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    • Rights: University of Waikato
    Published 10 June 2008, Updated 14 January 2016 Referencing Hub media
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    This animated video demonstrates the formation of hydrocarbons over millions of years in the depths of the Earth.

    Oil and gas forms in sedimentary1 ocean basins2. Dead plankton3 (organic4 plant and animal material) gets deposited together with mud, sand and other sediments5. In this early stage the plankton/sediment layer is called sapropel6. Through an increase in temperature7 the sapropel is converted into kerogen8. Folding or faulting forms oil traps9 where oil and gas can accumulate in the pore space of a source rock10 below the trap otherwise it will escape to the earth's surface as seepage11. The liquid or gas that forms is known as 'hydrocarbons

    Transcript

    Formation of today’s fossil fuel12 - the beginning - approximately 400 million years ago

    Dead plankton sinks to the ocean floor and gets deposited.

    Plankton sinks to the ocean floor and is mixed with fine mud (clay13). The resulting sediment14 (plankton, mud, anaerobic bacteria15) is now called sapropel.

    Sapropel is being buried by younger sediments, which increases the pressure16 (weight17) and temperature of the sapropel layer.

    At about 50°C (corresponding to 1-2 km burial) the sapropel is converted into kerogen (through anaerobic bacteria18 and chemical processes).

    The sediment layers may be folded as a result of tectonic movements in the basin19.

    More sediments fall on top, increasing the pressure and temperature on the kerogen.

    At 80-120°C (corresponding to 3-5 km burial) the conversion of kerogen into liquid pertorleum occurs.

    The oil migrates out of the kerogen into sandstone20 reservoir21 (storage beds).

    Between 120-150°C natural gas forms and migrates from the kerogen into the sandstone reservoir.

    In some sedimentary basins salty water may also migrate into the sandstone reservoir and underlie the oil.

    An oil or gas field forms when the liquid or gas moves through the pores and spaces of permeable22 rock and collects within the pore spaces under an impermeable23 trap.

    1. sedimentary: A type of rock formed after the deposition, compaction and cementation of sedimentary material produced by either the weathering and erosion of the Earth’s surface, biological organisms (shells) or chemical precipitation (ooids). Examples of sedimentary rocks are sandstone, mudstone, limestone and coal.
    2. ocean basin: The large depressions of the sea floor. Sediments can accumulate and build up here.
    3. plankton: A group of marine organisms including single-celled and multi-celled organisms.
    4. organic: 1. Molecules that contain carbon and that have a biological origin. 2. Grown using natural processes with nutrients from natural sources.
    5. sediments: Material that settles to the bottom of a liquid. In geology, it describes the solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water or ice.
    6. sapropel: A dark sediment layer with a high proportion of organic matter that can be found at the bottom of lakes and oceans.
    7. temperature: A measure of the degree of hotness or coldness of an object or substance. Temperature is measured with a thermometer calibrated in one or more temperature scales. Kelvin scale temperature is a measure of the average energy of the molecules of a body.
    8. kerogen: A mixture of solid organic chemical compounds found in sedimentary rocks. When heated, it forms the base for the formation of oil and gas
    9. oil traps: Also called traps. This refers to accumulations of petroleum that cannot escape sideways (laterally) or upwards (vertically) because of a geological feature (such as folds or faults) that contain layers of rock material that are impermeable, meaning the oil cannot seep through.
    10. source rock: The rock material of origin from which, in the case of oil formation, hydrocarbons like oil will form.
    11. seepage: The slow movement of a liquid or gas through small holes or cracks in a porous material.
    12. fossil fuel: Materials such as coal, oil and natural gas formed from the fossilised remains of plants that lived many millions of years ago. Often burned as fuel – although this releases large amounts of CO2, which contributes to global warming. Fossil fuels are also not renewable – there is a limited amount.
    13. clay: A naturally occurring fine-grained material formed from the chemical weathering of feldspar minerals found in rocks.
    14. sediments: Material that settles to the bottom of a liquid. In geology, it describes the solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water or ice.
    15. anaerobic bacteria: Bacteria which grow in the absence of oxygen.
    16. pressure: The force per unit area that acts on the surface of an object.
    17. weight: Force due to gravity acting on an object, measured in newtons.
    18. bacteria: (Singular: bacterium) Single-celled microorganisms that have no nucleus.
    19. basin: In geology, this means a depression of large size that may be caused by erosion or earth movements. Often you can’t see a basin on the surface as it has become filled in with other sediments or full of water.
    20. sandstone: A sedimentary rock formed from grains of sand (0.1–2.0 mm in size) held together by a natural cement. Sandstone can be formed under water or from wind-blown sand on land.
    21. reservoir: 1. A natural or artificial structure that stores water. 2. An underground accumulation of natural gas or petroleum oil.
    22. permeable: A surface that allows fluids and gases to pass through.
    23. impermeable: A layer that can not be infiltrated, usually because it has not got any or enough pores.

