Page 145 - Physical Principles of Sedimentary Basin Analysis

6.7 Sediment maturity and vitrinite reflectance 127

(T + 273) 1−n
n
G X (T ) = λ 0 298 . (6.99)
(1 − n)
Exercise 6.13 We will now compute the stationary geotherm when the heat conductivity
is both temperature- and pressure-dependent. The pressure dependence is normally given
by the factor (1 + α 0 p), and the temperature- and pressure-dependent heat conductivity
then becomes

λ(p, T ) = λ(T )(1 + α 0 p) (6.100)
where λ(T ) may be any heat conductivity as a function of temperature. Assume that the
pressure is p = gz and show that z as a function of temperature then becomes
1 α 0 g
z = (1 + α 0 gz m ) exp (G(T ) − G(T m )) − 1 (6.101)
α 0 g q m
where G(T ) is the function (6.95).
Solution: The starting point is Fourier’s law (6.93), which now becomes
dz
λ(T ) dT = q m . (6.102)
(1 + α 0 gz)
The integration (6.94) then gives

q m 1 + α 0 gz
G(T ) − G(T m ) = ln (6.103)
α 0 g 1 + α 0 gz m
and we are almost done.

6.7 Sediment maturity and vitrinite reflectance
Vitrinite is the most important thermal indicator in sedimentary basins and it is routinely
measured in a large number of exploration wells by the oil companies. Vitrinite reflectance
(VR) values can be grouped into intervals with respect to hydrocarbon generation like oil
maturation and gas maturation, and then provide directly a measure of the maturity of
sediment samples.
Vitrinite is one of the primary components of coals and it is common in sedimentary
rocks that are rich in organic matter. It becomes thermally altered in response to the influ-
ence of temperature over time, and the degree of alteration is measured as reflectance
(Tissot and Welte, 1978). The simplest approach to quantify the VR uses the TTI-
concept introduced by Lopatin (1971), which is the time–temperature index defined for
a temperature history T (t) as

t

TTI(t) = 2 aT (t )+b dt . (6.104)
0
TTI as an integral is a generalization of the observation made by Lopatin (1971) that reac-
◦
tion rates double by every step in temperature of 10 C. The parameter a is therefore