Organic Matter by

Abbas falah gharib

PhD student

Osmangazi University

The most important types of organic matter in geosphere are the fulvic and humic acids,

humin, kerogen (in coal, oil shales, source rocks or scattered in the sediments), bitumen,

oil, gas and graphite.

The particulate organic matter in sedimentary sequences ranges from disseminated

occurrences of organic particles to concentrated organic matter in coals. The classification

of the organic matter based on H/C and O/C atomic ratios (van Krevelen, 1993)

distinguishes three main types of kerogens (Type I, Type II and Type III) that initially were

associated to specific geological settings.

Type I and Type II will be dealth with next, but before that Type III will be explained

shortly.

Coal, usually described as Type III kerogen, is a combustible sedimentary rock composed

of lithified plant debris. This plant debris was originally deposited in a swampy

depositional environment. The prolonged burial of the peat at depths of up to several

kilometers, compaction, pressure and the influence of elevated temperatures for long

periods of time (million years) are known as the coalification process that change peat into

coal. Coal did not appear until the Devonian period due to the lack of terrestrial plants,

although some organic matter derived from marine algae occurs in Precambrian

sedimentary rocks.

Kerogene

Kerogene is a

fossilized mixture of insoluble organic material that, when heated, breaksdown into petrole-

um and natural gas. Kerogene consists of carbon, hydrogen,oxygen, nitrogen, and sulfur an

d forms from compacted organic material,including algae, pollen, spores and spore coats, a

nd insects. It is usually found in sedimentary rocks, such as shale(Greek wax and -gen,

birth).

Types of kerogen

The type of kerogen present determines source rock quality. The more oil prone a kerogen,

the higher its quality. Four basic types of kerogen are found in sedimentary rocks. A single

type or a mixture of types may be present in a source rock, it divided into four types

depending on the chemical properties of kerogen .

Kerogen type

Predominant

hydrocarbon potential Amount of hydrogen

Typical depositional

environment

I Oil prone Abundant Marine

II Oil and gas prone Moderate Mixture

III Gas prone Small Terrestrial

IV

Neither (primarily

composed of vitrinite)

or inert material

None Terrestrial(?)

Kerogen and hydrocarbon potential I

II

• Types of kerogen and Ro

Depending on the microscopic properties of kerogen

• 2. Sapropelic Kerogen

• (spores and pollens)

1. Humic Kerogen 2. Sapropelic Kerogen 3. Kerogen constituted by (woody fragments, and then (spores and pollens) Amorphous Organic Matter Vitrinite and others coal macerals) (unstructured, unrecognizable OM)

Photomicrograph showing Type I kerogen assemblage

Photomicrograph showing Type II kerogen assemblage

Kerogen maturity

The hydrocarbon potential of organic carbon depends on the thermal history of

the rocks containing the kerogen. Both temperature and the time at that

temperature determine the outcome. Medium temperatures (< 175 C) produce

mostly oil and a little gas. Warmer temperatures produce mostly gas.

Vitrinite reflectance (Ro) is used as an indicator of the level of organic

maturity

Macerals are the microscopic organic components typically

identified in coals. They derive from terrestrial, lacustrine

and marine plant remains, and their appearance is a function

of the parent material, of initial decomposition before and

during the peat stages and also of the degree of evolution

undergone. Macerals are distinguished from one to another

on the basis of their physico-optical properties and universal

acceptance is given to the ICCP classification of macerals in

three groups: liptinite, inertinite and huminite/vitrinite.

Maceral groups

Main components of maceral groups

Photomicrographs: 1,2,3,5: reflected white light; 4,6: fluorescence.

1,2) Bituminous coal with vitrinite, inertinite and liptinite. Carboniferous age

3,4 ) Telohuminite and resinite (liptinite) in a carbonaceous shale. Cretaceous.

5,6) Oil shale (Tasmanite algae, Liptinite, right image). Jurassic.

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