Environmental Geology 2/3

Name the three measurements made during down-hole logging
1. Porosity 2. Gamma ray spectroscopy 3. Resistivity
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How does porosity affect measurements?
The higher the porosity the higher the possible oil and natural gas content of a reservoir rock, it is also possible to interpret what fluids are present in the pore spaces- brine, gas or oil
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What is gamma ray spectroscopy?
It counts the gamma rays emitted form the rocs as a result of natural radioactive decay -potentail source rocks such as black oil shales and mudstones give a high gamma ray count
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What is resistivity?
This measures resistance of the rock to the flow of electricity- water is the main conductor present in rocks and gives a low resistance- the presence of hydrocarbons results in high resistance
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What is a natural resource?
Any natural material that is useful and valuable
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What is a reserve?
The amount of the resource that can be extracted at a profit using existing technology
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What is a proven reserve?
Those that we are certain are there
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What is a probable reserve?
Those that we are reasonably certain are there
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What is a possible reserve?
Ones that may become economic in the future if conditions are favourable
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Name 3 ways that oil and natural gas reserves go up
1. More are discovered 2. Technology improves so more ca be extracted from existing reservoirs 3. Smaller oilfields become profitable as prices rise
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Name 3 ways oil and natural gas reserves can go down
1. Continuous extraction from Earth 2. Calculations of reserves are incorrect 3. Smaller oilfields become uneconomic if prices fall
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How are reserves estimated?
Made on the basis of incomplete geological data, volatile market prices and require the use of very sophisticated mathematical modelling
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What is primary recovery?
Where oil initially gushes to the surface under natural pressure and is then pumped out
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How is the oil extracted?
The reservoir rock is drilled into and a production well is established. The well must be quickly capped to prevent blowouts and oil spills
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What is a blowout?
Occurs when oil gushes uncontrolled to the surface
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Name 3 ways in which the oil gushes to the surface due to
1. Gases in the oil coming out of the solution 2. Expansion of gas above oil 3. The hydrostatic pressure of the water in pore space beneath the oil
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What percentage of oil can be recovered by primary recovery?
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How can the reservoir rock affect the amount of oil recovered?
The porosity and permeability of the rock are ket factors determined by grain size, roundness and sorting. The amount of matrix or cement will be factors
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What is secondary recovery?
Where water is injected below or natural gas is injected above the oil to maintain the pressure
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Name the 5 secondary recovery techniques?
1. Water flood drive 2. Gas cap drive 3. Thermal methods 4. Detergents 5. Bacteriological methods
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What is a water flood drive?
Water is injected into reservoir rock beneath the oil to maintain the pressure
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What is a gas cap drive?
Natural gas, carbon dioxide and nitrogen gas are injected into the reservoir rock above the oil to maintain the pressure. Some gases dissolve in the oil lowering the viscosity
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What are thermal methods?
Steam injections can be used to increase the temperature and lower the viscosity of the oil
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How are detergents used?
Reduce the surface tension of the oil to loosen it from the grains
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What are the bacteriological methods?
Developed to digest and breakdown large hydrocarbon molecules to decrease their viscosity
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What is an oil spill?
Unintentional release of hydrocarbons due to human activity- spills may occur during extraction or transportation of oil
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Why are blowouts dangerous?
They usually catch fire and burn ferociously. There are specialist companies which deal with blowouts. The only way of extinguishing the fire is to drill a relief well to reduce the pressure at depth so the fire can be put out with water
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How can oil fires cause atmospheric pollution?
Smoke from burnt crude oil contains a cocktail od dangerous chemicals including particulate matter, sulfur dioxide, carbon monoxide, benzenes and dioxines
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Name one way in which the disposal of oil rigs can cause environmental problems
They are usually sunk in deep water which can be damaging. However some oil rigs can be used as artificial reefs for coral and fish
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What is the most economical way of transporting oil and natural gas?
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How do pipelines work?
They are made of steel and plastic. The oil and gas is kept flowing along the pipeline by a series of pumping stations. They are usually above ground but may be buried in built-up and environmentally sensitive areas and dangerous areas
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Name a famous pipeline
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Name 3 construction challenges that the pipeline presented
1. Passes through remote, mountainous and environmentally sensitive areas 2. The area is underlian by permafrost which becomes boggy and unstable 3. Passes over active faults that are prone to earthquakes
