SOUVENIR COMMITTEE - Birbal Sahni Institute of Palaeobotany

Training Programme on “PALYNOLOGY IN FOSSIL FUEL EXPLORATION”
[ September 10-17, 2012 ]
FUNDAMENTALS OF COAL PETROLOGY
ALOK K. SINGH
Rajiv Gandhi Institute of Petroleum Technology, Rae Bareli-229316
asingh@rgipt.ac.in, singhalokk@yahoo.co.in
It is a well instituted concept that coal is heterogenous organic sedimentary rock constituted
dominantly of organic matter along with variable amounts of assorted minerals. The mineral matter
occurs essentially as discrete mineral species having an extremely variable particle size and wide range of
concentration. The organic constituents of coal are derived from the plant kingdom and subjected to
intricate chemical and biochemical processes before their proper stratification and carbonization. These
processes induce significant chemical and physical alterations in the original plant material and
eventually transform them into a number of physically distinct entities, which can be identified and
described at megascopic and microscopic scale. The organic constituents of coal at megascopic scale are
called 'Lithotypes' Whereas the optically distinguishable entities are called 'maceral’. Scientists have
described four different types of lithotypes in bituminous coals on the basis of their physical
dissimilarities such as colour, luster, texture and type of stratification. These are vitrain, clarain, durain
and fusian.
The macerals of coal are equivalent to mineral in rocks, even though the criteria for a natural
solid to be called mineral do not hold good for macerals. A particular maceral may differ in optical
properties and chemical composition in coals of various ranks. Based on morphology and other optical
properties, macerals of coal are broadly classified into three groups, viz. Vitrinite, Liptinite and Inertinite.
Further these group macerals can be classified into different submacerals. International Committee for
Coal Petrology (1971,75, 1994 and 1998) has suggested a scheme for maceral classification which is
being followed for coal microscopy or coal petrology throughout the world. Apart from macerals, coals
constitute variable quantity of inorganic matter called mineral matter. A wide spectrum of minerals is
reported from coals but the most dominant minerals in coals are clays, carbonates, silica and various
forms of sulphur (sulphide, sulphates and organic sulphur). In addition to the identifications of coal
constituents as maceral and mineral matter, the constituents are also characterized and quantified as group
macerals or maceral associations called ‘microlithotype’ and also as maceral-mineral associations called
‘corbominerite’. There are certain rules set by the I.C.C.P. (1971 &75) for this kind of characterization.
Accordingly microlithotypes are identified on the basis of two conventions- the minimum band with 50 x
50 microns and the so -called 5% rule. The microlithotype may be monomaceral, bimaceral or trimaceral
depending upon whether the macerals of one group, two groups or all the three groups are involved in the
creation of a 50 micron band respectively.
The science of coal petrology (coal petrography), deals the study of megascopic and the
microscopic constituents of coal in qualitative and quantitative terms. Coal petrology is applied to the
studies of the depositional environments of coals, correlation of coals for geological studies, and the
investigation of coals for their industrial utilization. However, for any modern coal industry, the coal
petrography has become the prime tool to characterize coals and decide about their utilization. Presently,
it is more an applied science, which has become indispensable in solving the chemico-technological
issues in context of use and quality control of coals in coking plants, briquetting plants and power
stations. It is also playing very important role in evaluating a coals for beneficiation (coal preparation for
downstream utilization). Recently, significance of coal petrology has been demonstrated in coalbed
methane exploration and in potential CO2 sequestration into the coal seams. Besides, it plays an
important role in all the oil exploration programs, especially for the optical characterization of the source
rock and also for detecting their thermal maturation.
Birbal Sahni Institute of Palaeobotany, Lucknow
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