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      sedimentary

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      2. A type of rock formed after the deposition, compaction and cementation of sedimentary material produced by either the weathering and erosion of the Earth’s surface, biological organisms (shells) or chemical precipitation (ooids). Examples of sedimentary rocks are sandstone, mudstone, limestone and coal.

      organic

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      3. 1. Molecules that contain carbon and that have a biological origin.

      2. Grown using natural processes with nutrients from natural sources.

      temperature

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      4. A measure of the degree of hotness or coldness of an object or substance. Temperature is measured with a thermometer calibrated in one or more temperature scales. Kelvin scale temperature is a measure of the average energy of the molecules of a body.

      source rock

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      5. The rock material of origin from which, in the case of oil formation, hydrocarbons like oil will form.

      clay

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      6. A naturally occurring fine-grained material formed from the chemical weathering of feldspar minerals found in rocks.

      weight

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      7. Force due to gravity acting on an object, measured in newtons.

      sandstone

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      8. A sedimentary rock formed from grains of sand (0.1–2.0 mm in size) held together by a natural cement. Sandstone can be formed under water or from wind-blown sand on land.

      impermeable

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      9. A layer that can not be infiltrated, usually because it has not got any or enough pores.

      ocean basin

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      10. The large depressions of the sea floor. Sediments can accumulate and build up here.

      sediments

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      11. Material that settles to the bottom of a liquid. In geology, it describes the solid fragments of inorganic or organic material that come from the weathering of rock and are carried and deposited by wind, water or ice.

      kerogen

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      12. A mixture of solid organic chemical compounds found in sedimentary rocks. When heated, it forms the base for the formation of oil and gas

      seepage

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      13. The slow movement of a liquid or gas through small holes or cracks in a porous material.

      anaerobic bacteria

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      14. Bacteria which grow in the absence of oxygen.

      bacteria

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      15. (Singular: bacterium) Single-celled microorganisms that have no nucleus.

      reservoir

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      16. 1. A natural or artificial structure that stores water.

      2. An underground accumulation of natural gas or petroleum oil.

      plankton

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      17. A group of marine organisms including single-celled and multi-celled organisms.

      sapropel

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      18. A dark sediment layer with a high proportion of organic matter that can be found at the bottom of lakes and oceans.

      oil traps

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      19. Also called traps. This refers to accumulations of petroleum that cannot escape sideways (laterally) or upwards (vertically) because of a geological feature (such as folds or faults) that contain layers of rock material that are impermeable, meaning the oil cannot seep through.

      fossil fuel

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      20. Materials such as coal, oil and natural gas formed from the fossilised remains of plants that lived many millions of years ago. Often burned as fuel – although this releases large amounts of CO2, which contributes to global warming. Fossil fuels are also not renewable – there is a limited amount.

      pressure

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      21. The force per unit area that acts on the surface of an object.

      basin

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      22. In geology, this means a depression of large size that may be caused by erosion or earth movements. Often you can’t see a basin on the surface as it has become filled in with other sediments or full of water.

      permeable

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      23. A surface that allows fluids and gases to pass through.

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