4.2.1. Lithology

The lithology in the basement reservoirs of carbonate rocks includes dolostone, limestone, and a small amount of mudstone [39]. The development of tectonic fractures is divided into two categories: (1) only one or several sets of tectonic fractures that are developed in the rocks, and they are often regular and can be accurately measured and counted (Figure 7a); and (2) multiple sets of tectonic fractures that are interwoven into a network, which makes it difficult to measure and count each fracture, creating a fracture zone (Figure 7b). Observations and statistical analysis of different lithologies depict that the fracture zone is developed in both dolostone and limestone, but barely in mudstone (Figure 12). Based on borehole image logs, the ratio of the fracture zone thickness to the rock thickness in dolostone is 18.1%, while in limestone, it is 12.5%. Moreover, the linear density of tectonic fractures in the borehole image logs, excluding the fracture zone, was also analyzed (Figure 12). Hereof, the average linear density of tectonic fractures in dolostone is found the largest, which can reach 6.8 m<sup>−</sup>1, followed by limestone, and finally mudstone, 4.1 m<sup>−</sup><sup>1</sup> and 1.3 m<sup>−</sup>1, respectively. Also, in the outcrops, tectonic fractures are more developed in dolostone than limestone, while tectonic fractures in mudstone have the least development degree (Figure 13). All of these indicate that lithology controls the abundance of tectonic fractures in the target layers.

Lithology controlling fracture development is essentially due to the rock composition, particle size, and particle arrangemen<sup>t</sup> [61,62]. Both dolomite and calcite are the major minerals in carbonate rocks, but the Young's modulus of dolomite (8.71 × 104–14.18 × 10<sup>4</sup> MPa) is higher than calcite (5.69 × 104–8.82 × 10<sup>4</sup> MPa) [63–65]. Therefore, under the same stress conditions, dolostone with higher dolomite content is more likely to develop tectonic fractures than limestone with higher calcite content. However, the abundance of these minerals (calcite and dolomite) in the mudstone is very low, hence tectonic fractures are developed poorly. Based on thin sections, limestone is more likely to host dissolution fractures than dolostone and mudstone due to its higher calcite content [66].

**Figure 12.** Schematic diagram comparing the fracture zone frequency and fracture linear densities of tectonic fractures in different lithologies. The fracture zone frequency refers to the ratio of the fracture zone thickness to the rock thickness based on borehole image logs.

**Figure 13.** Tectonic fractures in different lithologies of carbonate rocks in the outcrops.
