*3.3. Sill Intrusions in the Basin*

As mentioned above, earlier studies have pointed out the importance of host-rock temperatures at time of emplacement for the resulting thermal effects of intrusions [8,13,16–18]. In our study two sills, ~50 m thick, one in the hanging wall and one in the footwall, have been modeled to intrude the basin with increasing time lapse after fault slip. Figure 16 shows the results for sills intruding at time of fault slip, 1 Myr and 10 Myr after fault slip. There is a clear thermal instability on both sides of the fault zone. However, due to the masses of faulted rock and larger deposits of sediments in the hanging wall section, the thermal effects of the fault displacement are more pronounced here. Therefore, the largest differences in the thermal effects of the sills as time passes are expected to be found here.

**Figure 16.** Temperature results for sills intruding with different timing in relation to fault slip. Temperatures to the left are at time of intrusion, in the middle, 10 kyr after intrusion, to the right, 100 kyr after intrusion.

At the time of intrusion, the temperatures in the close proximity of the sills increases dramatically (cf., [25]). As time passes the area with increased temperatures grow and consequently the temperatures starts to fall. The thermal effect of sills has a quick rise and fall within the first 1 Myr after intrusion.

Modeling results at 1000 years after fault slip show that there is a temperature difference up to 40 ◦C on either side of the fault zone (Figure 4). Results also show that as time passes the basin regains steady state (Figure 6a–c), which for most scenarios in this study occur somewhere between 3 and 22 Myr after fault slip. For sills intruding at time of fault slip the host-rock temperature effects are lower compared to sills intruding into a basin with a time lapse after fault slip, which have temperatures closer to that of a basin in steady state. A consequence is that sills intruding with a time lapse in relation to fault slip have higher background temperatures and the thermal effects will be more prominent (Figure 16). These thermal differences are still present 100 kyr after sill intrusion. In the hanging wall there is a temperature difference around 40 ◦C between the cases where the sills intrude at time of fault slip and 10 Myr after fault slip (Figure 16). The highest potential host-rock temperature effect of intruding sills will be in a steady state basin.

The largest thermal differences between basins with faults restored in different ways, was found between basin with fault restored by vertical shear to basin with non-restored fault (Figure 15b). Therefore, we have tested the possible influence of a non-restored fault basin with sills on the calculated thermal effects. As the thermal differences between a basin restored by vertical shear to a basin with a non-restored fault was found in the footwall part of the fault zone (Figure 15b), this is also the area where it is expected that possible differences in thermal effects of sills will be found (Figure 17a). However, the thermal results of the basin with sills and non-restored fault show very small thermal differences from basin with sills and fault restored by vertical shear (Figures 16 and 17a). The most pronounced thermal difference is found between the scenarios where sills intrude 1 Myr after fault slip. These small temperature differences will not significantly influence the maturation calculations for the two scenarios.

Sedimentary basins worldwide normally consist of changing layers of different lithologies with contrasting properties. A homogeneous shale basin with a lower thermal conductivity, such as the average conductivity used in this study (Table 2), result in a warmer basin (Figure 10a,b) and

consequently gives a higher host-rock temperature effect for intruding sills. This emphasizes that the basin lithologies and their thermal conductivities have large influence on the temperatures in the basin. We have here tested a case where the sills intrude into a shale basin with a sandstone layer (Figure 12a). The results show no differences for the sills intruding into the basin at time of fault slip (Figure 17b). However, as time passes (1 Myr and 10 Myr after fault slip) the presence of the sandstone layer makes a difference in the resulting thermal effects of the sill intrusions. Sills intruding 10 Myr after fault slip cause thermal differences in some areas up to 20 ◦C in both the footwall and hanging wall 10 kyr after sill intrusion (Figures 16 and 17b).

**Figure 17.** (**a**) Calculated temperatures 10 kyr after sills intrude into a basin with non-restored faults. (**b**) Calculated temperatures 10 kyr after sills intrude into a shale basin with a sandstone layer as shown in Figure 12a left.

As the results in Figures 16 and 17 show, the thermal impact of sill intrusions are more pronounced when emplaced into a basin closer to thermal steady state. This more pronounced thermal impact also influences to a small degree the maturation levels around the sills, especially the area above the sills in the hanging wall (Figure 18a). Reports from onshore and offshore sedimentary basins with magmatic intrusions show that sills often occur in clusters (e.g., [6,9,52,67]). It is considered that multiple sills intruding at different levels within a certain time frame thermally impacts a larger rock volume of the basin compared to one single intrusion (e.g., [8,21,68]). To replicate a basin with clusters of sills, two more sills have been added to the model, one on either side of the fault zone. The thermal effect of sill swarms intruding with different timing relative to fault slip have a large impact on the matured organic material, particularly in the area between the sills on both sides of the fault zone (Figure 18b). In the heterogeneous basin, the warmer host-rock temperatures compared to the homogeneous basin, result in more transformed organic material on both sides of the fault zone for sills intruding at time of fault slip, as well as for sills intruding 10 Myr after fault slip. However, the maturation differences between the homogeneous and heterogeneous basins are more prominent when the sills intrude 10 Myr after fault slip (Figure 18b).

**Figure 18.** (**a**) Calculated maturation for one sill intruding on either side of the fault zone. (**b**) Calculated maturation for two sills intruding on both sides of the fault zone. Left side show results for sills intruding at time of fault slip. Right side show results for sills intruding 10 Myr after fault slip.
