Synthetic Case

A synthetic case of arbitrary dimensions shows that in a homogeneous basin, stresses build up at and around the fault tips when subject to compression and extension (Figure 10). Red and yellow colors in the models represent areas where stresses concentrate, while blue colors are areas of small to no stress accumulations. Fractures may open and increase the permeability close to the sides of the

fault tips where the tensile stresses concentrate (Figure 10). The direction of the fractures will follow the direction of σ1 (maximum principal compressive stress) and open in the direction of σ3 (minimum principal compressive stress or maximum principal tensile stress) [111]. Initiation of fault movement may occur at the fault tips where shear stresses accumulate, and as the shear stresses are larger at the upper tip, this is the area most likely for fault slip initiation (Figure 10).

**Figure 10.** Stress results for the synthetic case of a fault zone with arbitrary dimensions in a homogeneous basin. The tensile and shear stress developed in a homogeneous basin with one fault subject to horizontal compressive stress of 5 MPa (indicated by horizontal arrows). The host rock has a Young's modulus value of 10 GPa, the damage zone is 1 GPa, and fault core 0.1 GPa.

As shown in Sections 3.3.1–3.3.3, magmatic sills will influence the diagenetic processes in their proximity (see Figures 5–7). In order to include this impact in the synthetic model, a second model featuring the same fault, but with one sill surrounded by a diagenetically altered area was added. The results (Figure 11) show that in addition to accumulating shear and tensile stresses at the fault tips, due to compression, stresses mainly build up in the sill and in the area modified by diagenesis surrounding the sill. This occurs as stresses tend to build up in layers/zones of stiffer rocks in contrast to softer rocks in heterogeneous basins (e.g., [111,118,119]). These shear stress accumulations are also present in the fault zone, particularly where the sill crosses the fault, and consequently, potential fault reactivation may start in this area. The accumulated shear stresses in the sill and diagenetically modified area may result in linking of present weaknesses and potentially initiate growth of new faults within this area, if the shear strength of the rock is exceeded [111]. Tensile stresses concentrating in the area modified by diagenesis along the fault zone and within the diagenetically altered damage zone may, if the tensile strength of the rock is exceeded, open fractures [111]. Similar results are obtained for the same model subject to extension, however, the resulting tensile stresses will be much larger compared to when subject to compression (not shown here) and accordingly the chance of opening fractures will increase.

**Figure 11.** The resulting tensile and shear stresses of a fault with sill intruded through the fault zone with a surrounding area of diagenetic alteration when subject to horizontal compressive stress of 5 MPa (indicated by horizontal arrows). Dimensions are arbitrary in this synthetic model.
