Page 147 - Physical Principles of Sedimentary Basin Analysis

6.7 Sediment maturity and vitrinite reflectance 129
Table 6.3. The kinetics for vitrinite reflection
proposed by Sweeney and Burnham (1990).

A i E i x i,0
[s −1 ] [kJ mole −1 ] [–]
1e+13 142.351 0.03
1e+13 150.725 0.03
1e+13 159.098 0.04
1e+13 167.472 0.04
1e+13 175.846 0.05
1e+13 184.219 0.05
1e+13 192.593 0.06
1e+13 200.966 0.04
1e+13 209.340 0.04
1e+13 217.714 0.07
1e+13 226.087 0.06
1e+13 234.461 0.06
1e+13 242.834 0.06
1e+13 251.208 0.05
1e+13 259.582 0.05
1e+13 267.955 0.04
1e+13 276.329 0.03
1e+13 284.702 0.02
1e+13 293.076 0.02
1e+13 301.450 0.01

called easy-ro. Table 6.3 shows that easy-ro has 20 steps with the same Arrhenius prefactor
A = 10 13 −1 , and that the activation energy comes in the step size E = 8.374 kJ mole −1
s
(2 kcal mole −1 )from E = 142.4kJ mole −1 (34 kcal mole −1 ) until E = 301.5kJ mole −1
(72 kcal mole −1 ). One particular feature of Table 6.3 is that the weighting factors add up
to 0.85 and not 1. The reason was a lack of observations corresponding to high activation
energies when the model was made. Therefore, there is an extra step i = 21 with initial
fraction x i = 0.25, which can be considered inert (x i (t) = x i,0 = 0.25). When this extra

step is subtracted from the transformation ratio (6.109) we get that Tr = 0.85 − i x i for
easy-ro. The easy-ro model has the parameters c =−1.6 and d = 3.7, and the minimum
and maximum values for VR are therefore %Ro = 0.2 and %Ro = 4.7, respectively. An
alternative to the extra step is to normalize the steps in the easy-ro model by writing

x i

d Tr = d (0.85 − x i ) = 0.85 d 1 − ( ) = d (1 − x ) (6.111)
i
0.85
i i i

where the new d-parameter is d = 0.85 d = 3.145 and the normalized easy-ro fractions

are x = x i /0.85.
i