How do oil and gas form, move and become trapped, and what makes a working petroleum system?
Hydrocarbons: the petroleum system (source rock, maturation, migration, reservoir rock, trap and seal); the formation of oil and gas from organic-rich source rocks by burial and heating; the properties needed in a reservoir (porosity and permeability) and a seal (low permeability); the types of trap (structural and stratigraphic); the formation of coal from plant material with increasing rank.
A focused answer to the OCR H414 dot point on hydrocarbons. Covers the petroleum system (source rock, maturation, migration, reservoir, trap and seal), the formation of oil and gas by burial and heating, the porosity and permeability needed in a reservoir, low-permeability seals, structural and stratigraphic traps, and the formation of coal with increasing rank.
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What this dot point is asking
OCR wants you to describe the petroleum system (source, maturation, migration, reservoir, trap and seal), to explain how oil and gas form by burial and heating of organic-rich source rocks, to explain the porosity and permeability needed in a reservoir and the low permeability of a seal, to describe structural and stratigraphic traps, and to describe the formation of coal with increasing rank.
The answer
The petroleum system
For oil or gas to accumulate, five elements must all be present and correctly arranged in space and time:
- Source rock. A fine-grained, organic-rich rock (typically a shale or mudstone) in which dead organic matter is preserved as kerogen.
- Maturation. As the source rock is buried, it heats up; through the oil window it generates oil, and at greater depth and temperature, through the gas window, it generates gas.
- Migration. Being less dense than the pore water, the oil and gas move out of the source rock and upwards through permeable pathways.
- Reservoir rock. A porous and permeable rock (for example a sandstone) that stores the hydrocarbons and lets them flow.
- Trap and seal. A trap (a geometry that halts upward migration), capped by an impermeable seal (cap rock) that prevents escape.
Reservoir and seal properties
- Porosity is the proportion of pore space, controlling how much hydrocarbon a reservoir can store.
- Permeability is how well the pores connect, controlling how easily fluid can flow into the reservoir and to a well. A rock can be porous but impermeable, so a reservoir needs both.
- A seal needs very low permeability (for example shale or evaporite) so hydrocarbons cannot pass through and escape.
Types of trap
- Structural traps. Formed by deformation: an anticline (oil rises into the crest), or a fault that juxtaposes reservoir against seal.
- Stratigraphic traps. Formed by the original geometry of the rocks: a reservoir pinch-out, a reef, or truncation beneath an unconformity.
The formation of coal
Coal forms from plant material that accumulated in oxygen-poor swamps and was buried. With increasing burial, heat and pressure, the rank rises and the carbon content increases: peat, then lignite, then bituminous coal, then anthracite. Higher rank means more carbon and a higher energy content.
Examples in context
Example 1. North Sea anticlinal traps. Mature source shales generated oil that migrated upward into porous sandstone reservoirs, trapped in anticlines beneath impermeable shale and evaporite seals, a textbook structural petroleum system.
Example 2. Coal rank and burial. A coal seam buried more deeply reaches a higher rank (for example anthracite rather than lignite), so its rank records the depth and temperature of burial, linking economic geology to basin history.
Try this
Q1. Name the five elements of a petroleum system. [2 marks]
- Cue. Source rock, maturation, migration, reservoir rock, and trap with seal.
Q2. Explain why a reservoir rock must be permeable as well as porous. [2 marks]
- Cue. Porosity lets it store hydrocarbons, but permeability (connected pores) is needed for the hydrocarbons to flow into the reservoir and out to a well; a porous but impermeable rock cannot yield them.
Q3. Put the coal ranks in order of increasing carbon content. [2 marks]
- Cue. Peat, lignite, bituminous coal, anthracite.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/01 20196 marksDescribe the five elements that must be present and correctly arranged for oil to accumulate in a reservoir (a working petroleum system).Show worked answer →
A level-of-response answer; name and explain each element and the need for correct timing.
- 1. Source rock
- A fine-grained, organic-rich rock (for example a shale) in which organic matter (kerogen) is preserved.
- 2. Maturation
- Burial heats the source rock through the oil window (and at greater depth the gas window), converting kerogen into oil and gas.
- 3. Migration
- The oil and gas, being less dense than the pore water, move out of the source rock and upwards through permeable pathways.
- 4. Reservoir rock
- A porous and permeable rock (for example a sandstone) that can store the hydrocarbons in its pore spaces and allow them to flow.
