OIL AND GAS Flashcards
Formation of hydrocarbons
- plankton in a shallow marine basin
- they die and are buried without decaying, forming carbon rich rock - e.g black shale
- this is the source rock, as the organic material forms sapropel
- Black shales are buried, converting sapropel into kerogen
- Temp and pressure rise, maturation occurs between 50-200°C
- this turns kerogen into petroleum
- petroleum is less dense than water, so it migrates upwards through bed dip until trapped or escapes
Oil and gas window
- shows optimum temperature for production
- oil - 100°C
- gas - 140°C
diagram 21
Anticline trap
Structual
- cap rock layer above reservoir rock
- petroleum can spill from sides if full
diagram 22
Fault trap
Structual
- dip same direction as fault
- fault brings cap rock next to reservoir
diagram 23
Salt dome trap
Structual
- evaporites have low density, so rise and form dipairs
- evaporites are impermeable, so hydrocarbons accumulated in dipping reservoir rock
- can cause anticline trap above
diagram 23
Unconformity trap
Stratigraphic
- unconformity is cap rock
- cap rock beside reservoir rock aswell
diagram 23
Lithological traps
Stratigraphic
Due to lateral variation along beds
- pinch out trap
- reef trap
-fossilised limestone reef is porous
diagram 23
Fate of petroleum
Lost
- through eroded cap rock, fault, etc
Destroyed
- metamorphism, igneous activity, Deep burial, etc
Drilling fluid / mud
High density fluid pumped down boreholes:
- keeps drill bits clean
- removes rock fragments
- maintains hydrostatic pressure, preventing fluid entering borehole, preventing blow out
Blow out - caused by pressure, forcing oil up a borehole
- can catch fire or cause pollution
Primary recovery
- oil comes to surface naturally due to pressure
- gas in oil comes out of solution
- nodding donkeys used after natural pressure is released
- recovers 25% of oil
Secondary recovery
- Water flood drive - water injected under oil to maintain pressure
- steam injection - increases temp which lowers viscosity
- bacteria - breaks down long hydrocarbons to lower viscosity
- 25% of oil is unrecoverable
Difficulty with extracting oil
- deep water
-anchoring rig is difficult - very viscous
-doesn’t flow - in impermeable rock
-doesn’t flow - small oil field
-not economic to exploit
Northern North Sea
- Europe and North America were joined in pre-jurassic
- Early rifting occurred (Y shaped/graben between Scotland + Norway)
- rift flooded and Kimmeridge clay deposits built up. Plankton created a source rock
- rift was infilled by delta of permeable Brent sandstone
- burial and maturation of clay produced petroleum which migrated up-dip into reservoir rock
- Faulting and salt mobilisation created traps
Southern North Sea
- source of unconventional petroleum (high production cost + environmental impact)
- Carboniferous coal (delta-top)
- Burial under permian desert sandstone (reservoir)
- covered by Zechstein salt (cap)
