Notes · GeologyUK · A-Levels

Earth materials and natural resources

This chapter applies geology to the resources society depends on. It covers metalliferous ores and how they are concentrated, industrial minerals and construction materials, the formation and trapping of hydrocarbons and coal, the reservoir properties of porosity and permeability together with groundwater, and the methods used to prospect for and evaluate these resources.

5 sections~18 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 2

T·0999 / 13

Exam profile

AO1 · Describe the types, formation and properties of mineral, hydrocarbon and water resourcesAO2 · Calculate grade, concentration factor and porosity, and apply reservoir and trap conceptsAO3 · Analyse and evaluate the exploration for, and the economics and sustainability of, resources

Operators:describeexplaincalculateinterpretevaluateassess

basic level

AS-Level expects you to describe ores, hydrocarbons and groundwater and how they form and are found.

higher level

The full A-Level requires you to calculate grade and porosity, apply trap and reservoir concepts, and evaluate resource exploitation.

Depth

Text

Contents · 5 sections

Earth materials and natural resources

Metalliferous ores and their formation#

StandardWJEC/Eduqas A level Geology (Component 3) — Earth materials and natural resources

A hydrothermal vein deposit

Fig. 1A hydrothermal vein: hot, mineral-rich fluids rise through fractures and precipitate ore and gangue minerals as they cool, forming a narrow but rich deposit along a structure.

An ore is a rock or mineral deposit from which a metal can be extracted at a profit; the valuable ore minerals are mixed with worthless gangue minerals (commonly quartz and calcite) that must be separated. The grade of an ore is the concentration of the wanted metal, usually given as a percentage or in parts per million, and the cut-off grade is the lowest grade that is economic to mine under current conditions. Whether a deposit is an ore therefore depends not only on geology but on the metal price, the extraction cost and the technology available, so a body of rock can become, or cease to be, an ore as economics change.

Because most metals are very rare in the crust, an ore represents a natural concentration of a metal far above its average crustal abundance, and the size of this enrichment is the concentration factor — the grade divided by the average crustal abundance of the metal. Metals that are already relatively common, such as iron and aluminium, need only modest enrichment to form an ore, whereas rare metals such as gold or mercury must be concentrated thousands of times. The whole of economic geology is, in effect, the study of the geological processes that achieve these concentrations.

Several processes concentrate metals. Hydrothermal processes are the most important: hot, mineral-rich fluids, driven off a cooling intrusion or heated groundwater, circulate through the crust and precipitate metal sulfides in fractures and faults as they cool, forming veins, or disseminated deposits spread through a large volume of rock. Magmatic segregation concentrates dense metal-rich minerals as they crystallise and settle in a magma chamber; placer deposits concentrate dense, resistant minerals (such as gold and cassiterite) where flowing water sorts them by density; and secondary enrichment concentrates metals by weathering, which dissolves and redeposits them at depth.

The type of process controls the form of the deposit and hence how it is found and mined. Hydrothermal veins are narrow but rich and follow structures such as faults, so they are traced by mapping structures; disseminated deposits are low-grade but very large and are mined in huge open pits; placers lie in river and beach sands and are dredged or panned. Understanding how a deposit formed therefore guides both exploration and extraction, and linking a named deposit type to its forming process is a standard AO1 and AO2 task.

\text{concentration factor} = \frac{\text{ore grade}}{\text{average crustal abundance}}

Concentration factor

The concentration factor measures how far a metal has been enriched above its average crustal abundance to form an ore; rare metals need very large concentration factors to be economic.

Worked example

Grade, cut-off and concentration factor

A copper deposit has a grade of 2.0% copper. The average crustal abundance of copper is about 0.0055% and the cut-off grade is 0.5%. Calculate the concentration factor and state whether the deposit is economic.

Compare with the cut-off
The grade (2.0%) is well above the cut-off grade (0.5%), so the deposit is economic to mine.
Calculate the concentration factor
Concentration factor = ore grade / crustal abundance = 2.0 / 0.0055.
$\text{concentration factor} = \frac{2.0}{0.0055} \approx 364$
Interpret
The copper has been concentrated about 364 times above its average crustal level, which some geological process (here likely hydrothermal) achieved.

Result: The deposit is economic (2.0% is above the 0.5% cut-off) and the copper is enriched about 364 times above crustal abundance.

Typical mistakes

Treating 'ore' as a purely geological term; whether a deposit is an ore depends on economics (price, cost, technology) as well as grade.
Confusing the ore minerals (which contain the wanted metal) with the gangue (the worthless minerals mixed with them).

