4.2 Oil And Gas Flashcards
What do oil and gas originate from?
Planktonic animals
Plankton die and sink to bottom of the sea and accumulate
More and more material is deposited compressing the plankton
Creates an organic rich black shale -> source rock
What changes occur to the source rock?
Organic matter increase in temperature 30-60C into kerogen, which is a solid from a hydrocarbon
At 90C it changes to a liquid state -> oil
At 150C it changes to a gas
Why is it known as a source rock?
A rock that has produced oil and gas
What is the migration of oil and gas?
Oil and gas are less dense so migrate through rocks into a porous and permeable reservoir rock
Sandstone and limestone are common reservoir rocks
What is an antiform trap?
Oil and gas migrate from source rock and are trapped in the hinge of the fold beneath the impermeable cap rock or seal rock
What is a fault trap?
Reservoir rock has been moved next to impermeable cap rocks/seal rocks which trap the oil and gas
What is a salt dome trap?
Salt is less dense than rocks
It rises up to form domes/diapers
Salt is impermeable and traps the oil and gas
What is a lithological trap/Unconformity trap?
The reservoir rock is next to the cap rock/seal rock due to “pinch out” of the reservoir rock or an uncomformable boundary
What are seismic surveys?
Good at sea and on land
Air gun from survey ship/thumper truck sends out seismic waves to different layers in the sea bed to find oil and gas
Waves bounce back and hit hydrophones (geophones on land) and records waves by their 2 way travel time
Records are interpreted and sent back to the survey ship
Waves don’t bounce off water
Explain borehole correlation?
Microfossils tell drillers where they are in the geological record.
This helps them to decide whether to drill further or stop drilling.
It can also be used as an indicator to the quality and potential amount of oil and gas.
What is fracking?
How do you remove the gas from the source rock?
Shale gas forms in organic rich shales which have been deeply buried.
Shale is trapped due to low permeability of source rock and impermeable beds above.
To remove the gas high pressure fluids are pumped into the source rock to create fractures.
During fracking the fluid displaces the gas and sand grains mixed in the fluid hold the fractures open allowing for gas to escape.
Explain what the problems are with fracking?
Water pollution - to access the shale gas you have to drill through the water table. People are worried that fracking fluid and gas could pollute the water supply.
Earthquakes - fracking by definition produces earthquakes. However, these are very minor (approx. 1Mw) and little noticeable effects. However, public perception of earthquakes is doom and gloom.
