Page 236 - Petroleum Geology

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seeds, pollen and bark (as in cannel coal).
Bacteria themselves can form a significant biomass. ZoBell (1964) found
up to 0.5 km m-3 in Holocene sediment, but the concentration decreased
rapidly with depth.
The two dominant sources of organic matter differ in some general, but
important respects. The atomic H/C ratio is higher in marine planktonic or-
ganic matter, 1.7-1.9 compared to 1 .O--1.5 for terrestrial organic matter.
The former is more aromatic; the latter is more aliphatic. Microbial action
may induce further changes, including the liberation of sulphur.
During burial to greater depths, the sediment with its contained organic
matter is subjected to increasing temperatures and pressures, and the volatile
and more soluble components and products of early diagenesis are lost to the
pore water. The portion that is insoluble in organic solvents is called herogen,
of which three main types have been recognized (Tissot et al., 1974):
Type-I herogen: derived from organic matter with lipids, dominantly ma-
rine, with H/C ratios greater than 1.5. It is composed dominantly of aliphatic
chains, with some aromatic chains. It is considered to be a good source material
for oil and gas.
Type-11 herogen: derived from marine organic matter mainly, with H/C
ratios smaller than for Type I. It is composed of aromatic and naphthenic
rings, and considered to be quite a good source material for oil and gas.
Type-111 herogen: derived from terrestrial higher plants mainly, with H/C
ratios usually less than 1.0. Not a good source material, it may generate gas
rather than oil.
During burial, these kerogens are altered, with trend$; to lower H/C and
lower O/C ratios (Fig. 10-1) due to the generation and liberation of hydro-
carbons, carbon dioxide, and water. The degree of alteration of the kerogens
is considered to be the prime measure of maturity in a petroleum source
rock. A source rock is not mature until the COz, HzO, and some compounds
of nitrogen, sulphur and oxygen have been released; and alt,hough some hydro-
carbons may be released at this stage, most are released later. Geochemists
call the processes of the first stage diagenesis, and those of the second stage,
prior to metamorphism, catagenesis.
The rate of the conversion reactions is a function of temperature and, per-
haps, pressure. It is also probably affected by minerals withcatalytic properties,
so the combined process is usually lumped together under the term thermo-
catalysis.
It seems to the author that the matter of catalysis in source rocks and dur-
ing primary migration has received too little attention in recent years. The
early catalytic cracking units of refineries used natural Fuller’s Earth (hydrated
aluminium silicates) as the catalyst to improve the yield of high-octane-num-
ber gasoline and reduce the gas yield. A great deal of laboratory work has
shown that most of the clay minerals, and nickel, are catalysts in petroleum
reactions and that splitting of hydrocarbon molecules can take place in the