Oil and Gas Flashcards
What is petroleum?
A naturally occuring viscous hydrocarbon mixture of crude oil, natural gas and semi solid asphalt.
What are the main requirements for the formation of oil and gas?
1) A source rock
2) Maturation
3) Migration
4) A reservoir rock
5) A cap rock
6) A trap
How do source rocks form?
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.
What is maturation?
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.
When does most oil and gas form?
Most oil form between 50 and 100 degrees and most gas forms between 100 and 200 degrees.
What factors control migration?
1) Permeability of the rocks
2) Pressure
3) Density differences
4) Viscosity of the oil
What is a reservior rock?
A porous and permeable rock capable of storing and yeilding large quantities of oil and gas.
What is a cap rock?
An impermeable layer of rock that prevents the migration of oil and gas from a reservoir rock.
What are traps?
The geological setting that allows oil and gas to be concerntrated in one place, making them more economic to extract.
How does an anticline form a trap?
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.
How does a fault trap oil?
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.
How do unconformities form traps?
These form if reservoir rocks below an angualr unconformitiy are overlain by cap rocks. Oil and gas will be trapped just beneath the unconformity.
How do salt domes create traps?
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.
How does lithology form traps?
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.
How can petroleum be destroyed?
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.
How can petroleum be lost?
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.
What is a seismic reflection survey?
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.
What is a gravity survey?
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.
What is exploration drilling?
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.
What are the advantages of Geophysical surveys?
1) Quicker and cover a larger area
2) Provides continuous data
What are the disadvantages of Geophysical surveys?
1) No samples are collected
2) Still requires interpretation
What are the advantages of exploration drilling?
1) Direct Evidence- provides actual rock samples
2) Precise depth information is obtained
What are the disadvantages of exploration drilling?
1) Very expensive
2) Boreholes are spaced out so may miss important info
What are reserves?
The amount of the resource that can be extracted at a profit using existing technology.
