ORIGIN OF Petroleum

Formation, Migration ,and Accumulation of Petroleum

ORIGIN OF PETROLEUM

To be considered are these 3 stages:

•Formation of Petroleum

•Migration of Petroleum

•Accumulation of Petroleum

Formation of Petroleum

There are 2 theories concerning the formation of petroleum:•The Inorganic theory•The Organic theory

Inorganic Theory

• Berthelot (1860) & Dmitri Mendeleev (1902):• Iron carbide in the earth’s mantle would react with

percolating water to form methane.• FeC2+ 2H2O →CH4+ FeO2• This theory is called Deep-Seated Terrestrial Hypothesis.

Inorganic Theory

Sokoloff (1890):

•Hydrocarbons precipitated as rain from the original nebular matter from which solar system was formed.

•The hydrocarbons were the ejected from earth’s interior onto surface rocks.

•This theory is called Extraterrestrial Hypothesis

Problems with Inorganic theoryProblems with Inorganic Theories :•No field evidence that inorganic processes have occurred in nature.•Commercial accumulations are restricted to mainly sedimentary basins.•Accumulations are absent from igneous and metamorphic rocks.

Organic Theory

• Early marine life forms living on earth were primarily plankton (rich in hydrogen & carbon)

• Over 95% of living matter in the ocean is plankton.

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• As these plankton died, their remains were captured by the process of erosion and sedimentation.

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• Successive layers of organic-rich mud & silt covered preceding layers of organic-rich sediments & overtime created layers on the sea floor rich in the fossil remains of previous life.

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• Thermal maturation processes (decay, heat, pressure) slowly converted the organic matter into oil & gas over periods of millions of geologic years.

• Conversion of the organic material is called Catagenesis.

• It usually occurs under anaerobic conditions

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• Clay & silt are carried together with the dead organic remains & deposited under deltaic, lacustrine & marine conditions to form Source rocks.

• Black-coloured, organically-rich shales deposited in a non-oxidizing, quiet marine environment are considered the best source rocks.

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• Thermal alteration of kerogen forms crude oil by increasing the carbon contents.

• At shallow depths (< 3,000 ft), bacteria actions on organic materials form Biogenic Gas(natural gas).

• At great depths (high temperature & overburden), Thermogenic Gasis formed.

• Later stages of thermogenesis will form wet gas and condensate.

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• Supports for organic hypothesis:• Carbon & hydrogen are the primary constituents of

organic material, both plant & animal.• Nitrogen & Porphyrinsare found in organic matter

& in many petroleum.• Porphyrins are chlorophyll derivatives in plants &

blood derivatives in animals

Migration of Petroleum

• Produced hydrocarbons migrates upward from the deeper, hotter parts of the basin through permeable strata into suitable structures.

• There are 2 stages of migration:• Primary Migration –Kerogen transformation causes

micro-fracturing of the impermeable & low porosity source rock, allowing hydrocarbons to move into more permeable strata

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• Secondary Migration –The generated fluids move more freely along bedding planes and faults into a suitable reservoir structure.

• Migration can occur over several tens of kilometers in lateral directions

Diagram

Diagram

Accumulation of Petroleum• Accumulation & storage occur when the migrating

fluids encounter an impermeable shale or dense layer of rock.

• This is called a Trap.• After accumulation, the fluids tend to stratify

according to their relative densities:• Gas• Oil• Water

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• If the migrating fluids do not encounter a trap, they tend to flow to the surface or deposited on the ocean floor.

• Examples are:Seepages

• Escaping natural gas

• Bituminous lakes

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Reservoirs are composed of either:Clastic formation –sandstone reservoirs made from silicates (quartz, SiO2).

Carbonate formation –carbonate reservoirs made from detritus(coral or shell fragments).

Reservoirs must be:•Porous

•Permeable

•Trapped.

Types of Traps

Anticlinal Traps•Result of ductile crustal deformations.

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• Fault Traps Result of brittle crustal deformations

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• Stratigraphic Traps Impermeable strata seal the reservoir.