Transcript Oil and Gas - Geology Rocks

Oil and Gas
Describe and explain the origin of oil
and natural gas and migration from
source-rock to reservoir-rock under a
cap rock.
 Define and recognise the trap
structures; anticline, fault, salt dome,
unconformity and lithological.
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Formation of Oil and Gas
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They are formed by the partial
decomposition of:
Marine planktonic organisms
(plants/algae and bacteria).
This decomposition usually happens at
low temperature < 200C, above this the
plant matter would completely
decompose.
Where does the energy come from
initially?
The Sun.
Formation of Oil and Gas 2
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This partial decomposition begins when
plankton accumulate on the sea floor.
This accumulation needs to be in a low
energy environment often in deep basins.
In this environment anaerobic (like coal)
conditions occur and the bacteria that do
exist can only partially break down the
organic material.
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Sometimes deeply buried coal can
also break down to form gas.
STAGES OF OIL FORMATION
Stage 1:
Organic mud.
 Present day forms in the Black Sea
and contains 35% organics.
 Draw a simplified version of
diagram 2 page 2.
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Stage 2:
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The organic mud is
buried and
compressed.
Forms a black “oil”
shale.
For example the
Jurassic (140mya)
Kimmeridge shale,
Dorset.
Stage 3/Maturation:
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If heated and compressed
enough the organic
material changes to
kerogen (diagrams 4 & 5
page 3).
With further burial chains
of C and H break off the
kerogen to form heavy oil.
As heating continues
chains break off the heavy
oil to produce light oil and
finally break further to
produce gas (Often
Methane).
Maturation 2
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The process of oil and
gas formation from a
rock is called
maturation.
Petroleum can exist as:
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natural gas
crude oil
asphalt (a solid).
Petroleum consists of H
and C with varying
amounts of O, S and N,
usually only consisting
of a total of 10%.
Maturation 3
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If in stage 3 the depth of
burial is shallow then
heavy crude oil will form.
With deep burial light
crude oil will form as the
greater temperatures and
pressures will break
down the long
hydrocarbon chains into
smaller lighter molecules
such as gas.
Maturation in the North Sea
Therefore the type of oil
and gas varies
depending on depth of
burial.
 In the N. Sea:
1. Oil forms at 3 - 4.5 km
depth
2. Gas forms at 4 - 6 km
depth
3. >6 km the kerogen is
carbonised with no
hydrocarbons.
 Look at diagram 6 page
3.
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REQUIREMENTS FOR OIL
AND GAS TO SURVIVE
UNDERGROUND
SOURCE ROCK:
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Rock in which the oil
formed.
Oil Shale.
Not necessarily where
it is found.
As this rock is
compacted more the
liquid and gas can be
squeezed out and
migrates up slope, as
it is less dense than
the surrounding solid
rock.
Oil Migration
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The usual route is via
permeable rocks (see
diagrams 7, 8 & 9
page 4), lines of
weakness such as
faults, joints and
unconformities.
This is a slow
process, probably
only a few Kms. in a
million years.
RESERVOIR ROCK:
CAP ROCK:
OIL TRAPS