Page 149 - Physical Principles of Sedimentary Basin Analysis

6.7 Sediment maturity and vitrinite reflectance 131

1000 million years. These rates are typical for deposition and burial in sedimentary basins
◦
◦
and they give oil generation in the temperature interval from 100 C to 150 C. The easy-ro
model can also be used on much larger heating rates – rates that are practical for laboratory
◦
◦
experiments. Figure 6.11b shows the VR when the heating is from 0 C to 500 C over
time spans of 1, 10, 100 and 1000 days. It is possible to generate a series of vitrinite
measurements in some tens of days in the laboratory if the temperature goes up to 500 C.
◦
The decay of each fraction x i as a function of temperature can be approximated as
follows when the heating is at a constant rate

x i (T ) = x i,0 exp − N i (F i (T ) − F i (T 0 )) (6.113)
2
where F i (T ) = (T/T i ) exp(−T i /T ), T i = E i /R and N is the (dimensionless) number
A i T i
N i = (6.114)
Q
where Q is the heating rate. (See Note 6.2 for details.) The temperature at the beginning
of the heating is T 0 , which must not be confused with temperature T i corresponding to
activation energy E i . Temperature and time are linearly related during heating at constant
time-rate, and it is more convenient to use temperature instead of time, because time may
vary by a large number of decades. The use of approximation (6.113) makes it simple to
compute the VR for heating at a constant rate.
Vitrinite is important in oil exploration because it tells us whether a formation or stratum
is in the oil window or not. It can answer similar questions such as where a stratum is in
the oil window or when it was in the oil window. In immature areas where exploration
wells have not been drilled and vitrinite has not been measured it is still possible to make
assessments of the basin maturity by modeling the burial history – assuming that the most
important formation boundaries have beeen mapped by seismic surveys. The temperature
history can be computed numerically and with the associated VR using reasonable litho-
logical properties and a reasonable heat flow history. Figure 6.12a shows an example of
a burial history with the corresponding temperature history, and Figure 6.12bshowsthe
present day VR computed with the easy-ro model. The modeled VR has a good match
against VR observations in this case.
There have been some attempts to extract the heat flow history from VR observa-
tions assuming that the burial history with lithological properties is sufficiently accurately
known. These efforts have shown that it is very difficult (or nearly impossible) to obtain
the heat flow history – except for around the temperature maximum.
Note 6.2 The decay of fraction i is given by the first-order equation (6.108). This equation
can be rewritten as
dx A i
=− exp(−T i /T ) dT =−N exp(−1/u) du (6.115)
x Q
with N = A i T i /Q, using that T (t) = T 0 + Qt and that u = T/T i . The integration of
2
exp(−1/u) is then approximated by u exp(−1/u) as shown in Note 11.1.