Active revision

Explain why the same body of low-grade copper rock might be an ore when the copper price is high but not when it is low.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — mineral resources (British Geological Survey)

Industrial minerals and construction materials#

FoundationWJEC/Eduqas A level Geology (Component 3) — Earth materials and natural resources

Types of Earth resource

Fig. 2The main categories of Earth resource: metallic ores, industrial and construction minerals, energy (hydrocarbons and coal) and water.

Not all mineral resources are metal ores; industrial minerals and construction materials are extracted in far greater bulk and are the foundation of the built environment. Aggregate — crushed rock, sand and gravel — is the most-quarried material of all, used in concrete, roads and railways; because it is low in value but needed in huge quantities, it must be worked close to where it is used, which is why quarries and gravel pits are so widespread. Suitable aggregate must be hard, durable and unreactive, so fresh, strong rocks such as igneous rock and well-cemented sandstone or limestone are favoured.

Limestone is a particularly versatile industrial rock: it is quarried for aggregate, burnt to make lime and cement, and used in steelmaking and agriculture. Clay is dug for bricks, tiles and, with limestone, for cement. Evaporites are valuable industrial minerals in their own right: rock salt (halite) is used for road de-icing and as a chemical feedstock, and gypsum for plaster and plasterboard. These materials are won by quarrying or by solution mining, and their value lies in bulk and utility rather than in a scarce element.

The geological requirements for these resources are about quality and quantity rather than exotic concentration. An aggregate resource needs a large volume of consistent, strong, unweathered rock with little overburden to remove; a cement resource needs limestone and clay of the right chemistry close together; a building stone needs a rock that is attractive, durable and can be cut into blocks. Assessing whether a rock is fit for a purpose — an unreactive aggregate for concrete, a durable stone for a building — is a practical, examinable application of the rock and mineral knowledge from earlier chapters.

These resources also raise the clearest environmental and planning issues in the subject, because they are worked at the surface, in bulk, and often near where people live. Quarrying creates dust, noise, traffic and visual scars and can affect groundwater, so extraction is balanced against amenity and conservation through planning controls, and worked-out quarries are restored to nature reserves, lakes or landfill. Weighing the need for these unglamorous but essential materials against their local impacts is a recurring evaluation theme.

Worked example

Selecting a construction material

A builder needs an unreactive, durable aggregate for concrete and a stone that can be cut into attractive blocks for facing a building. Recommend rock types and justify each choice.

Aggregate
For concrete aggregate choose a hard, unweathered, unreactive rock such as a strong igneous rock or well-cemented limestone, so it bears load and does not react with the cement.
Building stone
For facing choose an attractive, durable stone that cuts into blocks, such as a good sandstone or limestone (or granite for high durability).
Justify
The choices are governed by fitness for purpose: strength and inertness for the aggregate, durability and appearance for the facing.

Result: Use a hard, unreactive igneous rock or limestone as aggregate, and a durable, attractive sandstone, limestone or granite as building stone.

Typical mistakes

Overlooking bulk materials; aggregate and limestone are extracted in far greater quantities than metal ores.
Forgetting that aggregate must be worked close to where it is used because it is low in value relative to transport cost.

Active revision

Explain the geological properties that make a rock a good concrete aggregate, and why gravel pits tend to be near towns.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — industrial minerals (British Geological Survey)

Hydrocarbons and coal#

AdvancedWJEC/Eduqas A level Geology (Component 3) — Earth materials and natural resources

An anticlinal oil and gas trap

Fig. 3An anticlinal trap: buoyant hydrocarbons migrate up from the source rock into the reservoir and collect in the crest beneath the cap rock, with gas over oil over water.

Oil and gas form from the remains of microscopic marine organisms (plankton) that accumulate in oxygen-poor mud, so that the organic matter is buried and preserved rather than decayed. As this organic-rich source rock is buried, rising temperature 'cooks' the organic matter through the oil window (roughly 60 to 120 degrees Celsius) into oil, and at higher temperatures into gas — the process of maturation. Too little heat and the organic matter is immature; too much and it is destroyed, so hydrocarbons form only within a particular depth and temperature range, an important control on where they are found.

Once formed, oil and gas are less dense than the water that saturates the rocks, so they migrate upward and sideways through permeable rock until they either reach the surface and are lost or are stopped and collected. A commercial accumulation needs four things together: a source rock to generate the hydrocarbons, a porous and permeable reservoir rock to hold and yield them, an impermeable cap rock (seal) to stop them escaping, and a trap — a geological structure that holds the hydrocarbons in place. Missing any one of these and there is no field, which is why exploration is a search for all four in combination.

Traps take several forms. A structural trap is formed by deformation: the classic anticlinal trap collects buoyant oil and gas in the crest of an up-fold beneath the cap rock, and fault traps seal the reservoir against an impermeable bed across a fault. A stratigraphic trap is formed by the original geometry of the rocks, such as a reservoir that pinches out or is capped by an unconformity. In every case the oil and gas, being lighter, sit at the top of the reservoir with gas above oil above water, a layering that itself confirms the buoyancy-driven trapping.

