*4.3. Example from the Vøring Area*

For the transect VB-2-87-B in the Vøring Basin, the influence of sills and diagenetically altered area (as given in Sections 3.3.1–3.3.3) was included to investigate its effect on the stress accumulations. The mid-Norwegian margin is presently experiencing compression. However, this is not a specific study of the Vøring area, therefore, to study the response to compressive and tensile horizontal stresses the model was subjected to both. The area to the left (Figure 12a) with several faults and sills, is of particular interest as it displays the interaction between faults, sills, and areas modified by diagenesis. As the smectite to illite transition zones (Figure 7) show boundaries much like the one of opal A to opal CT (Figure 6) and quartz diagenesis (Figure 5), this transition was not included in the study, as the results are expected to be more or less the same as for the opal CT transition zone and quartz diagenesis.

**Figure 12.** (**a**) Stress results for the Vøring profile when the sills are excluded, and diagenesis is disregarded. The results show the shear stresses due to 5 MPa compressive horizontal stress (indicated by horizontal arrows). The white box indicates the area enlarged in (**b**) and (**c**), the red box indicates the area enlarged in Figures 13 and 14. (b) The same scenario as in (a) but with the tensile stresses due to compression. (c) The same scenario as in (a) and (b), but with tensile stresses due to 5 MPa extension (indicated by horizontal arrows).

**Figure 13.** Stress results for the basin with 50 m thick sills subject to horizontal compressive stress of 5 MPa. The area and scale are shown by the red box in Figure 12. (**<sup>a</sup>**,**b**) The results for the basin when the sills and the transition zone of opal CT are accounted for. (**<sup>c</sup>**,**d**) The results when sills and quartz diagenesis are accounted for. White circles indicate areas where the sill does not penetrate the fault zone, however, parts of the fault zone have area altered by diagenesis.

**Figure 14.** Stress results for the basin with 50 m thick sills subject to horizontal tensile stress of 5 MPa. The area and scale is shown by the red box in Figure 12. (**<sup>a</sup>**,**b**) Results for basin where sills and the transition zone of opal CT is accounted for. (**<sup>c</sup>**,**d**) Results for when sills and quartz diagenesis is accounted for. The white circle to the right indicates the area where the sill does not penetrate the fault zone, however, parts of the fault zone have area altered by diagenesis. This area concentrates stresses, and when high enough, may contribute to fault reactivation and fracture development. The sills are 50 m thick.

The transformation zone of opal A to opal CT starts when the porosity reduction is around 20% (cf. Figure 6; [23]) and is modeled to end around 200 m deeper (cf. [41]). Five different scenarios were tested and subject to horizontal compressive and tensile stresses of 5 MPa: (1) the basin with both sills and diagenesis disregarded (Figure 12), (2) the basin with sills but diagenesis disregarded, (3) the basin without sills but with opal A to opal CT transition zone included, (4) the basin with sills and opal A to opal CT transition zone included (Figures 13 and 14) and, (5) the basin with sills and quartz diagenesis (Figures 13 and 14).

Common results for all the models, whether subject to compressive or tensile stress, is that stresses build up at the fault tips (like the synthetic models), and in particular at the lower tips (Figure 12a–c). This is due to the larger contrast in rock stiffness between the fault zones and the basement compared

to the contrast between the fault zones and the host rock (cf. Table 3). Shear stresses are more or less the same with regard to location and strength of accumulations, whether they are subject to compression or extension. However, the tensile stresses are much lower for the models subject to compression (Figure 12b) than those subject to extension (Figure 12c).

When subject to both compression and extension, the synthetic models show that both shear and tensile stresses mainly concentrate in the sill and area modified by diagenesis, also where these features cross the fault zone (Figure 11). Figure 13 shows the results for the Vøring basin subject to compression and when sills and the opal CT transition zone are included (Figure 13a,c), as well as when sills and the quartz diagenetically modified area are accounted for (Figure 13b,d). For all cases, areas of increased tensile stresses are few and small, but potential fractures may open along the fault zone to the right and increase the permeability if the stresses become high enough (Figure 13c,d). In addition, areas indicated by white circles in Figure 13c,d, show elevated tensile stresses related to where the sill and diagenetically modified area cross the fault zone. Fractures may open or remain open if stress concentrations grow high enough, thereby contributing to increased permeability in these areas. The shear stresses tend to build up in the areas modified by diagenesis and in the sills. Where the sills cross the faults, stress concentrations are, without exceptions, particularly high (Figure 13a,b) and may reactivate the fault if they exceed the rock's shear strength of 1–12 MPa. Elevated shear stresses in the opal CT transition zone and area modified by quartz diagenesis, may potentially initiate growth of new faults if the stresses become high enough, and thereby contribute to increased permeability.

When the basin is subject to extension the shear stresses concentrate in the same location with the same strength (Figure 14a,b), as when subject to compression (Figure 13a,b), for all scenarios. If the shear stresses exceed the shear strength of the rocks, fault slip will be initiated in some areas (colored red and yellow). However, the tensile stresses are much larger for the basin subject to extension (Figure 14c,d) as opposed to compression (Figure 13c,d). The entire sills tend to concentrate tensile stresses, not just when crossing fault zones (Figure 14c,d). Additionally, the diagenetically altered area of both opal CT and quartz accumulate higher tensile stresses compared to their surroundings. The high concentrations of tensile stresses show that the basin is prone to open fractures and increase the permeability in several areas. Due to the particularly high stress concentrations in the sills and the areas that have undergone diagenetic transitions, fractures are expected to first open in these places.
