The `thermal flux' parameter in the Heat Flow Group File allows for the input of a constant heat flux entering at the base of the basin, or the input of an existing thermal flux file (filename.therm) that allows the user to vary the heat flow through time.
Strata2.1 has the capability of calculating the temperature distribution of each time step and the thermal evolution (integrated time-temperature history) of the simulations. A simplistic steady-state model is used. Heat transport is assumed to occur only vertically. The only heat source is the flux supplied at the base of the basin. The temperature distribution is calculated from:
where, k is the thermal conductivity (W/moK), T is temperature (degrees), and q is the heat flux (W/m2). Because this is a steady-state problem, given q and k we can calculate the temperature gradient at any point. The last boundary condition we set is the temperature at the sediment surface. From this we can calculate the temperature at any location.
The thermal conductivity is assumed to vary as a function of lithology and porosity.
where, fsand is the fraction of the matrix that is sand, fshale is the fraction of the matrix that is shale, and f is the porosity. ksand, kshale, and kfluid are the thermal conductivities of the matrix. We work in MKS units. Typical thermal conductivities are listed below (Turcotte and Schubert, 1982) :
The `thermal flux' parameter allows the user to specify a constant heat flux entering at the base of the basin. Typical continental heat fluxes are 56.5 mW/m2 (Turcotte and Schubert, 1982). Typical oceanic heat fluxes are 78.2 mW/m2 (Turcotte and Schubert, 1982). Note: syn-rift heat fluxes are typically higher than continental heat fluxes (Turcotte and Schubert (1982), Waples (1985)). No thermal calculations will be performed if there is not a positive value set for the `thermal flux.' The resultant temperature contours from a continental heat flux of 56.5 mW/m2 are show in Figure 13.
Figure 13: Contour display of temperature profile, illustrating the temperature field for one simulation (temp.dat). Note: Temperature contours are displayed at 10o intervals, vertical exaggeration is set to 10, and skip lines is set to 49. The thermal flux of 0.056 is specified in temp.dat and can be changed in setbasin under the Heat Flow group file.
Thermal flux files can be used if the user wishes to generate a more realistic simulation with changing heat fluxes over time.
Example of thermal flux file (nsea.therm):
| 0 | 0.08 |
| 60e6 | 0.08 |
| 61e6 | 0.05 |
| 165e6 | 0.05 |
Thermal flux file notes:
1) the first column is the age with 0 as the beginning of the simulation
2) the second column is the thermal flux in M/T^3
3) fluxes between given points are linearly interpolated
4) Note - unlike the subsidence file, time is now positive
We simulate an example from Waples (1985), using a subsidence file (nsea.sub) based on the history of Well North 30/4-1, Viking Graben (North Sea) and the heat flux shown above. The flux drops from .08 (M/T3) to .05 at 6.1 million years into the simulation. Click on
Figure 14.a:Contour display of thermal gradient from the simulation nsea.dat at 59 MY. Skip lines was set at 17, vertical exaggeration at 50, and the gradient contoured at .001. The thermal flux file nsea.therm and subsidence file nsea.sub were used for the simulation.
Figure 14b Same contour display for the nsea.dat simulation at 62 MY. Note that the base contour is .02 (not .04) and there are fewer contours.
Strata calculates the Time-Temperature Index (TTI) as: 2 (T - 100) / 10Dt to estimate the thermal maturation of the basin. This is a continuous version of the empirical equation: Dt*2n, where n(T) has been crudely found to be approximated by:
The basic assumption here is that the reaction rate will double when temperature increases by 10oC. (In fact, the actual quantity calculated by the simulator is S 2 (T - 100) / 10Dt, i.e. a continuous interpolation of the above.)
The values of the TTI indicate how much the sediment has matured in that temperature interval. The sum of the TTI is thus the total maturity of the sediment. The following table shows the interpretation of the sum TTI values. (Note that they are typically given in megayears, not years.)
Continuing the North Sea example, we used a present day percent of vitrinite reflectance value calculated for the well North 30/4-1 of 1.3 at a deep location in the basin. According to a correlation table from Waples (1980), this corresponds to a TTI value of approximately 160. Change skip lines to forty-nine, vertical exaggeration to 25, and toggle
Figure 15: Contour display of sum TTI from library file nsea.dat. Skip lines is set to 49 and sum TTI is contoured at 10. Note that the contour interval is 20, therefore sum TTI near the bottom of the basin is around 160. (vertical exag = 25)
8.3 Time-Temperature Index
Temp. interval (oC) <n 30-40 -7 40-50 -6 50-60 -5 60-70 -4 70-80 -3 80-90 -2 90-100 -1 100-110 0 110-120 2
Sum TTI <Interpretation 10 ma early oil generation 40 peak oil generation 75 late oil generation 180 wet gas (has liquid generation) 900 dry gas
Last Modified: 01:49pm EST, February 22, 1996 - Steven E. Nelson
