OIL AND GAS Flashcards

1
Q

Formation of hydrocarbons

A
  • 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
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2
Q

Oil and gas window

A
  • shows optimum temperature for production
  • oil - 100°C
  • gas - 140°C

diagram 21

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3
Q

Anticline trap

A

Structual

  • cap rock layer above reservoir rock
  • petroleum can spill from sides if full

diagram 22

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4
Q

Fault trap

A

Structual

  • dip same direction as fault
  • fault brings cap rock next to reservoir

diagram 23

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5
Q

Salt dome trap

A

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

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6
Q

Unconformity trap

A

Stratigraphic

  • unconformity is cap rock
  • cap rock beside reservoir rock aswell

diagram 23

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7
Q

Lithological traps

A

Stratigraphic
Due to lateral variation along beds

  • pinch out trap
  • reef trap
    -fossilised limestone reef is porous

diagram 23

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8
Q

Fate of petroleum

A

Lost
- through eroded cap rock, fault, etc

Destroyed
- metamorphism, igneous activity, Deep burial, etc

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9
Q

Drilling fluid / mud

A

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

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10
Q

Primary recovery

A
  • 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
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11
Q

Secondary recovery

A
  • 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
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12
Q

Difficulty with extracting oil

A
  • deep water
    -anchoring rig is difficult
  • very viscous
    -doesn’t flow
  • in impermeable rock
    -doesn’t flow
  • small oil field
    -not economic to exploit
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13
Q

Northern North Sea

A
  • 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
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14
Q

Southern North Sea

A
  • source of unconventional petroleum (high production cost + environmental impact)
  • Carboniferous coal (delta-top)
  • Burial under permian desert sandstone (reservoir)
  • covered by Zechstein salt (cap)
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15
Q

Unconventional petroleum - oil shale

A
  • fine grained sedimentary rock containing kerogen which did not mature into petroleum
  • processed by steam Injection to make oil
  • can be mined and converted into synthetic oil
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16
Q

Unconventional petroleum - Athabasca tar sands

A
  • Canada
  • area of boreal forest and muskeg bog
17
Q

Uncoventional petroleum - Orinoco oil belt

A
  • Venezuela
  • 90% of global reserve for heavy crude oil
  • mixed with water to make various fuels
18
Q

Unconventional petroleum - frozen gas hydrates

A
  • present on ocean floor sediments and permafrost
  • produce methane gas when melted
19
Q

Wytch Farm

A
  • Europe’s largest onshore oilfield
  • Poole harbor, Dorset
  • source - Jurassic blue lias clay
  • reservoir - Jurassic Bridport sandstone
  • estimated 480 million barrels

Environmental factors:
- screening with trees
- ecological surverys asses impact
- fracking is not permitted
- monitor air + groundwater

20
Q

Finding hydrocarbons - mud logging

A
  • mud brings small rock chippings
  • examined by mud loggers for rocky type and microfossils
  • correlate geological sequence
21
Q

Finding hydrocarbons - downhole logging

A

Wireline survey

  • geophysical Instruments mounted in a sonde
  • sonde enters borehole on a wireline
  • as its pulled back up, it measures:
  • Porosity -storage capacity of reservoir
  • Gamma Ray -source rocks have high gamma Ray counts
  • Resistance -hydrocarbons have high resistance
22
Q

Finding hydrocarbons - seismic reflection

A

Land based

  • heavy thumper to generate vibrations (e.g vibrotruck)
  • change in rock type reflects waves
  • geophones detect reflected waves

Water based

  • air guns generate vibrations
  • many hydrophones detect reflected waves (faster data gain)
  • ships are less restrictive than trucks
23
Q

Finding hydrocarbons - gravity survey

A
  • gravimeters measure variations in GFS
  • data is corrected for effects of latitude, altitude, topography (bouger map)
  • remaining variation is due to density of rock
  • positive anomaly = excess mass (anticline, horst)
  • negative anomaly = deficit mass (salt dome)