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Origin of Hydrocarbons
Origin of Hydrocarbons
• Two theories of origin
– Inorganic theories
»Deep seated
»Extra-terrestrial
–Organic theory
Deep seated hypothesis
Deep seated hypothesis
– Dmitri Mendeleev, father of periodic table
– Proposed that metallic carbides deep within the earth reacted at
high temperatures with water to form acetylene (C2H2) which
subsequently condensed to form heavier hydrocarbons.
– This reaction was readily reproduced in the laboratory.
– They rely on the fact that earth’s store of methane, atleast, is
primitive and a feature of the origin of the solar system
Evidences
• Solid petroleum bitumen occurrence in igneous environments
• Micro-inclusions of petroleum hydrocarbons have been identified in the minerals of alkaline igneous rocks.
• Association of HC with Hydrothermal systems• Bitumens in several active volcanoes(etna etc)• Some of the oilfields location (Indonesia,
Mexico)
Extraterrestrial Hypothesis
• W. Sokolof proposed a cosmic origin.• It was precipitated as rain from the original
nebular matter from which the solar system was formed.
• Subsequently it was ejected from the earth’s interior into the surface rocks
Evidences
• The present and the past atmosphere of the Earth.
• Critical discovery of (carbonaceous chondrite) meteorite
• The presence of hydrocarbons on other planets.
Evidences
Organic origin
• Oil and gas are made of a mixture of different hydrocarbons.
Organic origin
• Similar to the materials essential for life such as Proteins, fats and fatty acids.
• Fossils led to the anologies with marine life• Whale oil and fish oil• Further similarities with coal• Close association between oil and sediments• Biomarkers- compounds of undoubted organic
derivation. (waxes, porphyrins and steranes)• Carbon isotope C13 and C12
Accumulation of Organic matter
en.wikipedia.org/wiki/Image:Ceratium_hirundinella.jpg
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Accumulation of Organic matter
• Today, most plankton can be found where deep ocean currents rise to the surface
• This upwelling water is rich in nutrients and causes the plankton to bloom
• Blooms of certain plankton called dinoflagellates may give the water a red tinge
© Miriam Godfrey
Dinoflagellate bloom
Accumulation of Organic matter
• The renewal of fresh nutrient rich water doesnot take place.
• The water become layered
• The deposition takes place below the thermocline
Accumulation of Organic matter
• When the plankton dies it rains • down on sea bed to form an • organic mush
• Sea bed
• en.wikipedia.org/wiki/Image:Nerr0328.jpg
• If there are any animals on the• sea bed these will feed on the• organic particles
Accumulation of Organic matter
• However, if there is little or no• oxygen in the water then animals• can’t survive and the organic• mush accumulates• Where sediment
contains • more than 5%
organic matter, • it eventually forms a
rock • known as a Black
Shale
The summary of the Process of accumulation
Source Rocks
Source Rocks
• Shale and Limestones
• Two major divisions in lithology
• A sandwiched layer of organic matter in between the layers of sediments.
KerogenOrganic carbon (Weight percent) Quality
0.0.5 Poor
0.5-1 Fair
1-2 Good
2-4 Very good
Above 4 Excellent
Kerogen
• The TOC may reach 20 percent or more by weight.
• It is highest in coals and rich oil shales.• The TOC has two parts:
– Soluble in organic solvents is bituminous(bitumen)– Insoluble, nonextractable residue from the initial
transformation of OM is Kerogen
Kerogen
• Carbonates and shales contain organic matter.
• Carbonates contain much less OM than do shales.
• But the OM of the carbonates is much richer in HC than that of shales
Elemental Data For Kerogen
Peters, 1986
Type I This type of kerogen is characterized by having a high initial
hydrogen to carbon atomic ratio (H/C) of 1.5 or more, and a low oxygen to carbon atomic ratio (O/C) of less than 0.1.
Type I kerogen has a hydrogen index greater than 300 and an
oxygen index less than 50.
