Chapter 9 - Oil, gas and basins Flashcards
What is a viable source rock?
fine grained
organic rich
deposited in low energy anoxic marine environment
What is a viable resevoir rock?
porous
permeable
deposited in a high energy environment
or fracture after lithification
what is a viable cap rock?
impermeable
includes evaporites
what is the overall process of forming gas and oil?
- plankton die and sink to seabed
- burial in mud and partial decay to form sapropel
- maturations to form kerogen and petroleum
- oil and natural gas in oil shale source rock
(5. surface oil seep) - oil and natural gas stored at top of resevoir rock
- impermeable cap rock traps oil and gas
what is the ranges of the oil window?
50-100 derees
(50-200)
what is the ranges of the gas window?
100-200
when and how are hydrocarbons destroyed?
above 200 degrees:
- deep burial
- regional metamorphism
- igneous intrusion
- volcanic activity
what are the types of traps?
anticline traps
fault traps
salt dome traps
unconformity traps
lithological traps
where is most british oil found?
in the Mesozoic and Cenozoic sedimentary basins
what is the main source, resevoir and cap rocks in the northern basin of the north sea?
Source - Late Jurassic Kimmeridge clay
Resevoir - marine sandstone + fractured chalk
Cap - clays and traps
what formed the source rocks in the northern basin?
Jurassic rifting formed deep marine basins
where is gas and oil found in the north sea basin?
Northern basin - oil
Southern basin - gas
what is the source, resevoir and cap rocks in the southern basin?
Source - coals and organic shales
Resevoir - permian dune sandstone
Cap - permian evaporites
what are some geophysical exploraition techniques?
seismic reflection surveys
gravity surveys
exploration drilling
- mud logging
- core sampling
- down-hole logging
- stratigraphic correlation
how do seismic reflection surveys work?
artificial waves reflect on layer boundaries
detected by geophones and hydrophones
how to gravity surveys relate to oil and gas?
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
what is mud logging?
rock chips sieved from dilling mud
what is down-hole logging?
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
what is primary recovery?
initial rise of oil under natural pressures
‘nodding donkeys’ added to extract 20-30% of oil
what are the difficulties associated with primary recovery?
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
what are the processes in secondary extraction?
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
What are some cases of unconventional petroleum?
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
what is hydraulic fracturing?
219
