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9.6 Option — Organic geology: 2. The environment, and process of coal and petroleum formation
| Syllabus
reference (October 2002 version) |
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|---|---|---|
| 2. The environment,
and process of coal and petroleum formation |
Students learn to:
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Students:
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Extract from Earth and Environmental Science Stage 6 Syllabus
(Amended October 2002). © Board of Studies, NSW.
[Edit 14 Oct 04]
Prior learning: Science Stages 4–5
Syllabus: Outcomes 4.11 (content 4.11.1 c ), Outcome 5.11 (content 5.11.1 b).
analyse
information from secondary sources, including resource maps to:
- identify coal producing locatilies,
- identify petroleum producing locatilies
The information on coal and petroleum localities is available in science text books and on electronic sources such as CD ROMs and the Internet.
One source is Petroleum
Data. 
The NSW Minerals Council.
This site allows you to investigate coal deposits and oil and gas accumulations in various sedimentary basins in NSW.
outline the characteristics of coal forming environments
- Coal forms during the accumulated decay of plants (vegetable matter) in the absence of oxygen.
- Coal deposits are commonly formed in environments such as river flood plains, lakes, swamps, deltas and coastal environments, where the water is relatively still and stagnant, with bacteria using up all the oxygen.
gather and process information from secondary sources to analyse the similarity between environments in coal and petroleum producing localities
- Use sources such as digital technologies, including
the Internet, encyclopaedias and text books on organic geology to gather
information on coal and oil producing environments.
- Process the data by organising it so you can
extract the relevant information. Make sure you have information on how the
coal and oil were produced, including the possible organisms that were changed
to form the coal and oil and how this process occurred.
- Analyse the information to find relationships between coal and oil producing localities.
discuss the process of coalification – transforming vegetable matter into peat and coal
- The process of coalification is measured by the term rank.
- Rank advance is the degree of coalification.
- The higher the rank the greater the carbon content of the coal.
- Coalification is the process by which plant material has been progressively altered through peat, lignite, sub-bituminous, and bituminous coals to anthracite.
The table below shows the processes and what the new form is called.
| Form of coal/process | Description of Process | |
|---|---|---|
| Peat Plant debris, spongy mass, large amounts of water in it’s pores) |
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| ↓
diagenesis |
Biochemical
transformation with mild temperature and pressure |
|
| lignite (moisture driven out, ..brown coal) |
||
| ↓ catagenesis | Rank advance | |
| Bituminous
coal (transformed into harder, darker coal) |
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| ↓ metagenesis | Geochemical transformation with deep burial and high temperature and pressure | |
| Anthracite (hard, black coal with high carbon content) |
describe the characteristics of petroleum forming environments
- Ideal conditions for petroleum forming environments
are periods of high global sea level, where the continental margins are covered
by oceans.
- Large quantities of sediment settle to the bottom
along with dead marine organisms.
- Layers of sediment cover the organisms over a period
of thousands to millions of years.
- The marine organisms eventually decay into simpler
organic compounds.
- Heat and pressure transform the compounds into petroleum.
outline the maturation of petroleum – diagenesis, catagenesis and metagenesis
| Accumulation
of organic matter |
Temperature |
| ↓
diagenesis |
|
| Bacterial gas (biogenic gas, methane (CH4 ), produced by bacterial decay) |
burial
temperature 30-80oC |
| ↓
diagenesis |
|
| Immature
heavy oil (break down of organic matter into simpler organic compounds) |
|
| ↓ catagenesis | burial
temperature 80-150oC |
| Wet
gas and oil (heavy oil is transformed into simpler organic compounds, e.g. petroleum range hydrocarbons and gas) |
|
| ↓
catagenesis |
|
| Condensate (thermal breakdown of oil into lighter range hydrocarbons) |
|
| ↓
metagenesis |
burial
temperature 150-200oC |
| Dry
gas (thermogenic gas, CH4, produced by thermal decay) |
|
outline the process of oil and gas migration
- Much oil and gas move away or migrate from the source
rock, where they formed. This is triggered both by natural compaction of the
source rock and as more oil and gas form, pressure builds up. The oil and
gas move through minute pores and cracks which may have formed in the source
rock towards more permeable rocks above or below in which the pressure is
lower.
- Oil, gas and water migrate through permeable rocks in which the cracks and
pore spaces between the rock particles are interconnected and are large enough
to permit fluid movement .Fluids cannot flow through rocks where these spaces
are very small or are blocked by mineral growth; such rocks are impermeable.
Oil and gas also migrate along some large fractures and faults which may
extend for great distances if, or when as a result of movement, these are
permeable.
- Oil and gas are less dense than the water which fills the pore spaces in rocks
so they tend to migrate upwards once out of the source rock. Water is always
present below and within the oil and gas layers.
- Migration is a slow process, with oil and gas travelling
between a few kilometres and tens of kilometres over millions of years. But
in the course of many millions of years huge amounts have risen naturally
to sea floors and land surfaces around the world . Visible liquid oil seepages
are comparatively rare, but traces of natural oil seepage can often be detected
if sought. See
Oil and gas for Britain for diagrams to illustrate some of these processes.
describe the features of source rocks, reservoir rocks and cap rocks
- A source rock is an impervious shale with a high amount
of organic matter.
- When hydrocarbons are generated they exist as individual
molecules dispersed throughout the source rock.
- A reservoir rock is a porous and permeable rock such
as sandstone.
- Hydrocarbons migrate from source rocks to reservoir
rocks, by a process of primary and secondary migration.
- A cap rock (seal) is a dense impervious rock such as shale, dense limestone, strongly cemented sandstone or quartzite. Pore size must be too small for fluids to enter.
analyse the conditions under which petroleum accumulates in structural and stratigraphic traps
- When the petroleum reservoir rock layers are folded or faulted, they become petroleum accumulation sites.
- The accumulation is a trap site.
- There are four basic trap types; Anticline, Fault trap, Unconformity and Impermeable Barrier.
- The petroleum (oil and gas) is lighter then the water it is mixed with. Therefore it floats to the highest point such as fold apex and faults.
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