Oil and gas

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Origin of Hydrocarbons
Decayed plants and bacteria (most plankton) dies and then sinks to the sea floor. Oil
comes from plankton but if it is buried deeply enough it can also form gas (land plants
can form gas if they're buried deeply enough). Gas can
pass through porous and permeable rocks but oil is a
viscous liquid so it can only pass through permeable.
Planktonic organisms die and sink to the sea floor where there are anoxic
conditions (because of the lack of movement of circulation water creating
anaerobic, stagnant water) that prevent decay and preserve the organic
matter. This bacterium is buried and due to the heating of the earth's crust
can be warmed up and compressed as more sediments are deposited on top
of it. COMPRESSION AND HEATING. Kerogen is the first formed, and then
with increasing heat the carbon and hydrogen atoms break away to form
heavy oil and then continuing you get light oil and then gas.
Example North sea oil forms at 34.5km depth and gas at 46km depth
Migration of Oil and Gas
Migration Definition the movement, in response to hydroclastic pressure, of crude oil
from the source rock to the reservoir rock through permeable
As hydrocarbons form they create a higher pressure in the source rock pore spaces, the
liquid oil and gas is less dense than its source rock and so migrates upwards.
The oil moves through water, rock and even faults. Migration is a very slow process and
is stopped when the oil and gas reach impermeable layers which act as seals.
Factors needed for the formation of petroleum
Clay/mud to encase plankton
Clay/mud that can be compacted by later compression
Low energy conditions to allow dead microorganisms to settle and accumulate
Anoxic/ anaerobic conditions
(So organic material does not decay by oxidation)
Burial by more material to pressure the organic material and provide weight for
later compaction

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Oil Reservoirs
Oil reservoirs are found when permeable and porous rock holds oil and gas. Some rocks
are impermeable and block the upward passage of oil and gas. And therefore get
Reservoir rocks have lots of interconnected holes called pores. These absorb the oil and
gas like a sponge. Permeability can increase if the rock is fractured or weathered,
sometimes the oil migrates. The formation of oil increases the volume of the material
within the rocks.…read more

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Oil Extraction
Mainly extracted on land, but due to depleting supplies, humans started looking to the
sea. Initially, oil was harvested from shallow regions however as demand increased
companies began looking to deeper waters and therefore had to design economically
viable offshore structures that were safe to use and maintain.…read more


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