Energy Resources

Concealed Coalfield

A concealed coalfield is where coal-bearing strata are below the surface and covered by younger rocks.

Reservoir Rock

A reservoir rock must have a high porosity to be able to store significant amounts of petroleum, it must also have a high permeability to allow oil to migrate into it and be extracted. The properties of a reservoir rock are the same as the properties of an aquifer. Suitable reservoir rocks include poorly cemented sandstones, limestones and fractured chalk.

Seismic Profile

A seismic profile shows the subsurface layering and can be interoperated by geologists to identify possible traps.

Coal Mining

Coal has been mined for thousands of years, initially where it outcropped and the seam was followed into the ground parallel to the bed. These adit mines gradually developed into full underground mines.

Directional and Extended Reach Drilling

Directional/extended reach techniques are used so that a number of wells from one well head can tap the reservoir rock over a large area.

Gamma-Ray Spectroscopy

Gamma rays emmited by rocks as a result of radioactive decay are counted. Potential source rocks give a high gamma ray count as they are mineralogically and chemically diverse where as reservoir rocks give a low reading as they are mineralogically and chemically simple.

Gravity Surveys

Gravity surveys can be land-based or airborne. AN instrument called a gravimeter measures small variations in the Earth's gravitational field strength in milligals (mGal). Survey point safe tied to each location via GPS. Maps are plotted with lines joining points of equal gravitational field strength and anomalies can then be identified.

Two-Way Time (TWT)

Seismic profiles are commonly constructed using two-way time, which is the time taken for a seismic waves to travel to a reflective layer and return to the surface

Acid Mine Drainage

Sulphur is a common impurity within coal and when mixed with rainwater creates weak sulphuric acid. The acid also dissolves and mobilises heavy metals e.g. Pb, Ni and Cd within the rocks which may enter the water supplies. Acidic waters also destroy vegetation.

Tar Sands

Tar sands (technically know as bitumen sands) are semi-solid fractions of petroleum mixed with clay and sand. Originally the oil/tar sands would have been conventional crude oil, however, over time bacteria have converted it to bitumen.

Depleted Oil and Gas Reservoirs

These are reservoirs from which all the recoverable oil and gas has been extracted, leaving underground rocks capable of storing gas. They are ideal as the geology is already well known and it is cost effective as the site is already drilled with equipment left over from extraction.

Water Flood Drive

Water is injected into the reservoir rock beneath the oil to maintain the pressure.

Oil and Natural Gas Reserves Increase If...

•Exploration discovers more. •Technology improves so more can be extracted from existing wells •Smaller oil fileds become economic when prices rise.

Blowout

A blowout is when oil gushes up the well and flows out onto the surface. Production wells must be quickly capped off to prevent blowout and oil spills.

Extraction of Geothermal Energy

A borehole is drilled into the rock. As super-heated water rises up the borehole, the reduction in pressure causes it to flash to steam. The steam may be used to directly drive turbines or passed through a heat exchanger to heat freshwater and generate steam, reducing the problems of corrosive volcanic waters. The water is then pumped back into the ground after use to eliminate waste disposal problems and help extend the life of the well.

Coal Seam

A coal seam is an economically viable layer of coal, usually 1-2m thick.

Exploration Drilling

A cylindrical drill commonly studded with diamonds is used to drill boreholes. The drill bit is cooled and lubricated by drilling mud, the density of this mud is controlled by adding Barite which is dense and allows the mud to reach the bottom of the hole. Rock chips and/or continuos drill core can be recovered and material is examined and logged. Continuous drill core is very expensive and only used in critical locations.

Sapropel

After plankton accumulate on the sea bed anaerobic bacteria cause them to partially decay forming an organic rich mud called a Sapropel. Diagenesis of a sapropel forms an organic rich sedimentary rock called a source rock, for example black oil shales and mudstones. Their dark colour represents their high carbon content and their fine grain size represents the low energy conditions of deposition.

Adit Mines

An adit mine is a horizontal tunnel dug into the hillside.

Exposed Coalfield

An exposed coalfield is where coal-bearing strata outcrop at the surface.

If Source Rocks Are Impermeable How Does The Oil Escape?

