Download HYDROCARBONS (OIL AND GAS)

HYDROCARBONS (OIL AND GAS)
Refs. Britains Offshore Oil and Gas
Geoscience p. 154
Crude Oil is 85% carbon, 12% hydrogen
Natural gas is mainly methane
In order for them to form, there must be
 A source rock
 The correct amount of heating of source rocks
 A path for migration of the hydrocarbon (rocks must be permeable)
 A reservoir rock
 A cap rock
 a trap
Origin of Hydrocarbons (source rock = rock containing sufficient organic material
to generate oil and gas)
Mostly from decayed plants and bacteria – recycled energy from the sun, stored as
hydrocarbons. Oil is mostly from decayed plankton, but if the plankton are buried more
deeply, they form gas. Land plants may also form gas if they are buried deep enough.
Coal remains in situ, but gas and oil can migrate. Gas can pass through porous and/or
permeable rocks. Oil is a viscous liquid, so movement is only through permeable rocks.
Process of oil and gas formation
Temperatures within the earth’s crust increase with depth, so sediments and plant
materials warm up as they are compressed by younger deposits. Increasing heat first
leads to the formation of kerogen ( a dark woody substance). With increasing heat, carbon
and hydrogen atoms break away from the kerogen to form heavy oil. At higher
temperatures still light oils and then gas are formed. In the North Sea, oil forms at 3 – 4.5
km depth and gas at 4 – 6 km. When source rocks start to generate oil or gas, they are
said to be mature. The North Sea is still sinking , so new oil and gas is still being formed.
MIGRATION OF OIL AND GAS
Migration = movement of oil and gas away from the source rocks.
Migration is triggered by:
 Compaction of the source rock
 Process of oil and gas formation – as hydrocarbons separate from kerogen during oil
and gas generation, they take up more space and create pressure in the source rock.
Oil and gas squeeze through pores in the source rock and into other rocks where
pressure is lower.
Oil and gas tend to migrate upwards as they are lighter than water and rock. They move
through permeable rocks via pores and cracks or even faults. Migration is a very slow
process and is stopped when the oil and gas reach impermeable layers which act as
seals.
Oilfields and gasfields are areas where hydrocarbons have become trapped in
permeable reservoirs, e.g. porous sandstone or fractured limestones. A cap rock is the
impermeable rock above the reservoir rock which seals in the oil and gas. A trap is a
place where oil or gas collects. There are two types of trap:
Structural traps (rocks have been folded, faulted or domed over a salt dome)
Stratigraphical traps ( e.g. where a reservoir rock is sealed by im