How do oil and gas form, migrate and become trapped?
The formation of hydrocarbons from organic-rich source rocks, the requirements of a petroleum system (source, reservoir, cap rock, trap, maturation and migration), the main trap types (anticlinal, fault, stratigraphic, salt dome), and the principles of exploration and recovery.
A focused WJEC and Eduqas A-Level Geology G4 answer on how oil and gas form from organic-rich source rocks, the elements of a working petroleum system (source, reservoir, cap rock, trap, maturation, migration), the main hydrocarbon trap types, and the principles of exploration and recovery.
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What this dot point is asking
G4 is the economic and applied half of interpreting the record, and hydrocarbons are its flagship topic. WJEC wants the full petroleum system: how oil and gas form, how they migrate, what holds them, and the trap geometries that make a field. This draws together sedimentary rocks (G1), structures (G2) and porosity, and underpins the resources and environment material.
The answer
How hydrocarbons form
Oil and gas form from the remains of microscopic marine organisms (mainly plankton) buried in fine sediment:
The petroleum system
For an economic accumulation, six things must coincide:
Porosity (the void space) controls how much a reservoir holds; permeability (the connectedness of the pores) controls how easily fluids flow. Both depend on grain sorting, packing and cementation set during diagenesis.
Trap types
- Anticlinal (structural) trap: cap rock folded into an arch over a reservoir; hydrocarbons collect at the crest.
- Fault trap: a fault seals the reservoir by juxtaposing impermeable rock against it.
- Stratigraphic trap: a sandstone pinch-out or an unconformity seals the reservoir by a change in the rocks.
- Salt dome trap: rising salt arches and seals the strata.
Exploration and recovery
Exploration uses seismic surveys (reflected sound waves imaging structures), gravity and magnetic surveys, and exploration wells. Recovery proceeds through primary (natural pressure), secondary (water or gas injection) and enhanced methods, never extracting all the oil in place.
Examples in context
The North Sea oil and gas province has Jurassic organic shales as source rocks, Jurassic and older sandstones as reservoirs, and both structural and salt-related traps sealed by shale and Zechstein salt, a textbook petroleum system. The Wytch Farm field in Dorset, onshore Europe's largest, produces from Triassic and Jurassic reservoirs in a faulted structure. Reef limestones form important reservoirs elsewhere because their original and fracture porosity can be high.
Try this
Q1. State the difference between a source rock and a reservoir rock. [2 marks]
- Cue. A source rock is organic-rich and generates hydrocarbons; a reservoir rock is porous and permeable and holds them.
Q2. Explain why a cap rock is essential to a hydrocarbon trap. [2 marks]
- Cue. An impermeable seal above the reservoir prevents the buoyant hydrocarbons from escaping to the surface.
Q3. Describe how an anticlinal trap holds oil and gas. [2 marks]
- Cue. A folded impermeable cap rock arches over the reservoir, so migrating hydrocarbons collect at the crest of the anticline beneath the seal.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20196 marksDescribe the conditions and rock properties needed for oil and gas to accumulate in an economic reservoir.Show worked answer →
Work through the elements of a petroleum system, because each is a separate requirement and a separate mark.
A source rock is needed: an organic-rich rock, typically a dark marine shale, deposited under anoxic conditions so the organic matter is preserved and later cooked.
Maturation is needed: burial to the oil window (about 60 to 120 degrees Celsius) converts the organic matter (kerogen) to oil, and deeper, hotter burial to gas.
A reservoir rock is needed: a porous and permeable rock, such as a well-sorted sandstone or a fractured limestone, to hold the hydrocarbons and let them flow.
Migration is needed: the oil and gas, being less dense than water, move upward and sideways out of the source into the reservoir.
A cap rock (seal) is needed: an impermeable rock, such as shale or salt, above the reservoir to stop the hydrocarbons escaping.
A trap is needed: a structural or stratigraphic configuration (such as an anticline) that holds the hydrocarbons in place, with gas above oil above water by density.
Markers reward source, maturation, reservoir (porous and permeable), migration, cap rock and trap, each correctly described.
WJEC Eduqas 20215 marksDescribe three types of trap in which hydrocarbons can accumulate.Show worked answer →
Name and describe each trap, because three distinct traps are required.
An anticlinal (structural) trap forms where an impermeable cap rock is folded into an arch over a porous reservoir; the hydrocarbons rise to the crest of the anticline and are held there beneath the seal.
A fault trap forms where a fault places an impermeable rock against the porous reservoir, sealing the up-dip edge so hydrocarbons are trapped against the fault plane.
A stratigraphic trap forms where the reservoir is sealed by a change in the rocks themselves, for example where a porous sandstone pinches out or is overlain by an unconformity capped by impermeable rock.
A salt dome trap (also acceptable) forms where rising salt pierces and arches the strata, sealing reservoirs against the impermeable salt.
Markers reward an anticlinal trap (fold beneath a cap), a fault trap (seal against a fault) and a stratigraphic trap (pinch-out or unconformity), each with the sealing mechanism.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)