As maturation takes place Kerogen turns into Petroleum. Kerogen is a waxy solid composed of many different hydrocarbon compounds, where as petroleum is a viscous liquid. The transformation of a solid into a liquid corresponds with an increase in volume. This increase in volume induces an increase in pore fluid pressure which causes the rock to crack, creating pathways for the oil and gas to escape.

Maturation of Petroleum

As the source rock is buried, pressure and temperature increase. Over time the organic matter breaks down to form a mixture of organic compounds of C, H, O, N and S called Kerogen and finally Petroleum. This process is called maturation

Salt Caverns

Evaporites are impermeable and therefore ideal for underground storage. Salt Caverns are constructed by injecting water to dissolve the salt and then pumping it out as brine, which can be used in the chemical industry. Salt caverns are ideal as natural gas can be replenished and removed rapidly as they are hollow and do not contain grains to restrict flow.

What Is Geothermal Energy?

Geothermal energy is energy extracted from hot subsurface rocks, magma or heated water/steam. It is used to heat buildings, provide hot water and drive turbines to generate electricity.

Hot Dry Rock Sources

Granite intrusions contain a higher concentration of radioactive heat producing elements. As granites are impermeable twin boreholes are drilled and high pressure water is injected to artificially fracture the rock. Cold water is then pumped down one borehole, heated by the hot rocks and rises up the second borehole.

Geothermal Aquifers

Hot water can be extracted from aquifers in deep sedimentary basins. Geothermal aquifers are inter-bedded with impermeable mudstones and shale's, which act as insulators. Hot water is pumped to the surface, passed through a heat exchange and used for space heating or hot water supply. The cooled water is then re-injected to maintain water pressure within the aquifer.

Volcanic Sources

In volcanic areas, magma at 700-1200°C heats the surrounding groundwater. The hot groundwater can be trapped in porous and permeable rocks overlain by impermeable cap rocks, which act as insulation. The pressure at depth usually prevents the water from boiling so it becomes super-heated.

Down-Hole Logging

It is possible to mount geophysical instruments on a wire or "Sonde" (probe) that is passed down the drill hole to record data as it is slowly pulled back up from the bottom.

Coalification

Like any other sediment, peat undergoes diagenesis due to the effects of heat and pressure during burial. The diagenetic process of turning peat into coal is called coalification. As peat undergoes coalification and is compacted by the weight of the overburden, water and other volatiles are squeezed out.

British Oil and Gas Reserves

Most oil and gas reserves in the UK are located offshore, particularly in the North Sea, but gas fields also occur in the Irish Sea. There are a few onshore facilities, the largest of which is Wytch Farm in Dorset. Nearly all the British reserves are in Mesozoic (Triassic, Jurassic, Cretaceous) and Cenozoic (Tertiary, Quaternary) age sedimentary basins.

Gas Cap Drive

Natural gas (carbon dioxide or nitrogen) are injected into the reservoir rock above the oil to maintain the pressure.

How Can Petroleum Be Destroyed or Lost?

Oil and gas may be destroyed if the temperature exceeds 200°C due to: •Heat from an igneous intrusion or volcanic activity. •Regional metamorphism. •Burial where the geothermal gradient results in high temperatures. Oil and gas may be lost from a trap by: •Erosion or removal of the overlying cap rock. •Escaping upwards along an unsealed fault plane.

Requirements For The Formation of Oil and Gas

Oil and gas originate in sedimentary basins. The main requirements to form economic accumulations are: •A source rock •Maturation •Migration •A reservoir rock •A cap rock •A trap

Sustainability of Oil and Natural Gas

Oil and natural gas are non-renewable energy resources. They are fossil fuels that formed millions or years ago. Once they are burnt they are lost as gasses into the atmosphere and cannot be recycled on a human timescale. As a result the use of fossil fuels is ultimately unsustainable.

Oil Shale

Oil shale is a general term for any fine grained sedimentary rock containing significant amounts of kerogen, that did not undergo enough maturation to produce petroleum. Oil shales can be processed by steam injection to produce oil that can be pumped out. Alternatively they can be mined and the kerogen converted into synthetic crude oil or burnt as a low grade fuel.