Coal forms by a different route, from the accumulation of land plant material in waterlogged, oxygen-poor swamps, so that it builds up as peat rather than rotting. Burial then transforms the peat through increasing rank — from peat to lignite (brown coal) to bituminous coal to anthracite — as heat and pressure drive off water and volatiles and concentrate the carbon, so a higher-rank coal has a higher carbon content and energy value. Great coal deposits, such as the Carboniferous Coal Measures, therefore record extensive ancient swamp forests, tying the resource directly to the environments and time scale of earlier chapters.

Worked example

Evaluating a prospect

A drilling company finds an anticline in which a porous sandstone is overlain by an impermeable shale, with an organic-rich mudstone buried beneath at temperatures around 90 degrees Celsius. Assess whether hydrocarbons are likely and explain each element.

Source
The organic-rich mudstone at about 90 degrees Celsius is in the oil window, so it can generate oil (maturation).
Reservoir and cap
The porous sandstone can hold and yield oil, and the overlying impermeable shale is a cap rock that stops it escaping.
Trap
The anticline is a structural trap that collects buoyant oil in its crest beneath the cap rock.
Conclude
All four elements are present, so the prospect is promising and worth drilling near the crest of the anticline.

Result: With a mature source, a porous reservoir, a sealing cap and an anticlinal trap all present, the prospect is promising; drill the crest.

Typical mistakes

Thinking oil forms in the reservoir; it forms in the source rock and migrates into the reservoir.
Confusing the order of the elements; without a cap rock and a trap the hydrocarbons escape however good the source and reservoir.

Active revision

Explain why an anticline with a porous sandstone sealed by an overlying shale, above an organic-rich source rock, is a promising place to drill for oil.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — oil and gas (British Geological Survey)

Reservoirs, porosity and hydrogeology#

AdvancedWJEC/Eduqas A level Geology (Component 3) — Earth materials and natural resources

An aquifer and the water table

Fig. 4An aquifer is a permeable, water-bearing rock; the water table is the top of the saturated zone. A well yields water from the saturated aquifer.

Whether a rock can hold and yield a fluid — oil, gas or water — depends on two properties. Porosity is the percentage of the rock's volume that is pore space, and it determines how much fluid the rock can store; permeability is the ability of the rock to let fluid flow through it, and it depends on whether the pores are connected. A good reservoir or aquifer needs both: high porosity to store the fluid and high permeability to yield it. A well-sorted sandstone with rounded grains has high porosity and permeability, whereas a clay may have high porosity but such tiny, poorly connected pores that it is almost impermeable.

Porosity is calculated as the volume of pore space divided by the total volume of the rock, expressed as a percentage, and is measured in the laboratory or estimated from wireline logs. It is controlled by the sorting, packing, shape and cementation of the grains: good sorting and rounding give high porosity, while poor sorting (fine grains filling the gaps between coarse ones), tight packing and cementation reduce it. Because cementation during lithification fills pore space, deeper, older and more cemented sandstones tend to have lower porosity, an important control on reservoir quality.

The same properties govern groundwater, the water held in the pores of rocks, which is a vital resource. A permeable, water-bearing rock is an aquifer; an impermeable rock that will not transmit water is an aquiclude or aquitard. The water table is the surface below which the rock is saturated, and it rises and falls with recharge from rainfall and abstraction from wells. Where an aquifer is confined between impermeable beds and the water is under pressure, a well drilled into it may flow at the surface without pumping (an artesian well), a direct application of the reservoir concept to water supply.

Managing groundwater, like managing any reservoir, means balancing what is taken out against what is replenished. If water is abstracted faster than the aquifer is recharged, the water table falls, wells run dry, the ground may subside, and near the coast salt water can be drawn into the aquifer (saline intrusion). Understanding porosity, permeability, the water table and recharge lets a geologist assess how much water an aquifer can sustainably yield, and the same reasoning applies to how much oil or gas a reservoir will produce — a unifying idea that links water supply and hydrocarbon production.

\text{porosity (\%)} = \frac{\text{volume of pore space}}{\text{total volume}} \times 100

Porosity

Porosity is the percentage of a rock's volume that is pore space; it determines how much fluid the rock can store, while permeability (whether the pores connect) determines how readily it flows.

Worked example

Calculating porosity

A cylindrical core of sandstone has a total volume of 120 cm^3. When fully saturated it holds 27 cm^3 of water in its pore spaces. Calculate its porosity and comment on its likely quality as a reservoir.

