever these rocks were sealed by a layer of impermeable
rock, the cap rock, the petroleum accumulating within the pore spaces of
the source rock was trapped and formed the petroleum reservoir.
However, when such conditions of trapping the petroleum within the
source rocks did not exist, oil gas moved (migrated ), under the effects of
pressure and gravity, from the source rock until it was trapped in another
capped (sealed) rock.
Because of the differences in density, gas, oil, and water segregated
within the trap rock. Gas, when existed, occupied the upper part of the
trap and water occupied the bottom part of the trap, with the oil between

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