Page 237 - Petroleum Geology

212
IH/C

I <‘

1-5

0 o/c,
0 0-1 0-2 0-3
Fig. 10-1. Kerogen evolution during burial in terms of atomic ratios of hydrogenlcarbon
and oxygenlcarbon. The dashed lines crossing the trends are approximate vitrinite reflect-
ance values (in percent). Trends are towards the elimination of hydrogen and oxygen with
increasing depths of burial. (After Tissot and Welte, 1978, p. 149, fig. 11.5.1.)

presence of catalysts at temperatures well below cracking temperatures
(McDermott, 1940; Brooks, 1948; Dobryansky, 1963; Louis, 1966; Hunt,
1967). The objection to catalysis as a geological process has been raised that
the minerals are covered by a film of water and so cannot act on the oil. The
evidence of petroleum reservoirs (Chapter 8) is that there may be a thin film
of adsorbed water, perhaps 2 or 3 molecules thick, separating petroleum
from solid surfaces. Adsorbed water, surely present in the catalytic cracking
unit, may inhibit catalysis but is unlikely to prevent it. An efficient catalyst
from a laboratory point of view is not required in nature: there is plenty of
time.
The point here is that different clay minerals may exist along a migration
path due to facies changes, and may exist in different petroleum source rocks,
and so may contribute to variations in the quality and quantity of petroleum
generated that are independent of temperature.
In view of all these variables and unknowns, it has little purpose to seek to
make such statements as “the principal zone of oil generation is at 80°C, or
1500 m” or any other temperature or depth. Palaeozoic source rocks could
have generated significant quantities of crude oil at 50°C, while Pliocene source
rocks might have required 120°C to generate the same amount. Likewise, the
depth of intense oil generation can be much deeper than 1500 m in young
rocks in areas of low geothermal gradient, and shallower in older rocks in
areas of larger geothermal gradient. The role of pressure is not considered to