4.2.3. Fault

Since the Paleozoic, the Jizhong Sub-basin has experienced three large-scale tectonic movements: Indosinian, Yanshan, and Himalayan movements, to form a typical multi-period structural superimposed sub-basin [39]. The sub-basin mainly has caused two-set extensional fault systems of NNE-SSW and NW-SE strikes, in addition to some near E–W strikes [34,42] (Figure 17). The characteristics of these faults indicate that the faults with NNE–SSW strikes are major ones that control the orientations of deep tectonic belts, and the faults with NW–SE and near E–W strikes are the lateral faults which segmen<sup>t</sup> the tectonic belts and control the formation and scale of deep reservoirs [44]. Interpretations of borehole image logs explain that tectonic fractures in these reservoirs are mainly NNE–SSW, NW–SE, and near E–W strikes overall. Nevertheless, in different locations of the sub-basin, fracture directions vary notably (Figure 17). In the northeastern and southwestern regions of this sub-basin, the dominant direction of tectonic fractures is the NNE–SSW strikes, while in the central part and near the transfer zones, the dominant direction of these tectonic fractures becomes more complicated where all the three sets (NNE–SSW, NW–SE and near E–W strikes) can appear.

**Figure 17.** The relationship between fault and fracture strikes. (**a**) Location of the Jizhong Sub-Basin in the Bohai Bay Basin in China (modified from Zhao et al., 2015) [9]. (**b**) Faults and tectonic fracture orientations in the carbonate rocks of the Jizhong Sub-basin. The fault data was modified from the Huabei Oilfield database. The data regarding fracture orientations is derived from the borehole image logs of 14 wells.

The main reason is that the NNE–SSW strike faults are the only one set that is developed in the northeastern and southwestern regions of this sub-basin and all the NNE–SSW, NW–SE, and near E–W strike faults are developed in the central area of the sub-basin. The orientation of the fractures and faults is consistent, and the dominant direction of these fractures is sub-parallel to the direction of faults or oblique at a smaller angle. Therefore, one can conclude that tectonic fractures in this sub-basin are associated with faults, and these faults have a significant influence on the fracture direction.