Its primary source is from algal sediments, such as lacustrine deposits.
Type I kerogen is also called alginite kerogen, containing high concentrations of alkanes and fatty acids.
It is the best source for oil-prone maturation, but unfortunately
it is very rare.
TYPES OF KEROGEN
Type II This type of kerogen has a relatively high H/C ratio (1.0 to 1.4)
and a low O/C ratio (0.09 to 1.5). Type II kerogen has a hydrogen index between 200 and 300,
and an oxygen index between 50 and 100. It consists of abundant moderate length aliphatic chains and
naphthenic rings. Ester bonds are common and sulfur is present in substantial
amounts.Type II kerogen is also called exinite, and is usually associated
with marine sediments, where autochthonous organic matter (bacteria, phytoplankton and zooplankton) have been deposited in a reducing environment.
It is a good oil or gas prone kerogen. It is more common than alginite.
Type III This type of kerogen has a relatively low H/C ratio (usually <1.0)
and low O/C ratio (0.2 to 0.3).
Type III kerogen has a hydrogen index below 300 and an oxygen index above 100.
It contains an important proportion of polyaromatic nuclei and
heteroatomic ketone and carboxylic acid groups. Aliphatic groups are a minor constituent, usually consisting of
longer chains originating from higher-order plant waxes.
The main source of this type of kerogen are continental plants found in thick detrital sedimentation along continental margins.
This type of kerogen is also called vitrinite. It is less favorable for oil generation, but will provide a source rock for gas.
CONTROLS ON TOTAL ORGANIC MATTER
• Productivity
• Grain size
• Sedimentation rate
• Oxidation/Reduction
ENVIRONMENT OF TRANSFORMATION
Mostly Hydrogen and Carbon are neededOxygen and Nitrogen should be removed
Not possible in oxygenated environmentsNo prolonged exposure to atmosphere, aerated watersOr subsurface waters carrying acids or bases, to elemental sulfur,Or vulcanicity or igneous activity
If the Oxygen content in water > 1mg/lAerobic decomposition of OM is very efficientOrganisms also play their part in destruction of OM
If the Oxygen is < 0.1 mg/lDecomposition of organic matter is slowAnaerobic bacteria may use the Nitrogen and SulphurAnoxic environments and quick burial is needed
Anaerobic decomposition is influenced by the:
Grain sizeCoarse grains OxygenatedFine grains Anoxygenated
Sedimentation rateSlow and discotinuousRapid
Conversion of organic matter
• The transformation of OM to kerogen proceeds from shallow depths of burial to depths of perhaps 1000m, with temperatures up to 50 degree centgrades.
• On further burial and heating, the large molecules crack to form smaller, lower molecular weight hydrocarbons (geomonomers), around 1000-6000 m depth and 50-175 degree centigrade temperature.
General Scheme for Hydrocarbon Formation
Tissot et al., 1974
Conversion of organic matter
• Initial products are H2O and CO2
• Hydrogen, methane and liquid products C13 –C30.
• Oxygen is lost most rapidly by dehydration and decarboxylation
• Carbon and Nitrogen are lost least rapidly.
Conversion of organic matter
• The progressive alteration of OM leads to two fractions:– A fluid product high in hydrogen, eventually
petroleum and natural gas.– A residue high in carbon, such as bituminous coal.– Increase in the atomic ratio of hydrogen-carbon in
the oils.
Conversion of Kerogen
Barker, 1996
Organic matter: 1%
• Kerogen 90%• Bitumen 10%
Precursors of Petroleum
• Humic- gas prone• Sapropelic- oil prone• Classification of Humic and sapropelic on the
basis of H:C :– Less than 0.8, predominantly humic.– Between 0.8 and 1.0, mixed– Above 1.0, predominantly sapropelic.
Porosity and Permeability
• Definition
• Formula
• Procedures
• Controlling Factors
Permeability
• Definition
• Formula
• Procedure
• Controlling factors.