aspects of organic geochemistry of nigerian coal university of ibadan

1.6.2 Quality of Organic matter
The quality of organic matter contained in rocks can be determined by
Optical and Physiochemical methods. These methods are briefly described
below:
1.6.2.1 Optical Methods
These methods are based on maceral recognition techniques. Maceral
examination can be carried out using reflected light microscopy of thin sections
of the whole rock or of isolated organic particles. Transmitted light microscopy
can also be used for isolated maceral concentrates. Shape and degree of
transmittance or reflectance, and also fluorescence under uv-illumination, can be
used to identify broad maceral groups (liptinite, exinite, vitrinite and inertinite).
1.6.2.2 Physico-chemical Methods
Kerogen is the disseminated organic matter in sedimentary rock that is
insoluble in non-oxidizing acids, bases and organic solvents. Elemental analysis
of kerogen concentrate from rock is the most reliable method of characterizing
the types or quality of organic matter. It is based on the major constituents (C, H,
O), which have been used to define main types of kerogen based on the plot of
H/C versus O/C in Van Krevelen diagram. The plot of Hydrogen index (HI) vs.
Oxygen index (OI) provides an analogue to the van Krevelen diagram. Both the
HI and OI can be obtained from Rock-Eval pyrolysis. Based on the plot of H/C
vs. O/C and HI vs. OI, kerogen can be classified into types I to IV which are
broadly equivalent to the maceral groups, liptinite, exinite, vitrinite and inertinite
respectively for coals (Killops and Killops, 1993).
Type I kerogen has a high H/C ratio and a low O/C ratio. It contains a
significant contribution from lipid material. Algal material and bacteria remains
are the main contributors of its organic matter. The kerogen is deposited under
anoxic conditions in shallow water environment (e.g. Lagoon and Lake) with
total 4.6% oxygen. The type I kerogen is relatively rare but has high oil potential.
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