Oil and gas in rocks
Adam Vonk explains that, to find hydrocarbons, scientists first need to identify a source rock that has been buried deep enough so that the temperature and pressure drive off or separate the hydrocarbons from the kerogen – the organic material the hydrocarbons were formed from. Once they have separated from the rock, the hydrocarbons will move into an oil trap to form a reservoir. A trap is an area that the oil collects in.
This process is called migration. Reservoirs are porous rocks. Once the hydrocarbon is in the reservoir rock, it is then trapped by an impermeable rock layer, called a seal rock. The seal rock stops the hydrocarbons migrating further. All this must happen deep within the Earth to avoid the hydrocarbons degrading in quality.
Jargon alert
Each field of science has its own language – here are a few of the terms Adam uses:
Source rock – a rock that contains the raw materials from which hydrocarbons will eventually form. They are rich in organic matter.
Kerogen – the type of organic material in the source rock – the chemical compounds that make up the organic matter.
Hydrocarbons – a molecule made up of hydrogen and carbon. Fuels such as oil and natural gas are hydrocarbons.
Porous – having tiny holes or pores that a fluid or gas can enter. Porous rocks allow fluids to seep into them.
Points of interest
Hydrocarbons can also migrate along fault lines, as well as traps – why is this so?
Transcript
ADAM VONK
The recipe for finding hydrocarbons is fourfold. First of all, you need a source rock which contains this kerogen material. That needs to be buried to a certain depth within asedimentary basin to drive off those hydrocarbons from that kerogen. So that is typically like 3, 4, 5, 6, 7 kilometres, and you’re essentially trying to get into that oil window of pressureand depth. And then once those hydrocarbons are driven off, then we have a process called migration. The hydrocarbon is being produced over here, it will flow up and up, and accumulate into a trap. Also, the migration might follow up a fault line in the Earth – they often form conduits for the migration of hydrocarbons. And then we need a reservoir – a reservoir is a porous rock, they might be sandstones, which have a good porosity, or maybe alimestone that’s got a lot of pore space between the grains in which they can accumulate. So that is the reservoir rocks. And then, once the hydrocarbon is in that reservoir of rock, we need something that caps it, called a seal rock, to actually stop those hydrocarbons escaping out of that reservoir rock. So we need a rock with relatively low porosity and permeability, and that acts like a blanket to seal in those hydrocarbons. And that all needs to happen at a certain depth in the Earth to actually be away from the Earth's surface. Because as hydrocarbons actually come closer to the Earth's surface, we get a bit of biodegradation of the hydrocarbons. So essentially they rot.
Acknowledgements:
Richard Sykes – GNS Science
Eurico Zimbres
James Shook