Migration

Once formed the petroleum undergoes migration from the source rock to a reservoir rock. The main factors controlling migration are: •Permeability of rocks - There must be permeable rocks for the petroleum to travel through. •Pressure - Oil & gas naturally move down the pressure gradient from high to low pressure. •Density differences - Petroleum is less dense than water so will percolate upwards until it reaches an impermeable layer. •Viscosity of the oil - Higher temperatures lower the viscosity of the oil. The lower the viscosity the easier it is to flow.

Opencast Coal Mining

Opencast coal mining takes place from quarries or open pits at the surface. All the overburden is removed and the depth of this overburden is critical to the economics of the mine. The overburden is waste rock and piled up as a spoil heap. Blasting is then used to break the coal up and large machines called drag line excavators are used to extract it. After mining is complete the land can be restored by backfilling the pit with the overburden.

Safety Consideration

Opencast mining is safer than underground mining. Despite improvements in safety, underground mining is still dangerous. The main dangers are flooding, tunnel collapse, cave-ins and explosions caused by methane gas.

Economic Considerations

Opencast mining is very efficient and high production rates can be achieved up to 200m, but underground mining can achieve similar results if the geology is good. It is also cheaper as the setup and running costs are lower and only a small work force is required. Although machinery is expensive it is cheaper than that needed for underground mining. Also ventilation equipment is not required and thinner seams can be mined at a profit.

The Origin of Peat and Coal

Peat and coal form from the remains of terrestrial vegetation. Land plants are only found from the Silurian Period (c. 425Ma) onwards. Most coal formed during the Carboniferous (c. 360-300Ma).

Requirements For the Formation of Coal

Peat and coal originate in deltaic sequences. The main requirements for formation are: •Hot, humid climate - Encourages rapid growth of trees, so most coal forms in equatorial and humid, tropical climates. •Anoxic conditions - Prevents decay of the plant matter. Anaerobic bacteria slowly change the plant matter into peat. •Rapid burial - Only deltaic sedimentary environments have sufficiently high sedimentation and subsidence to build up the large thickness of peat required to form economic quantities of coal.

What is Petroleum?

Petroleum is a naturally occurring, viscous hydrocarbon mixture of Crude Oil, Natural Gas and Asphalt. In addition to fuel petroleum forms the basis of the petrochemical industry. Products include plastics, paints synthetic fibres, rubber, pharmaceuticals and fertilisers.

Source Rock Formation

Petroleum is formed from the remains of microscopic marine organisms called plankton. Plankton live in the photic zone of the oceans and after dying accumulate on the seabed. The environment of deposition must be low energy so they can settle out of the water and anoxic so the plankton won't decay or be scavenged.

Rank

Rank describes the percentage carbon in a coal. The rank is a measure of the maturity of the coal. As the rank increases, a number of physical and chemical changes occur, the most important being the increase in carbon. As rank increases the calorific value or amount of heat energy released during burring increases.

Reserves of Oil and Natural Gas

Reserves are the amount of the resource that can be extracted at a profit using existing technologies.

Resistivity

Resistivity is the ability of a material to resist the flow of electricity. Water is the main conductor present in rocks and gives a low resistance. However the presence of hydrocarbons results in a high resistance.

Seismic Reflection Surveys

Seismic surveys can be land-based or ship-based. Artificial seismic waves are generated by dropping a heavy thumper mounted on a vibrotruck on land or by air guns at sea. The seismic waves travel into the Earth and are reflected at layer boundaries within the strata. The reflected waves travel back to the surface where they are recorded by an array of receivers called geophones on land or hydrophones at sea. Their locations accurately pinpointed using Global Positioning Systems (GPS).

Unintentional Oil Fires and Burning of Fossil Fuels

Short term impacts: •Release of Carbon Monoxide, which binds with haemoglobin and reduces oxygen carrying capacity •Release of soot, which causes respiratory problems •Release of Volatile Organic Compounds, e.g. Benzene which are carcinogenic. Long term impacts: •Release of large amounts of Carbon Dioxide, which may contribute to the enhanced greenhouse effect.

