Page 146 - Geology and Geochemistry of Oil and Gas

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Chapter 7

DISPERSED ORGANIC MATTER

7.1. ORGANIC MATTER INSOLUBLE IN ORGANIC SOLVENTS: KEROGEN

The content of kerogen in the rock–water–organic matter system is insignificant
and varies between fractions of 1% and a few percentage points. However, its sig-
nificance is greater than its content and may be compared to that of the accessory
minerals in petrography: kerogen enables one, albeit not always, to solve various
geochemical problems, including the oil and gas genesis.
The major part of the organic matter (OM) is represented by carbon, which is
usually used as the basis for geochemical calculations. There is a correlation between
the organic carbon content and lithology. The clarke values of the total organic
carbon (TOC) are: clays – 0.9%; siltstones – 0.45%; sandstones – 0.2%; and car-
bonates – 0.2% (Vassoyevich, 1984). This is a reflection of the depositional envi-
ronments at the time of original organic matter accumulation in sediments.
Historically, the appearance of organic matter in sediments goes back to
the origin of life on the Earth. Organic carbon is present in rocks as old as 4 to 4.5
billion years. Sidorenko (1978) suggested that the oldest accumulations of organic
carbon-rich formations occurred 3.7 to 3.5 billion years ago. He identified the
Priazov Series (Ukrainian Shield), Kola Series of the Kola Peninsula and the graph-
ite gneiss of the Siberian Platform as such formations. Traces of organic matter
are omnipresent in the Phanerozoic deposits. The dispersed organic matter can be
divided into two categories: (1) soluble in organic solvents and (2) insoluble in
organic solvents.
All living organisms comprise various combinations of the same carbon
compounds: lipids, proteins, carbohydrates and lignin (for higher plants). In 1920,
Potonie classified caustobioliths, according to their genesis, into humites and sa-
propelites. The humites are lignin-cellulose substances and liptobioliths, which form
from tar, wax, spore shells, and cuticles. The sapropelites are transformation prod-
ucts of the simplest plant and animal remains, with or without preservation of
original form. Same classification is applied to the dispersed organic matter. A
weakly substantiated is the assignment of the humites to the continental genesis and
the formation of various coals, and of the sapropelites to the major source matter for
the oil and gas formation. Ammosov et al. (1987) proposed one more class: organo-
fluidoliths, which are solid substances forming during migration of fluids (hydro-
carbons or coaly fluids).
Dispersed organic matter of rocks may be classified into insoluble and soluble in
organic solvents (chloroform, benzene, alcohol, methanol, acetone, ethanol, and
their mixtures).