What does G4 of WJEC A-Level Geology cover?

G4, Earth Materials and Natural Resources, is the fourth topic of the A2 section Interpreting the Geological Record. It covers the applied, economic geology: how oil and gas form, migrate and are trapped in a petroleum system; how metals are concentrated into ore deposits and how grade, cut-off grade and reserves decide whether to mine; how groundwater is stored in aquifers and how geology affects engineering works; and the formation of coal, the range of energy resources, and the environmental impacts and sustainable management of extraction.

What are the elements of a petroleum system?

Six elements must coincide for an economic oil or gas accumulation: a source rock (organic-rich shale) to generate hydrocarbons, maturation by burial to the oil or gas window, a porous and permeable reservoir rock to hold and transmit them, a migration path, an impermeable cap rock (seal) to prevent escape, and a trap to hold the hydrocarbons, with gas over oil over water by density. Traps may be anticlinal, fault, stratigraphic or salt dome.

What is the difference between grade, cut-off grade and reserves?

Grade is the concentration of the valuable metal in the rock, given as a percentage or grams per tonne. Cut-off grade is the lowest grade that can be mined and processed at a profit, set by the metal price and extraction costs. Reserves are the part of the total resource that is economic to extract now. Because the cut-off grade depends on price and cost, reserves are not fixed: a higher metal price lowers the cut-off and so increases reserves.

What makes a good aquifer, and what are artesian conditions?

A good aquifer is both porous (to store water) and permeable (to transmit it), such as sandstone or fractured limestone; an aquiclude like clay is porous but impermeable and yields no water. The water table is the top of the saturated zone. Artesian conditions occur where a permeable aquifer is confined between impermeable layers and dips from a higher recharge area, putting the water under pressure so it rises, or flows at the surface, when a well is drilled through the seal.

How can geological resources be used sustainably?

Most geological resources (fossil fuels, metal ores, uranium) are non-renewable, forming far slower than they are used, while geothermal heat and groundwater are renewable if not over-used. Extraction affects land, water and air. Impacts are reduced by restoring worked sites, treating mine drainage, recycling, and carbon capture and storage, which injects carbon dioxide into deep porous reservoirs sealed by an impermeable cap rock, using the same trapping principle as a petroleum trap.

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WJEC A-Level Geology G4 Earth Materials and Natural Resources: a deep dive on hydrocarbons, ores, water and energy

A deep-dive WJEC and Eduqas A-Level Geology guide to G4, Earth Materials and Natural Resources. Covers the petroleum system and hydrocarbon traps, the formation and evaluation of ore deposits, groundwater and engineering geology, and energy resources and sustainability including carbon capture and storage, with exam-style worked questions.

Generated by Claude Opus 4.815 min readWJECUpdated 2026-06-16

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What G4 actually demands
Hydrocarbons
Ores
Water and engineering
Energy and sustainability
Check your knowledge

What G4 actually demands

G4 is where geology meets society and the economy. Examiners want the science of how resources form, the economic vocabulary that decides whether they are exploited, and a reasoned view of environmental impact and sustainability. The recurring skill is applying the trapping and reservoir ideas across hydrocarbons, water and carbon storage, and applying grade and cut-off to a mining decision.

Each part of the topic has a dot-point page with worked questions; this overview connects them.

Hydrocarbons

Oil and gas form from organic-rich source rocks matured by burial, then migrate into porous, permeable reservoirs and are held by a cap rock in a trap. The six elements (source, maturation, reservoir, migration, seal, trap) must coincide, and traps may be anticlinal, fault, stratigraphic or salt dome. Porosity controls how much is held; permeability controls flow.

Ores

Metals are concentrated far above their crustal average by magmatic segregation, hydrothermal veins, secondary enrichment, placer and sedimentary processes. Grade is the metal concentration, cut-off grade the lowest profitable grade set by price and cost, and reserves the economic part of the resource, so reserves rise and fall with price.

Water and engineering

Aquifers store and transmit water and must be porous and permeable; confined, inclined aquifers give artesian flow. Sustainable abstraction must not exceed recharge. Rock and ground conditions control dams, tunnels and foundations, with soluble limestone, faults and dip causing leakage and failure.

Energy and sustainability

Coal forms in anoxic swamps and rises in rank with burial. Resources are renewable or non-renewable, and extraction affects land, water and air. Restoration, drainage treatment and carbon capture and storage reduce the impacts.

Check your knowledge

Attempt these under timed conditions, then check against the solutions.

List the six elements of a petroleum system. (3 marks)
Distinguish a source rock from a reservoir rock. (2 marks)
Define grade and cut-off grade. (2 marks)
State the two properties a good aquifer must have. (2 marks)
State the rank sequence of coal from lowest to highest. (2 marks)
Explain how carbon capture and storage uses a geological trap. (2 marks)

Sources & how we know this

WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)