The North Sea

The North Sea is a shallow continental sea that was subject to crustal extension during the Mesozoic when the Atlantic Ocean began to open. Faulting broke the area into a series of linear horsts and grabens that controlled sedimentation. The North Sea is split into two sedimentary basins by an east-west trending ridge or 'high'. The Northern North Sea basin contains oil and gas fields, whereas the Southern North Sea basin only contains gas fields.

The Northern North Sea Basin

The Northern North Sea basin has a variety of oil and natural gas fields. •The main source rock for oil is Jurassic age Kimmeridge Clay •Reservoir rocks are mainly marine sandstones and fractured chalk. •Cap rocks are mainly clays. •Oil and gas are contained by a variety of traps.

Issues Associated With Extraction of Oil

The extraction of petroleum leads to a reduction in pore fluid pressure. If the lithostatic pressure exceeds the pore fluid pressure then pore spaces will close as grains move closer together. This can create minor earthquakes and subsidence. However if pore fluid pressure exceeds lithostatic pressure fractures can be generated releasing the pore fluid pressure and leading to compaction and subsidence.

Geothermal Gradient

The geothermal gradient is the increase in temperature with depth. The average geothermal gradient is 25°C/km, varying from 10°C/km in continental shield areas to more than 70°C/km in volcanically active areas. Areas with the highest geothermal gradient tend to be at or near plate margins and this is where the prospects for geothermal energy are best.

Southern North Sea Basin

The hydrocarbon reservoirs in the Southern North Sea basin do not contain oil but only produce natural gas. •This is because the source rock for this area is the carboniferous coal measures. •Coal forms from the breakdown of vegetation so the source rock did not at any time contain plankton needed for oil. •Escaped gas is mainly held in desert sandstone reservoirs. •The cap rocks are often clays and evaporites and the traps are mainly salt domes and their associated anticlines.

The Coal Series

The rank of coal increases with depth and temperature, producing the coal series, which increases in rank form: • Peat→Lignite→Bituminous Coal→Anthracite

Stripping Ratio

The ratio of overburden removed to the amount of coal removed, e.g. a ratio of 20:1 means that for every 20m of overburden removed 1m of coal is removed. Opencast mining can take place up to 200m below the surface providing the stripping ratio does not exceed 20:1.

Cap Rock

The reservoir rock must be overlain by an impermeable cap rock. The cap rock prevents further migration petroleum. Without a cap rock the petroleum would continue to rise, eventually forming oil seeps or tar pits on the surface. Suitable cap rocks include clay, mudstone, shale and crystalline sedimentary rocks such as evaporites.

Benches

The sides an opencast mine are dug at an angle and stepped to increase stability and prevent collapse. The flat parts of the steps are called benches. The angle of the sides depends on: •Rock type- Weak incompetent rock like clays require shallow slopes. •Weathering - Heavily weathered rock will be weaker and require shallow sides. •Structures such as faults and joints weaken the rock. •Dip of the strata - Rocks dipping towards the pit are more prone to slip.

Travel Time

The time taken for the reflected waves to arrive back at the receivers is called the travel time.

Primary Recovery

To extract reserves the reservoir rock is drilled and a production well established. Initially oil comes to the surface under natural pressure caused by hydrostatic pressure of he water in pore spaces beneath the oil, gasses in the oil coming out of solution and the expansion of gas above the oil. After the natural pressure is released oil is pumped to the surface using nodding donkeys. Typically only 20-30% of the oil in a reservoir can be recovered by primary recovery.

Traps

Traps are where the geological setting allows oil and gas to be concentrated in one place, making them economic to extract. All traps require a porous and permeable reservoir rock to be overlain by an impermeable cap rock.

Longwall Retreat Mining

Two horizontal roadways are dug out from the main shaft to the furthest extent of the coal bed being extracted. A coalface (longwall) up to 400m long is established between the two roadways. A rotating machine called a shearer moves along the coalface, cutting slices of coal that falls onto a conveyor belt and is transported to the shaft and up to the surface. The roof is held up by closely spaced, mobile hydraulic steel supports called chocks, once a slice of coal is removed, the chocks are moved forward and the mined-out area is allowed to collapse. Mining takes place backwards, retreating towards the shaft.

