Oil and Gas

Question 1

What is petroleum?

Answer 1

A naturally occuring viscous hydrocarbon mixture of crude oil, natural gas and semi solid asphalt.

Question 2

What are the main requirements for the formation of oil and gas?

Answer 2

1) A source rock
2) Maturation
3) Migration
4) A reservoir rock
5) A cap rock
6) A trap

Question 3

How do source rocks form?

Answer 3

Microscopic organisms called plankton sink to the bottom of the ocean to accumulate. The environment must be low energy and anoxic so plankton will not decay. Anerobic bacteria cause partial decay of the plankton creating a sapropel. Burial of the sapropel results in the formation of the source rock.

Question 4

What is maturation?

Answer 4

As a source rock is buried it is subjected to compaction and an increase in temperature. The organic matter breaks down to form a mixture of compounds of C, H, O, N and S called kerogen and then petroleum.

Question 5

When does most oil and gas form?

Answer 5

Most oil form between 50 and 100 degrees and most gas forms between 100 and 200 degrees.

Question 6

What factors control migration?

Answer 6

1) Permeability of the rocks
2) Pressure
3) Density differences
4) Viscosity of the oil

Question 7

What is a reservior rock?

Answer 7

A porous and permeable rock capable of storing and yeilding large quantities of oil and gas.

Question 8

What is a cap rock?

Answer 8

An impermeable layer of rock that prevents the migration of oil and gas from a reservoir rock.

Question 9

What are traps?

Answer 9

The geological setting that allows oil and gas to be concerntrated in one place, making them more economic to extract.

Question 10

How does an anticline form a trap?

Answer 10

Oil and gas will be concerntrated in the top of the reservoir rock at the crest of the anticline provided it is overlain by a cap rock.

Question 11

How does a fault trap oil?

Answer 11

These form when movement along a fault plane results in a reservoir rock being moved adjeacent to an impermeable rock. This prevents the oil from escaping laterally and provided the strata are dipping the oil will migrate and be trapped at the top of the reservoir rock next to the fault.

Question 12

How do unconformities form traps?

Answer 12

These form if reservoir rocks below an angualr unconformitiy are overlain by cap rocks. Oil and gas will be trapped just beneath the unconformity.

Question 13

How do salt domes create traps?

Answer 13

These results from the presence of evaporites which have a lower density than the surrounding rock and rise upwards towards the surface. These uplift and piece the surrounding rocks to form salt domes. The overlying rocks are folded gently upwards into anticlines and because evaporites are crystalline they are impermeable and form a good cap rock.

Question 14

How does lithology form traps?

Answer 14

These result from varations in rock type. Fossilized limestone reefs make ideal reservoirs because they have good porosity and an abundance of life to produce the organic matter needed to produce petroleum. Sandstones that formed as a river channel, point bar or deltaic deposits often have a lens shape making small but common traps.

Question 15

How can petroleum be destroyed?

Answer 15

Oil and gas may destroyed if the temperature increases above 200 degrees due to:

1) Heat from an intrusion or volcanic activity
2) Regional metamorphism
3) Burial where the geothermal graident results in high temperature.

Question 16

How can petroleum be lost?

Answer 16

Oil and gas can be lost from a trap by:

1) Erosion and removal of the cap rock
2) Escaping upwards along an unsealed fault.

Question 17

What is a seismic reflection survey?

Answer 17

Artifical seismic waves are generated by explosions or vibrations. The seismic waves travel into the Earth and are reflected at layer boundaries with sedimentary sequences. The reflected waves travel back up to the surface where they are detected by geophones or hydrophones.

Question 18

What is a gravity survey?

Answer 18

An instrument called a gravimeter measures small vibrations in the Earth’s gravitational field strength. Maps are plotted with lines joining points of equal gravitational field strength and anomalies can be identified.

Question 19

What is exploration drilling?

Answer 19

A cylindrical drill is ised and the drill bit is commonly studded with diamonds to cut through rock for easily. The drill is cool and lubricated by drillung mud. Rock chips or continuous drill core can be recovered and material is exaimined and logged.

Question 20

What are the advantages of Geophysical surveys?

Answer 20

1) Quicker and cover a larger area

2) Provides continuous data

Question 21

What are the disadvantages of Geophysical surveys?

Answer 21

1) No samples are collected

2) Still requires interpretation

Question 22

What are the advantages of exploration drilling?

Answer 22

1) Direct Evidence- provides actual rock samples

2) Precise depth information is obtained

Question 23

What are the disadvantages of exploration drilling?

Answer 23

1) Very expensive

2) Boreholes are spaced out so may miss important info

Question 24

What are reserves?

Answer 24

The amount of the resource that can be extracted at a profit using existing technology.

Question 25

How can oil and gas reserves go up?

Answer 25

1) Exploration discovers more
2) Technology improves os more can be extracted.
3) Smaller oil fields become economic if prices rise.

Question 26

How can oil and gas reserves go down?

Answer 26

1) They are being extracted

2) Calculations of reserves are incorrect

Question 27

What is primary recovery?

Answer 27

A reservior rock is drilled into and the well must be capped off to prevent blowout and oil spills. After the natural pressure dies away oil is pumped to the surface using pumps called nodding donkeys.

Question 28

What factors affect recovery?

Answer 28

1) Viscosity of the oil
2) % of cement
3) Grain shape
4) Sorting
5) Grain size
6) Joints

Question 29

What are secindary recovery techniques?

Answer 29

1) Water flood drive
2) Gas cap drive
3) Thermal methods
4) Detergents
5) Bacteriological

Question 30

What are the main options for storing oil and gas?

Answer 30

1) Depleted oil and gas reservoirs

2) Salt caverns

Question 31

What are the advantages of using depleted oil and gas reservoirs?

Answer 31

1) The geology will already be well known and conditions for storage are present.
2) Cost effective
3) Equipment left over from when the reservoirs was in production will still be present

Question 32

How are salt caverns used?

Answer 32

Fresh water is injected to created the cavern, Brine is disposed of at sea or used in production of chemicals, Once fully formed the cavern is used to store natural gas.

Question 33

Why is there only oil in the Southern North Sea basin?

Answer 33

1) The source rock for this area is carboniferous coal measures.
2) Coal forms from vegetation therefore the source rock did not at any time contain the marine plankton

Question 34

How are oil and gad unsustainable?

Answer 34

Oil and gas are non renewable energy resources and cannot be recycled on a human timescale.