Page 150 - Geology and Geochemistry of Oil and Gas

120 DISPERSED ORGANIC MATTER

Fig. 7.1. Coal maceral evolution curves (modified after van Krevelen, 1961; also see Larsen and
Chilingar, 1967).

Generally speaking, it would be unrealistic to expect a single energy threshold for
complex natural compounds and numerous chemical reactions they undergo. It is
obvious that both initial and final values of energy threshold (and speed of processes
and composition of end products) will change depending on the type of mixture.
According to Glesston et al. (1948), the energy threshold (activation energy) is a
natural boundary at which the reactions begin. Until that boundary is exceeded, the
system may be considered as practically stable under the existing energy environ-
ment.
As far as the field observations are concerned, Price (1983) noted that the effect of
time factor was overestimated, because the difference in geothermal gradients in
different regions was not taken into account. Koshman (1985), however, disagrees
with this view. Also, very intriguing results were obtained in the laboratory studies
by Rohrback et al. (1984). They studied the effect of temperature on various se-
diments and found that vitrinite reflectance changes from 0.2 to 3.0% on heating
from 35 to 550 1C during the time interval of 1 to 15,000 h (which is just a drumbeat,
geologically speaking).
Thus, one may suggest that the alterations in the coaly matter occur rapidly at
first, after which the matter may preserve its acquired properties for a long time. An
inference from the latter might be that the alterations of coaly matter occur not as
much from the amount of heat received but from reaching the necessary temper-
atures. Andreyev et al. (1958) indicated that the duration of achieving the stable state
for organic matter is always much lower than the residence of the system in stable
state. Moskvin (1981) reached a similar conclusion: time does not affect the degree of
alteration of organic matter. Allen and Allen (1993, p. 284), on the other hand, do
not exclude the effect of time on the organic matter alteration. Lopatin (1983),
summarizing ideas of the time and temperature effects on the organic matter trans-
formations, prepared a graph (Fig. 7.2). As shown later, heat flow is not the only
energy source in the subsurface (Shvetsov, 1974; Trofimuk et al., 1981).
Out of all the components of coals presented in Table 7.1, vitrinite is considered to
be the most suitable for determination of paleotemperatures. Paleotemperatures are