How does oil and gas form, move and become trapped in the rocks?
Hydrocarbons and petroleum systems: the formation of oil and gas from organic-rich source rocks by burial and maturation; migration into porous and permeable reservoir rocks; the role of impermeable cap (seal) rocks and trap structures (anticline, fault, stratigraphic and salt traps); and the elements that must coincide for an accumulation to form.
A focused answer to the Eduqas Geology statement on hydrocarbons. Covers the formation of oil and gas from source rocks by burial and maturation, migration into reservoir rocks, the role of cap rocks and trap structures, and the elements that must coincide for a petroleum accumulation to form.
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What this dot point is asking
Part of the Earth materials and natural resources concept. Eduqas wants you to explain how oil and gas form from source rocks by burial and maturation, how they migrate into reservoir rocks, the role of cap (seal) rocks and trap structures (anticline, fault, stratigraphic, salt), and the elements that must coincide for an accumulation to form. This is the petroleum system: source, maturation, migration, reservoir, seal and trap, all correctly timed.
The answer
Source rock and maturation
Hydrocarbons form from organic matter (mainly plankton and algae) deposited with fine sediment in anoxic, low-energy conditions that prevent it decaying. This makes an organic-rich source rock (typically a dark shale). As the source rock is buried, temperature and pressure rise and the organic matter matures:
- In the oil window (roughly 60 to 120 degrees Celsius) it generates oil.
- At higher temperatures (deeper burial) it generates gas.
Too little burial leaves the organic matter immature; too much over-cooks it. So the timing and depth of burial matter.
Migration
Oil and gas are less dense than the formation water filling the pores, so they are buoyant and migrate upwards out of the source rock, through any porous and permeable rock, until something stops them. If nothing stops them, they reach the surface and are lost (a seep).
Reservoir, porosity and permeability
The hydrocarbons accumulate in a reservoir rock, which must have two properties:
Good reservoirs are well-sorted sandstones and fractured or vuggy limestones (high porosity and permeability).
Seal and trap
For the hydrocarbons to be held rather than escape, two more things are needed:
- A cap (seal) rock: an impermeable layer (shale, salt or unfractured mudstone) above the reservoir that stops further upward migration.
- A trap: a geometry that holds the accumulation. The main types are:
- Anticline trap: the reservoir is arched up and sealed over the crest (the commonest).
- Fault trap: a fault seals porous reservoir against impermeable rock.
- Stratigraphic trap: a lateral change (pinch-out, reef, unconformity) seals the reservoir.
- Salt dome trap: rising salt arches and seals the surrounding beds.
In a trap the fluids stack by density: gas on top, oil beneath, and water below the oil.
The elements must coincide
An accumulation forms only if all six elements are present and correctly timed: a source rock, sufficient maturation, a migration path, a reservoir, a seal, and a trap that existed before the hydrocarbons migrated. Miss any one (no seal, or the trap formed too late) and there is no field.
Examples in context
Example 1. North Sea anticline fields. Many North Sea oilfields are anticline traps: a sandstone reservoir arched up and sealed by overlying shale, charged by oil generated from a buried organic-rich source shale.
Example 2. Salt-dome traps. Where buoyant salt has risen through the overburden, it arches and seals the surrounding beds, creating traps on its flanks and crest, a common style in salt-rich basins.
Try this
Q1. State the six elements that must be present for a petroleum accumulation. [3 marks]
- Cue. A source rock, maturation, migration, a reservoir, a seal (cap rock) and a trap, all correctly timed.
Q2. Explain the difference between porosity and permeability. [2 marks]
- Cue. Porosity is the percentage of pore space (how much fluid the rock can store); permeability is the ability to transmit fluid (needing connected pores, so how easily it flows).
Q3. State the order in which gas, oil and water are arranged in a trap, and why. [2 marks]
- Cue. Gas on top, oil beneath, water at the base, because they separate by density (gas least dense, water most dense).
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 20196 marksDescribe the conditions and processes by which oil and gas form from a source rock and accumulate in a reservoir, naming the elements that must be present for an oilfield to exist.Show worked answer →
A levels-of-response answer; follow the petroleum system from source to trap.
- Source rock and maturation
- Oil and gas form from organic matter (plankton, algae) buried in fine-grained, anoxic source rocks (for example organic-rich shale). As burial increases the temperature, the organic matter matures: in the "oil window" (roughly 60 to 120 degrees Celsius) it generates oil, and at higher temperatures gas.
- Migration
- Because oil and gas are less dense than the formation water, they migrate upwards out of the source rock through porous and permeable rock until they are stopped.
- Reservoir and seal
- They accumulate in a reservoir rock with high porosity (to store the oil) and permeability (to allow flow), such as sandstone or fractured limestone, capped by an impermeable seal (cap rock, for example shale or salt) that stops further upward escape.
- Trap
- A trap (a geometry that holds the hydrocarbons), such as an anticline, a fault trap, a stratigraphic trap or a salt dome, concentrates the accumulation.
- The elements
- For an oilfield to exist, a source rock, maturation, migration, a reservoir, a seal and a trap must all be present and correctly timed.
Top-band answers track source to maturation (oil window) to migration to reservoir and seal to trap, and list the elements that must coincide.
Eduqas 20215 marksExplain the difference between porosity and permeability, and why a good reservoir rock needs both. Sketch the type of trap formed by an anticline.Show worked answer →
Define both properties, link them to the reservoir, then describe the trap.
- Porosity
- Porosity is the percentage of the rock's volume that is pore space (voids between or within grains); it determines how much fluid the rock can store.
- Permeability
- Permeability is the ability of the rock to transmit fluid, which depends on the pore spaces being connected; it determines how easily the oil can flow through and be produced.
- Why both are needed
- A reservoir needs high porosity to hold enough oil and high permeability so the oil can migrate in and be extracted. A rock can be porous but impermeable (for example clay or unconnected vesicles), which would store fluid but not yield it, so both properties are essential.
- Anticline trap
- In an anticline trap the reservoir bed is arched upwards and sealed by an impermeable cap rock over the crest. Buoyant gas collects at the very top, oil beneath it, and water below the oil, all held in the highest point of the fold beneath the seal.
Markers reward porosity as storage capacity and permeability as the ability to transmit fluid (needing connected pores), the point that both are required, and a correct anticline trap with gas, oil and water stacked by density under the seal.
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- Folds, faults and joints: fold elements (limb, axial plane, hinge) and types (anticline and syncline, symmetric, asymmetric, overturned, recumbent, monocline); fault types and the stress they record (normal from tension, reverse and thrust from compression, strike-slip and tear from shear); dip-slip versus strike-slip movement; throw, heave and the fault plane; joints as fractures with no displacement; and reading these structures on geological maps and cross-sections.
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Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)