Page 239 - Petroleum Geology

214

less than 60%; no commercial accumulation occurred when the percentage of
fixed carbon is greater than about 70% ; and gas occurred in the intervening
zone (White, 1915). There was thus observed a ranking of oil withdepthanal-
ogous to the ranking of coal with depth in many fields. This theory was con-
sidered to be widely applicable.
This line of thought, with the associated idea that petroleum was a distilla-
tion product of coal (a demonstrable process to some extent) went out of
fashion as a marine origin for oil became fashionable. But it has returned
with the increasing evidence that some petroleums can hardly have hada
marine source, and that some crude oils, particularly those with high wax
content, have an important component of vegetable organic origin (a matter
that we shall examine shortly). The diagenesis of vegetable organic matter to
petroleum has been well established since the work of Brooks and Smith
(1967, 1969). And the Groningen gas field, the largest in the world, with 58
Tcf (lo1* cubic feet; 1.6 X 10” m3) recoverable proven reserves, is believed
to be the result of distillation of Upper Carboniferous (Pennsylvanian) coals
(Stauble and Milius, 1970).
Brooks (1970) considered that crude oil generation does not begin until
the rank of the coal reaches 80% total carbon, dry mineral matter free; and
that oils are formed until the rank reaches about 85% ; and light oils with gas,
and dry gas, up to about 90% total carbon, dry mineral matter free. (The
relationship between fixed carbon and total carbon is not precise: 60% fixed
carbon is approximately equivalent to 83% total carbon, dry mineral matter
free; 65-86%, and 70-87% .)
White’s carbon ratio theory was revived and reclothed in the late 1950s
and early 1960s. The coal industry required a measure of rank in sedimen-
tary sequences that contained no coal, and vitrinite (a coal maceral) was
found to have a property that closely followed the rank of coals and was
used for ranking coals: its capacity to reflect light increased with increasing
rank. Largely through the work of M. Teichmuller (l958,1963),vitrinite reflec-
tance became accepted in the petroleum industry as a measure of the thermal
maturity of sedimentary rocks. The mean reflectance of many particles of
vitrinite is determined from measurements under standard conditions, and
the result recorded as R, or R, = x% (the suffix o indicating oil as the medium
in which the reflectance was measured). In general terms, it has been found
that only biogenic gas occurs where R, < 0.595, and the rocks are thermally
immature; oil occurs where 0.5 < R, < 1.3; wet gas and gas only occur where
R, > 1.3.
It is most important for the petroleum geologist to appreciate that vitrinite
reflectance is a measure of the thermal maturity of the sedimentary rock in
which it was found, whereas petroleum may accumulate at some distance
from its source rock. It is therefore important to phrase the statements con-
cluding the previous paragraph with care. It is an interpretation of empirical
observations that leads to statements such as “oil is to be expected in this