Underground Gas Storage

UK gas consumption is rising and to meet this demand natural gas is imported. Underground gas storage facilities are being built to store the gas until it is needed. Onshore facilities are more convenient but may have to be located in environmentally sensitive locations. Gas can be stored in depleted oil and gas reservoirs and salt caverns.

Unconventional Sources of Petroleum

Unconventional sources include oil shales, tar sands and coal-bed methane. Historically the exploration of these sources was unpopular due to higher production costs and greater environmental impacts. But as global reserves of oil have dwindled, more attention is being focused on them.

Causes of Gravity Anomalies

•A positive gravity anomaly is caused by an excess of mass or materials that are denser than their surroundings. Positive anomalies are usually associated with anticlinal traps. •A negative gravity anomaly is caused by a deficit of mass or materials that are less dense than their surroundings. Negative anomalies are usually associated with salt dome traps.

Spills of Light Fractions

•Covers seabirds leading to a loss of buoyancy and drowning. •Enters the plumage of birds and impacts upon their thermoregulation. •Blocks out sunlight leading to a reduction in photosynthetic algae (Phytoplankton), which in turn has knock-on effects for the ecosystem.

Environmental Issues

•Deforestation leads to carbon dioxide sinks and landslides as trees roots bind surfaces together and leaves act as interception surfaces. •Destruction of habitats and ecosystems. •Dust from blasting and mining cause's silicosis (lung disease). •Visual pollution from scar on landscape and noise pollution from blasting and transport trucks. •Burning of coal leads to carbon dioxide and soot emissions.

Geological Factors Affecting Coal Mining

•Faults displacing the coal seam - Throws of >1m can disrupt production as the machinery has to be moved. Also faults are zones of permeability and weakness that may cause flooding or collapse. •Folds and steep dips - Longwall mining can only take place horizontally and isn't possible if the coal seam dip at an angle of >5°. •Build-up of methane gas - Methane is highly flammable and can cause dangerous underground explosions. •Presence of hard and/or permeable sandstones - The shearer can be damaged by the rock and the mine might be flooded.

Why Do Older Rocks In the UK Not Contain Oil and Gas?

•Metamorphism in parts has raised temperatures denaturing the oil. •Intrusions of igneous rocks denatures the oil. •Older rocks are generally buried deeper raising the temperature and denaturing the oil.

Advantages of Offshore versus Land-Based Surveys

•Offshore surveys are particularly effective as a large number of hydrophones an be towed behind a ship. •Also a ships path is not restricted like vibrotrucks are on land.

Seam Splitting and Washouts

•Seam splitting - Where one seam splits into several thinner seams the seam is rendered uneconomic. •Washouts - Changes in the geology as a result of the river channel changing cause on the delta top, the coal seam is replaced by hard sandstones and conglomerate in places.

Where Does The Earth's Heat Come From?

•The planets internal heat was originally generated during its formation. This energy is known as gravitational binding energy, the energy generated when mass accretes. •Additional heat is continually generated by the radioactive decay of heat producing elements such as Uranium, Thorium and Potassium.

Secondary Recovery

•Thermal methods - Steam is injected to increase the oils temperature and lower the viscosity of the oil •Detergents - Used to reduce surface tension of the oil to loosen it from the grains •Bacteriological - Digest and break down large hydrocarbon molecules to decrease viscosity. Even after secondary recovery 20-30% of the oil still remains in the reservoir.

Oil and Natural Gas Reserves Decrease If...

•They are being extracted •Calculations of reserves are incorrect •Smaller oil fileds become uneconomic when prices fall.

Spills of Heavy Fractions

•Toxic heavy fractions sink to the seabed, where they persist for a long time and smother organisms. •The oil reacts with dissolved oxygen in the water and thus reduces the available oxygen for marine life.

Factors Affecting Recovery

•Viscosity of oil - Higher viscosity oils don't flow as easily. •% of matrix or cement - A Higher % will reduce pore space/storage capacity and increase grains per unit volume therefore increasing surface tension. •Grain shape - Rounded grains have a higher porosity •Sorting - Poorly sorted rocks contain less oil as smaller grains infill pore spaces. •Grain size - Smaller grains have a larger surface area producing more friction and reducing permeability. •Joints - Joints increase porosity and permeability allowing oil to be stored and transmitted.