kerogen and maceral

1. Organic Matter
by
Abbas falah gharib
PhD student
Osmangazi University

2. 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.

3. 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(?)

5. Kerogen and hydrocarbon potential
I

6. II

7. • Types of kerogen and Ro

8. 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)

9. Photomicrograph showing Type I kerogen assemblage
Photomicrograph showing Type II kerogen assemblage

10. 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

11. 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

12. Main components of maceral groups

14. 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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