How do rocks store and supply hydrocarbons and groundwater?
Hydrocarbons (oil and gas) form from buried organic matter, then migrate from a source rock into a porous, permeable reservoir rock where an impermeable cap rock and a trap (for example an anticline or fault) hold them; groundwater is stored in a porous, permeable aquifer beneath the water table and supplied to wells; both depend on the rock properties porosity (storage) and permeability (flow), and both can be over-exploited or polluted.
A focused answer to the Eduqas GCSE Geology statement on hydrocarbons and groundwater. Covers how oil and gas form, migrate and are trapped by a reservoir, cap rock and structure, how groundwater is stored in aquifers below the water table, and the key rock properties porosity and permeability.
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What this dot point is asking
Eduqas wants you to explain how hydrocarbons (oil and gas) form from buried organic matter, migrate from a source rock into a porous, permeable reservoir rock, and are held by an impermeable cap rock and a trap (such as an anticline or fault). You also need to explain how groundwater is stored in a porous, permeable aquifer below the water table and supplied to wells, and that both depend on the rock properties porosity (storage) and permeability (flow). The shared idea is that two simple rock properties control where fluids are stored and how they move, and both resources can be over-used or polluted.
The answer
Two key rock properties
Both oil and gas and groundwater depend on the same pair of properties:
- Porosity is the proportion of a rock that is pore space (gaps between or within the grains). It controls how much fluid the rock can store.
- Permeability is how easily fluid can flow through the rock, which depends on whether the pores are connected. It controls how fast fluid can move and be pumped out.
A useful reservoir or aquifer needs both: high porosity to hold the fluid and high permeability to yield it. Clay, for example, is porous but barely permeable, so it holds water but will not give it up.
How hydrocarbons form, migrate and are trapped
Oil and gas form and accumulate in a sequence of steps:
- Formation in a source rock. Tiny organisms (plankton) and plant matter are buried in fine sediment such as shale or mudstone. With burial, heat and pressure slowly turn the organic matter into hydrocarbons.
- Migration. Being less dense than water, the oil and gas are squeezed out of the source rock and migrate upward into a porous, permeable reservoir rock (such as sandstone or limestone) that can store and transmit them.
- Cap rock. An overlying impermeable layer (shale or salt) acts as a cap rock, stopping the hydrocarbons leaking to the surface.
- Trap. A trapping structure holds them in place: the crest of an anticline, or a fault that seals porous rock against impermeable rock. Within the trap, gas sits above oil above water, sorted by density.
If any element is missing (no source, no porous reservoir, no seal, no trap), the hydrocarbons disperse and there is no useful accumulation.
Groundwater and aquifers
Rain that soaks into the ground fills the pore spaces of permeable rock. Below a certain level the pores are completely saturated, and the top of this saturated zone is the water table.
Water is supplied to wells drilled below the water table. Because an aquifer needs to both store water (porosity) and yield it (permeability), the best aquifers are well-sorted sandstones and fractured limestones.
Over-exploitation and pollution
Both resources are vulnerable:
- Over-extraction. Pumping groundwater faster than rain replaces it lowers the water table, so wells dry up; near the coast it can draw in salt water (saltwater intrusion); and removing the water can cause the ground to subside. Oil and gas are finite and, once extracted, gone.
- Pollution. Aquifers can be contaminated from the surface (leaking chemicals, sewage, fertiliser), and the same permeability that makes a rock a good aquifer lets pollutants spread through it. Hydrocarbon extraction risks spills.
So both resources must be managed: extracted no faster than they are replenished (for groundwater) and protected from contamination.
Examples in context
Example 1. North Sea oil. North Sea fields have organic-rich shale source rocks, porous sandstone reservoirs, salt and shale cap rocks, and anticline and fault traps, the complete set of elements needed for a working petroleum system.
Example 2. A chalk aquifer. The chalk beneath south-east England is a major aquifer: porous enough to store huge volumes of water and permeable through its fractures, supplying wells and springs, but vulnerable to pollution and over-abstraction.
Try this
Q1. Define permeability. [1 mark]
- Cue. How easily a fluid can flow through a rock (depending on how well the pore spaces are connected).
Q2. Explain why an impermeable cap rock is essential for an oil trap. [2 marks]
- Cue. Oil and gas migrate upward; without an impermeable cap to seal them in, they escape to the surface instead of accumulating.
Q3. State one consequence of extracting groundwater faster than it is replaced. [1 mark]
- Cue. Any one of: the water table falls and wells dry up; saltwater intrusion near the coast; ground subsidence.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20216 marksExplain the conditions and rocks needed for oil and gas to accumulate in an underground trap, naming the role of each rock.Show worked answer →
Take the elements of a petroleum trap in turn: source, reservoir, cap and trap.
- Source rock
- Oil and gas form from organic matter (dead plankton and plants) buried in fine sediment such as shale or mudstone. Heat and pressure with burial turn the organic matter into hydrocarbons.
- Migration and reservoir rock
- Being less dense than water, the oil and gas migrate upward out of the source rock into a porous, permeable reservoir rock (such as sandstone or limestone) that can store and transmit them. Porosity provides the storage space; permeability lets them flow in.
- Cap rock
- An overlying impermeable cap rock (such as shale or salt) stops the hydrocarbons escaping to the surface.
- Trap
- A trapping structure (for example an anticline crest, or a fault that seals porous rock against impermeable rock) holds the hydrocarbons in place so they pool rather than disperse. Gas sits above oil above water, by density.
Markers reward the source rock (where they form), the porous, permeable reservoir (storage and flow), the impermeable cap rock (seal), and a trap structure (such as an anticline), with the role of each named.
Eduqas 20195 marksExplain the difference between porosity and permeability, and explain why a good aquifer must have both. State one consequence of over-extracting groundwater.Show worked answer →
Define both properties, show why an aquifer needs both, then give a consequence.
Porosity is the proportion of the rock that is pore space (gaps between or within grains). It controls how much water (storage) the rock can hold.
Permeability is how easily fluid can flow through the rock, which depends on whether the pores are connected. It controls how fast water can move through and be pumped out.
Why an aquifer needs both. A rock with high porosity but low permeability (such as clay) holds water but will not let it flow out, so it is useless as an aquifer. A good aquifer must be both porous (to store water) and permeable (to yield it to a well), like a well-sorted sandstone.
A consequence of over-extraction. Pumping water out faster than it is replaced lowers the water table, so wells dry up; near the coast it can draw in salt water (saltwater intrusion); and the ground can subside.
Markers reward distinct definitions of porosity (storage) and permeability (flow), the point that an aquifer needs both, and a valid consequence of over-extraction.
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Sources & how we know this
- WJEC Eduqas GCSE (9-1) Geology specification (teaching from 2017) — WJEC Eduqas (2017)