Chapter 9 - Oil, gas and basins

Question 1

What is a viable source rock?

Answer 1

fine grained
organic rich
deposited in low energy anoxic marine environment

Question 2

What is a viable resevoir rock?

Answer 2

porous
permeable
deposited in a high energy environment

or fracture after lithification

Question 3

what is a viable cap rock?

Answer 3

impermeable
includes evaporites

Question 4

what is the overall process of forming gas and oil?

Answer 4

1. plankton die and sink to seabed
2. burial in mud and partial decay to form sapropel
3. maturations to form kerogen and petroleum
4. oil and natural gas in oil shale source rock
   (5. surface oil seep)
5. oil and natural gas stored at top of resevoir rock
6. impermeable cap rock traps oil and gas

Question 5

what is the ranges of the oil window?

Answer 5

50-100 derees
(50-200)

Question 6

what is the ranges of the gas window?

Answer 6

100-200

Question 7

when and how are hydrocarbons destroyed?

Answer 7

above 200 degrees:
- deep burial
- regional metamorphism
- igneous intrusion
- volcanic activity

Question 8

what are the types of traps?

Answer 8

anticline traps
fault traps
salt dome traps
unconformity traps
lithological traps

Question 9

where is most british oil found?

Answer 9

in the Mesozoic and Cenozoic sedimentary basins

Question 10

what is the main source, resevoir and cap rocks in the northern basin of the north sea?

Answer 10

Source - Late Jurassic Kimmeridge clay
Resevoir - marine sandstone + fractured chalk
Cap - clays and traps

Question 11

what formed the source rocks in the northern basin?

Answer 11

Jurassic rifting formed deep marine basins

Question 12

where is gas and oil found in the north sea basin?

Answer 12

Northern basin - oil
Southern basin - gas

Question 13

what is the source, resevoir and cap rocks in the southern basin?

Answer 13

Source - coals and organic shales
Resevoir - permian dune sandstone
Cap - permian evaporites

Question 14

what are some geophysical exploraition techniques?

Answer 14

seismic reflection surveys
gravity surveys
exploration drilling
- mud logging
- core sampling
- down-hole logging
- stratigraphic correlation

Question 15

how do seismic reflection surveys work?

Answer 15

artificial waves reflect on layer boundaries
detected by geophones and hydrophones

Question 16

how to gravity surveys relate to oil and gas?

Answer 16

positive gravity anomaly - excess of mass due to anticline or uplifted fault block = potential traps

negative gravity anomaly - deficit of mass due to low-density salt dome

Question 17

what is mud logging?

Answer 17

rock chips sieved from dilling mud

Question 18

what is down-hole logging?

Answer 18

inserting a sonde which records data such as:

• Porosity: high is likely resevoir rock
• fluids: oil, gas or brine
• gamma ray spectroscopy:
  > source rocks have high gamma ray count
• electrical resistivity - hydrocarbons have high resistivity
• density: coal is low density

Question 19

what is primary recovery?

Answer 19

initial rise of oil under natural pressures
‘nodding donkeys’ added to extract 20-30% of oil

Question 20

what are the difficulties associated with primary recovery?

Answer 20

oil is viscous and has high surface tension at shallow depth
deeper means it is less viscous but rock is more liely to be compact and have less pore space

Question 21

what are the processes in secondary extraction?

Answer 21

water flood drive - water injected below maintains pressure

gas cap drive - natural gas, co2 or nitrogen injected in to resevoir rock above to maintain pressure, and dissolves to lower viscosity

steam injection - increase temp loweres viscosity

detergents - reduce surface tension

bacteria - digest and breakdown large hydrocarbons to decrease vsicosity

Question 22

What are some cases of unconventional petroleum?

Answer 22

oil shale - sufficient source rock but did not mature to produce oil (processed by steam injection)

Athabasca tar sands

orinoco oil belt

frozen gas hydrates

page 218

Question 23

what is hydraulic fracturing?

Answer 23

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