origin of hydrocarbons21

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

Plant plankton Animal plankton

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• 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


• The renewal of fresh nutrient rich water doesnot take place.

• The water become layered

• The deposition takes place below the thermocline


• 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


• 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


• 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.


• 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.

origin of hydrocarbons21

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