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Name the strategies which were used to stabilise the pipeline
1. Building it on supports above ground 2. Using refrigeration plants to pump cold brine into the pipes so the permafrost wouldn't melt 3. Rollers and shock absorbers for movement during earthquakes
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Name two methods of underground gas storage
1. Depleted oil and gas reservoirs 2. Salt caverns
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How are depleted oil and gas reserves used?
1. Reservoirs from which all recoverable oil and gas have been extracted- this leaves underground rocks capable of strong excess oil and gas. To maintain the pressure, 50% of the reservoir must be gas
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Name 2 advantage of the use of this method
1. The geology is known 2. There is still gas present so you don't have to inject as much gas
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How are salt caverns used?
The shape of the caverns aren't ideal so new cavities are created by dissolving the rock salt and pumping it out as brine. Natural gas can be replenished and withdrawn more quickly from salt caverns than from depleted hydrocarbon reservoirs
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Where is the largest onshore oilfield
Wytch farm, 3 oil fields which underlie Poole harbour Dorset
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Who is the oil field operated by?
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At what age are the oil fields around the British isles?
Mesozoic and Cenozoic age
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Why are older rocks less likely to to contain hydrocarbons?
Buried deeply and have low porosity due t the compaction that they have undergone and may have been subjected to conditions above 200
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When was the North sea oil discovered?
Early 1960s
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What is the North sea oil?
A shallow continental sea that was subjected to crustal extension during the mesozoic when the Atlantic ocean started to open. Faulting broke the area into a series of linear horst and graben and controlled sedimentation
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What is the main source rock in the Northern sea basin?
Jurassic age Kimmeridge clay
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What are the reservoir rocks in the Northern sea basin?
Marine sandstones and fractured chalk- cap rocks are mainly clays with a great variety of traps
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What do the Southern north sea basins do not contain?
Oil but only produce natural gas. This is because the source rock for this area is the carboniferous coal measures- the source rock did not at any time contain the marine plankton needed for oil formation, gas escaped from he coal as it formed
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What is the cap rock?
Often evaporites and the traps are mainly salt domes and their associated anticlines
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What is a non-renewable energy source?
Takes millions of years to form and cannot be replenished on human time scales
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What does unsustainable mean?
Resources that cannot continue in the future
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What happens when reserves fall?
Each year the production of both oil and gas steadily declining
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Why has unconventional petroleum sources been unpopular?
High production costs and greater environmental impacts
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What sequence do peat and coal originate from?
Deltaic sequences
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Name 3 requirements for the formation
1. Land plants 2. Anoxic environment 3. Rapid burial
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What period did most coal formed in?
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What climates encourage the growth of trees?
Hot and humid climate
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What prevents the decay of matter?
A swampy/boggy environment which ensures that is it anoxic, preventing the decay of the plant matter
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What environment has high amounts of sedimentation to build an economic coal seam?
deltaic sedimentary environments
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What is coalification?
The diagenetic process by which peat is turned into coal by the effects of heat and pressure during burial
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What is a coal seam?
A layer of coal
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What is the coal series?
A sequence of increasing rank from peat to anthracite
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What is in the coal series?
Peat-Lignite-Bituminous coal-anthracite
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What is the rank a measure of?
The maturity of the coal
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What is the most important change in the series?
The increase in carbon
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What happens when the percentage of carbon increases?
The calorific value or amount of heat energy released on burning increases
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What is the main coal used?
Bituminous coal
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What can bituminous coal make?
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How is coke made?
Made by baking bituminous coal at high temperatures without oxygen to drive out the volatiles
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What is coke used for?
Used in blast furnaces to produce iron and steel, it is also burned as 'smokeless' fuels
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What else can coal be used for?
Methan can be recovered from coal, coal can be converted into liquid fuels such as petrol but the process releases large amounts of carbon dioxide
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What is open cast coal mining
Mining from surface quarries
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How do they work?
The rock above is removed-this is called the overburden and is critical to the economics of the mining operation- the sides of the open pit are dug at an angle and stepped to increase stability and prevent collapse-the flat parts are called benches
