*5.2. Vitrinite Reflectance*

Vitrinite is an organic material from cell walls of woody plant tissue [27]. Vitrinite reflectance (VR) is a measure of the percentage of incident light reflected from the surface of vitrinite particles in a sedimentary rock. It is referred to as %Ro. Results are often presented as a mean Ro value based on all vitrinite particles measured in an individual sample. Vitrinite reflectance is a standard method for measuring the thermal maturity of sedimentary rocks and kinetic models of vitrinite reflectance are commonly used to constrain paleo thermal histories in basin and petroleum system modeling. BMT uses the EASY%RO model to calculate vitrinite reflectance [28]. Generally, the onset of oil generation is correlated with a reflectance of %Ro = 0.5–0.6% and the termination of oil generation with reflectance of 0.85%–1.1%. The onset of gas generation ('gas window') is typically associated with %Ro values of 1.0%–1.3% and terminates around 3.0%. The vitrinite reflectance was here modeled for cases with and without development of piercing salt structures for both models, and the results are shown in Figures 20 and 21.

In model 1, the increased conductivity of the salt and the resultant higher sediment temperatures above salt and lower temperatures below and around the salt clearly affects the modeled vitrinite reflectance (Figure 20). The same can be seen, but less pronounced, in Model 2 with the five columnar diapirs (Figure 21). The hydrocarbon potential may be lower in areas around penetrating salt structures, as cooling of the sediments around the stem of the salt structure lowers the maturation potential locally. This may be enhanced by erosion over the salt diapirs.

The temperature effects in the area above salt are not sufficient to have significant hydrocarbon potential in Model 1 (Figure 20). The model with salt has only <0.05% higher VR at shallow depths and increasingly lower VR towards the stem below salt with a maximum difference of 0.6%–1.0% lower VR at 7 km depth in the lower Triassic sequence.

**Figure 20.** Top: Modeled vitrinite reflectance for Model 1 at 18 Ma, without salt balloon (top left) and model with salt balloon (top right). Lower figure shows the difference in modeled vitrinite reflectance between the two models.

The calculated present day vitrinite reflectance difference between the models with and without salt structures for Model 1 and Model 2 are shown in Figure 21. The modeled "oil windows" and "gas windows" are indicated by light green and red colors for the no-salt models and with hatching on the salt-structure models. Models with salt diapirs have lower maturation in the basins between salt structures than models without salt diapirs. In general, the VR difference increases by depth from around 2 km depth, with lower Ro values (less mature sediments) in the salt-diapir models. Maturation is higher above salt diapirs, but only at shallow depths and has little impact on hydrocarbon potential.

In Model 1, a maximum depth difference of −1065 m at the base of the "gas window" is shown towards the salt stem. At Ro = 1.0 (base of "oil window" the maximum depth difference is −935 m (Figure 21a). The salt modeled option may still be gas generating in the Jurassic and upper Triassic below the salt structure, while the model without the salt balloon has a present day "gas-window" that only reaches down into Lower Cretaceous strata.

Along the profile in Model 2, the VR difference decreases upward, but the salt model has consistently lower %Ro values in the Triassic pod basins relative to the no-salt model. At a position 30 km along the profile %Ro values of 3.0 are found 600 m shallower in the no-salt model. As expected from the temperature modeling, the oil and gas windows are deeper between salt pillars than in the model without salt. Maximum depth of oil window is as much as 264 m deeper centrally in the Triassic pod at position 20 km in the salt model (marked A in Figure 21b). Above the salt structure at 33 km, the salt model reaches base of oil window (i.e., Ro = 1.1) at 203 m shallower depth than in the no-salt model (marked B). The resulting vitrinite reflectance models indicate that the Triassic sequences of Model 2 may still be gas-generating presently.

**Figure 21.** Differences in modeled Vitrinite Reflectance at present day geometry between models with salt (stippled lines) and model without salt diapirs (solid lines). "Oil-window" and "gas-windows" are indicated between %Ro values of 0.5–1.0 and 1.0–1.3, respectively. (**a**) model 1, (**b**) model 2.
