Page 148 - Physical Principles of Sedimentary Basin Analysis

130 Heat flow

Vitrinite is useful as a thermal indicator because it has similar kinetics to oil and gas
generation. The values of VR are therefore divided into groups with respect to the different
stages of hydrocarbon generation. One such grouping is

VR interval Stage
0.5–0.7 early mature oil
0.7–1.0 mid mature oil
1.0–1.3 late mature oil
1.3–2.6 main gas generation

which shows that oil generation takes place in the interval 0.5–1.3. Different source rocks
can behave quite differently with respect to oil and gas generation, and such a scheme is just
a rule of thumb. Nevertheless, measurements of VR immediately give valuable information
about the maturity of a sediment layer.
The simplest burial histories have deposition of sediments at a constant rate. If the heat
conductivity is also constant then we derive that the temperature of a stratum increases
linearly with time. Fourier’s law gives
qz qωt
T (t) = T surf + = T surf + (6.112)
λ λ
where T surf is the surface temperature, q is the heat flow, λ is the heat conductivity and
the ω is the burial rate, and the heating rate becomes Q = qω/λ. It is instructive to test
the easy-ro model for heating at a constant rate. Figure 6.11a shows VR from the easy-
◦
◦
ro model in the case of heating from 0 C to 250 C over time spans of 1, 10, 100 and
1000 days
4 1000 Ma 4
100 days
100 Ma
3 3
10 Ma 10 days
VR [%] 2 main gas generation 1 Ma VR [%] 2 main gas generation 1 day

late mature oil late mature oil
1 1
mid mature oil mid mature oil
early mature oil early mature oil
0 0
50 100 150 200 250 0 100 200 300 400 500
temperature [°C] temperature [°C]
(a) (b)
◦
Figure 6.11. (a) VR (easy-ro) for linear heating from 0 C to 250 C over four different time spans in
◦
million years. (b) VR (easy-ro) for linear heating from 0 C to 500 C over four different time spans
◦
◦
in days.