Identify the values
Volume of pore space = 27 cm^3 (the water it holds); total volume = 120 cm^3.
Apply the formula
Porosity = (pore volume / total volume) x 100 = (27 / 120) x 100.
$\text{porosity} = \frac{27}{120} \times 100 = 22.5\%$
Interpret
A porosity of 22.5% is good; if the sandstone is also well sorted so the pores connect (high permeability), it would make a good reservoir or aquifer.

Result: The porosity is 22.5%, a good storage capacity; with good permeability the sandstone would be a good reservoir or aquifer.

Typical mistakes

Treating porosity and permeability as the same; a clay can be porous yet nearly impermeable because its pores are tiny and poorly connected.
Forgetting that cementation during lithification reduces porosity, so deeper, older sandstones often make poorer reservoirs.

Active revision

Explain why a well-sorted sandstone makes a better aquifer than a clay of the same porosity.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — groundwater (British Geological Survey)

Prospecting and exploration#

StandardWJEC/Eduqas A level Geology (Component 3) — Earth materials and natural resources

The stages of exploration

Fig. 5Exploration proceeds from cheap, broad methods (mapping, remote sensing) through geophysics and geochemistry to expensive, decisive drilling, narrowing the target at each stage.

Finding a resource is a staged process that moves from cheap, broad reconnaissance to expensive, precise evaluation, narrowing the search at each step. It begins with geological mapping and the study of existing maps and satellite imagery to identify areas with the right rocks and structures — a source-and-trap setting for hydrocarbons, the right host rocks and structures for ore, a suitable aquifer for water. Only where the geology is promising are the more expensive methods deployed, so that money is not wasted testing unpromising ground.

Geophysical methods detect resources indirectly by measuring physical properties of the rocks from the surface. Seismic surveying, the workhorse of oil exploration, sends sound waves into the ground and records their reflections from rock boundaries to build a picture of the sub-surface structure, revealing traps at depth. Gravity and magnetic surveys detect the density and magnetic contrasts of buried bodies — a dense ore body or a buried structure — and electrical and electromagnetic methods detect conductive ore minerals or the presence of water. These methods survey large areas without drilling.

Geochemical methods detect the chemical traces a concealed deposit leaves in its surroundings. Systematic sampling of soils, stream sediments, water and even vegetation, analysed for anomalously high concentrations of the target metal, can point to a hidden ore body up-slope or up-stream of the sample; a stream draining a copper deposit, for example, carries elevated copper. Such surveys are relatively cheap and are used to focus the search onto small target areas for the final, decisive stage.

The final stage is drilling, the only method that provides direct evidence by bringing up cores of the rock at depth; it confirms the presence, grade and extent of a deposit or reservoir but is very expensive, so it is done last and only on the best targets. The whole sequence — mapping, then geophysics and geochemistry, then drilling — reflects a rational balance of cost against certainty, and evaluating the appropriateness and limitations of each method for a given resource is a genuine applied-geology skill the specification assesses.

Worked example

Choosing exploration methods

A company wishes to explore for oil in a sedimentary basin. Recommend an appropriate sequence of methods and justify the choice at each stage.

Reconnaissance
Begin with geological mapping and study of existing data to identify areas with a source-reservoir-cap-trap setting, cheaply narrowing the search.
Geophysics
Run seismic surveys to image the sub-surface structure and locate possible traps (such as anticlines) at depth, over large areas without drilling.
Drilling
Drill the best-defined trap to obtain direct evidence, confirming whether oil is present and testing the reservoir; this is done last because it is very expensive.

Result: Map to find a promising basin, use seismic surveys to locate traps, then drill the best target last to confirm the oil directly.

Typical mistakes

Suggesting drilling first; drilling is the most expensive method and is used last, only on the best targets identified by cheaper surveys.
Assuming geophysics finds the resource directly; it detects physical contrasts that indicate possible structures or bodies, which drilling then tests.

Active revision

Recommend, in order, the methods you would use to explore for a buried metal ore deposit, and justify why each is used at that stage.

Active recall

Recall the key points — then reveal.

Sources: WJEC/Eduqas A level Geology specification (WJEC / Eduqas) · British Geological Survey — exploration and prospecting (British Geological Survey)

References & sources

Sources

WJEC / Eduqas

WJEC/Eduqas A level Geology specification

British Geological Survey

Earth materials and natural resources

5 sections~18 min reading time3 competenciesLevel Foundation 1 · Standard 2 · Advanced 2

Earth materials and natural resources

Key points

Grade, cut-off and concentration factor

Key points

Selecting a construction material

Key points

Evaluating a prospect

Key points

Calculating porosity

Key points

Choosing exploration methods

Sources

Earth materials and natural resources

Key points

Grade, cut-off and concentration factor

Key points

Selecting a construction material

Key points

Evaluating a prospect

Key points

Calculating porosity

Key points

Choosing exploration methods

Sources