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Name 3 factors that affects the angle of the sides
1. Rock type 2. Weathering 3. Structures in rock
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How can rock type affect the angle of the sides?
Weak and incompetent rocks will require shallower side slope angles
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How can weathering affect the angle of the sides?
Heavily weathered rock will be weaker and need shallower sides
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How can structures in rocks affect the angle of the sides?
Faults and joints can weaken the rock and may need rock bolts, wire netting, shotcrete, rock drains or other ground improvement strategies
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What are spoil heaps?
Where the excess overburden is piled up on the ground
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What method is used to break the coal up?
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How is the coal collected?
Large machines called dragline excavators extract the coal
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What happens after mining?
The land can be restored by backfilling the pit with the overburden
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What is longwall mining?
Highly mechanised method of underground mining
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What is the importance of the shaft?
Dug at the surface and tunnels and roadways are driven out from the shaft. A ventilation shaft is essential
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What happens in longwall retreat mining?
Two horizontal roadways are driven out to the furthest point of the area of coal to be extracted which allows geological conditions to be assessed prior to mining
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Where is the coalface?
Established between the two roadways
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What does the shearer do?
Rotating machine moves to and fro along the coalface, cutting slices of coal. The coal then falls onto a conveyer belt and is transported to the shaft and up to the surface
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How is the roof held up?
Held up by closely spaced, mobile hydraulic steel supports called chocks. Once a slice of coal is removed the chocks are moved forwards and the mined out area is allowed to collapse
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Does mining take place backwards or forwards?
Backwards- retreating towards the shaft
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What happens when the shaft does not collapse immediately?
A cavern can open up, which is dangerous as it puts immense pressure on the chocks
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Name geological factors which can make underground coal mining difficult and uneconomic
1. Faults 2. Folds 3. Washouts 4. Seam splitting 5. Rock type 6. Methane
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How can faults make it difficult or uneconomic?
Faults can displace the coal seam. Throws as little as 1/2 cm can disrupt production as the machinery has to be moved upwards or downwards. A coal seam on 1 side may give way to hard deltaic sandstones which can damage the shearer
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How can folds and steep dips make it difficult/uneconomic?
Longwall mining takes place horizontally and is not possible if the coal seams dip at angle greater than about 5 degrees
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How can washouts make it difficult/uneconomic?
Washouts resulting from river channel switching on the delta top. the peat is eroded away and gravel s are deposited instead-disrupting coal seam and sandstones in its place
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How can seam splitting be a problem?
Where one thick seam splits into several thinner uneconomic seams. It results from different rates of subsidence in different part of the delta
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How can rock type be a problem?
Sanstones are hard and permeable so may allow flooding of the mine especially of the water table is high
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How can methane be a problem?
methane is highly flammable and can cause dangerous underground explosions
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Which method of coal mining is cheaper?
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Whee did coal form?
Formed in the top sets of cyclothems
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What is a cyclothem?
Repeated cycle of sedimentation
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What is a coal measure?
The name for the carboniferous-age coal-bearing strata
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Name the two types of coal field
Exposed and concealed coalfield
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What is an exposed coalfield?
Where coal bearing strata outcrops at the surface
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What is a concealed coalfield?
Where coal-bearing strata is below the surface overlain by younger rocks
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Name an environmental consequence of open cast coal mining
Short term problems of noise, dust and physical pollution during the mining operation. By law the hole has to be restored and backfilled
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Name an environmental consequence of underground coal mining
Long lived mining, spoil heaps used to blot the landscape, rain water can be contaminated
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What is an environmental consequence of longwall mining?
It can cause surface subsidence, it can also trigger small earthquakes, it can also generate acid mine drainage water
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Other cards in this set

Card 2


How does porosity affect measurements?


The higher the porosity the higher the possible oil and natural gas content of a reservoir rock, it is also possible to interpret what fluids are present in the pore spaces- brine, gas or oil

Card 3


What is gamma ray spectroscopy?


Preview of the front of card 3

Card 4


What is resistivity?


Preview of the front of card 4

Card 5


What is a natural resource?


Preview of the front of card 5
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