- 5. Trap and seal
- A trap (a geometry such as an anticline or fault) that halts upward migration, capped by a seal (an impermeable rock such as shale or evaporite) that prevents the hydrocarbons escaping.
- Timing
- The trap and seal must be in place before migration occurs, or the oil escapes. All elements must coincide for an accumulation.
Top-band answers name all five elements with their function and note that the trap and seal must form before migration.
OCR H414/01 20204 marksExplain why a good reservoir rock must be both porous and permeable, and why a seal rock must have low permeability.Show worked answer →
Define the two properties and tie each to its role.
- Reservoir: porosity
- Porosity is the proportion of pore space in the rock; high porosity means the reservoir can store a large volume of hydrocarbons in its pores.
- Reservoir: permeability
- Permeability is how well the pores are connected so fluid can flow through. High permeability lets the hydrocarbons migrate into the reservoir and flow to a well during production. A rock can be porous but not permeable (isolated pores), which would store but not yield hydrocarbons, so both properties are needed.
- Seal: low permeability
- The seal (cap rock) must have very low permeability so that hydrocarbons cannot pass through it; this traps them in the reservoir below and prevents them escaping to the surface.
Markers reward porosity (storage) and permeability (flow) for the reservoir, and low permeability for the seal as the barrier to escape.
Related dot points
- Groundwater: porosity and permeability and how they differ between rock types; aquifers, aquitards and the water table; confined and unconfined aquifers; the calculation of porosity from pore and total volumes; the use of a simple form of Darcy's law to relate groundwater discharge to hydraulic conductivity, hydraulic gradient and area; the issues of over-abstraction and contamination.
A focused answer to the OCR H414 dot point on groundwater. Covers porosity and permeability and how they vary between rock types, aquifers, aquitards and the water table, confined and unconfined aquifers, calculating porosity, using a simple form of Darcy's law for groundwater flow, and the issues of over-abstraction and contamination.
- Basin analysis: the definition of a sedimentary basin and the mechanisms of subsidence (thermal subsidence after lithospheric stretching, flexural loading and sediment loading); the concept of accommodation space and its control by subsidence and sea-level change; the main basin types (rift, passive-margin and foreland); the use of vertical facies successions and burial-history curves to reconstruct basin evolution.
A focused answer to the OCR H414 dot point on basin analysis. Covers the definition of a sedimentary basin, mechanisms of subsidence (thermal, flexural and sediment loading), accommodation space and its control by subsidence and sea level, the rift, passive-margin and foreland basin types, and the use of facies successions and burial-history curves to reconstruct basin evolution.
- Sedimentary environments: the concept of facies as a body of rock reflecting a particular depositional environment; sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks and desiccation cracks) and their interpretation; the characteristics of the main environments (fluvial, deltaic, shallow marine, deep marine and desert); the construction and interpretation of sedimentary logs to reconstruct environmental change.
A focused answer to the OCR H414 dot point on sedimentary environments. Covers facies, sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks, desiccation cracks) and their interpretation, the fluvial, deltaic, shallow-marine, deep-marine and desert environments, and how sedimentary logs reconstruct environmental change.
- Sedimentary rocks: the stages from sediment to rock (deposition, compaction and cementation as lithification); the classification of sedimentary rocks into clastic (by grain size, from conglomerate to mudstone), chemical (precipitates such as evaporites) and biogenic or biochemical (limestone and coal); the description of clastic texture using grain size, sorting and roundness.
A focused answer to the OCR H414 dot point on sedimentary rocks. Covers lithification (deposition, compaction and cementation), the clastic, chemical and biogenic or biochemical classes, the grain-size scale from conglomerate to mudstone, and how clastic texture is described using grain size, sorting and roundness.
- Ore formation: the processes that concentrate metals into economic mineral deposits (hydrothermal vein and disseminated deposits, magmatic segregation, placer deposits and residual deposits); the conditions and host rocks typical of each; the distinction between ore and gangue and the idea that a deposit is economic only if the metal is concentrated well above its crustal average.
A focused answer to the OCR H414 dot point on ore formation. Covers hydrothermal vein and disseminated deposits, magmatic segregation, placer deposits and residual deposits, the conditions and host rocks of each, the distinction between ore and gangue, and the requirement that a metal be concentrated well above its crustal average to be economic.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)