coal microlithotypes and their usage in interpreting deposition environment

COAL MICROLITHOTYPES AND THEIR USAGE IN INTERPRETING DEPOSITIONAL ENVIRONMENT

BY

OLATINPO, OLUSEGUN AYOBAMI14/68ET003

OUTLINEIntroduction

Coal Microlithotypes

Microlithotype Analysis

Significance of Coal Microlithotypes forInterpreting Depositional Environment

Summary and Conclusion

7/11/2015 OLUSEGUN OLATINPO (2015) 2

Introduction• Coal is described as an organic sedimentary rock

usually found in layers with other sedimentary

rocks such as shale, limestone and sandstone

• It is a combustible rock

• The major chemical elements in coal are: carbon,

hydrogen, oxygen, nitrogen, and sulphur


CONTD

The process of coal formation from organic

compounds includes two distinct phases:

1. Biochemical phase

plant material – peat – lignite

2. Geochemical phase

lignite – bituminous coal – anthracite


CONTD


Over time, the chemical and physical properties of the plant remains

were changed by geological action to create a solid material.

COAL MICROLITHOTYPES Microlithotype was developed to permit the designation of rock

types within coal that are at a microscopic scale and their definition

is based on maceral percentages.

Microlithotypes are natural associations of macerals in coals at

microscopic level.

In addition to the maceral content, 20–60% (vol.) of silicate or

carbonate minerals or 5–20% (vol.) sulfide minerals redefines

themicrolithotype as a carbominerite.

All macerals have a suffix "-inite" and microlithotypes have the

suffix "-ite".


CONTDMacerals are the basic and relatively homogenous

organo-petrographic entities of coal which by theirchemical composition and physical characteristicsdetermine its properties and utilization.

They are normally classified into 3 groups:Liptinite-Hydrogen rich and show lowest reflectance

Vitrinite-Oxygen rich and show medium reflectance

Inertinite-Carbon rich and show highest reflectance

Macerals are the coalified remains of various planttissues or plant-derived substances existing at the timeof peat formation.


TABLE 1: ILLUSTRATION OF MACERAL GROUPS AND SOME MACERALS


CONTD The formation of macerals from plant remains during the early

stages of peat accumulation depends on th: type of plant community,

climatic and

ecological controls, and

conditions of the depositional environment

In polished sections under the microscope using incident light, macerals are identified on the basis of their optical properties.

Thus, layers of a particular maceral group greater than 0.05 mm in thickness and consisting of greater than 95% of a particular maceral are termed after the maceral group e.g vitrinite maceral group forms vitrite microlithotype.


CONTDMicrolithtypes are classified into three groups based on

number macerals they contain :

Monomaceral- type consist of one maceral group examples are vitrite (over 95% vitrinite) and inertite(over 95% inertinite)

Bimaceral- consist of 2 maceral groups examples are clarite, durite and vitrinertite.

Trimaceral-consist of 3 maceral groups examples are duroclarite, clarodurite and vitrinertoliptite.


Microlithotypes of Hard Coals

(Data from Taylor et al., 1998)


CONTD


Fig. 1: Diagrammatic representation of Microlithotype Classification ( Bustin et al 1983)

CONTD The chemical properties of microlithotypes are very similar to those of the

predominating macerals

Their physical properties, however, are related not only to those of the

macerals but also to the combined effect of the association.

The microhardness of bi- and trimaceralic microlithotypes is always

higher than that of monomaceralic associations.

The density of the microlithotypes varies with rank, maceral composition,

and size, as well as the form and quantity of associated minerals.

The degree of heterogeneity in a microlithotype is also important in its

technological behavior, particularly in carbonization, combustion, and

gasification


CONTD

G


The maceral, mineral, and microlithotype

composition of a coal seam may change over

short distances both vertically and laterally, in

response to the conditions existing during the

formation of the original peat swamps. These

changes can be quantified by petrographic

assessment of the microlithotypes in relevant

coal samples.

MICROLITHOTYPE ANALYSIS It is used to determine the relative proportions of the various microlithotypes and

coal-mineral associations

Although microlithotype analysis is carried out in a similar manner to maceralanalysis, a suitable 20-point reticule must be placed in one of the oculars of themicroscope as a substitute for the micrometer or cross-hairs.

Two conventions (ICCP, 1963) must be observed: The minimum bandwidth of the association to be measured must be 50microns,

and

Macerals present in the association in amounts smaller than 5% should bedisregarded (the 5% rule).

Each observation on a 20-intersection reticule is regarded as one point in theanalysis, and each intersection on the reticule represents 5% of the total number ofintersections (20), providing guidance in use of the 5% rule.

For a complete microlithotype analysis, at least 500 points should be measured, andthe results should be expressed as volume percentages. Microlithotype analysis isless accurate than maceral analysis.


Significance of Coal Microlithotypes forInterpreting Depositional Environment

Maceral data needs to be considered in the light of different swamptypes and resulting coal microlithotypes in Teichmuller (1962), the coalfacies diagrams of Diessel (1986).

The swamp and moor types described by Teichmuller (1962) indicatethat characteristic plants occur in each swamp type and plant remainscan be identified petrographically by maceral and microlithotypeanalysis.

Three swamp types illustrated on this basis include Forest swamp

Reed swamp

Open water swamp


CONTD The forest swamp is characterised by large contents of

structured vitrinite associated with spores, cuticles,resinite, and frequent fusinite. Vitrite and clarite rich invitrinite are believed to form in this environment with ahigh groundwater level

The reed swamps occur at the transition into open waterswamp and this characterized by a decrease of structuredvitrinite, decomposed vitrinite and spores. More ofinertinite, cuticuloclarite, and duroclarite are found here.

The open water swamps are dominated by algae andsubaquatic plant communities (especially floating plants).Alginite, clarodurite, durite and carbargillite, andassociated washed-in cuticles and spores may be found


h


Fig. 2:Facies diagram of the seams with reference to lithotypes A, B, C, and D (Mastalerz, 1992)

CONTD


Fig 3: Coal depositional enviroments based on the composition of Microlithotypes (free of mineral matter) Singh and Shukla (2004)

KEYA: Lacustrine,B: Fluvial,C: Brackish water,D: Upper deltaic, andE:Lower deltaic environments.

SUMMARY AND CONCLUSION Vitrite is usually derived from stems, branches, and liquified tree roots where

trees were protected from oxidation by high water level.

The association vitrite and clarite, particularly liptinite poor clarite, suggestsa strongly decomposed forest litter of wood and bark which can degrad tohumic detritus.

Liptinite rich clarites are derived from reed peat and reed lignitesintermingled with subaquatic deposits.

Clarites often display microlayers and suggest formation under wet conditionsnormally associated with vitrinitic carbargillites and syngenetic pyrite.

Durites are derived from a subaquatic ooze of oxidized peat characterized bya low spore content and low water table.

Vitrinertites are characteristic of frequent swamp dessication.


SUMMARY AND CONCLUSIONFor geological research on coal basins and for an

evaluation of coal seam quality it is important toknow the quantitative composition of a coal interms of the macerals (and minerals in somecases) or maceral groups and microlithotype.

This is because differences in maceralcomposition may indicate differences in chemicalcomposition and consequently differences in thetechnological properties of a coal likeliquefaction, carbonization, and combustion.


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coal microlithotypes and their usage in interpreting deposition environment

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