**4. Results**

## *4.1. Fracture Characteristics*

This study distinguished three genetic types of natural fractures, including tectonic, pressuresolution, and dissolution fractures, in the basement reservoirs of carbonate rocks in the Jizhong Sub-Basin [39,52,53]. Among these, tectonic fractures have a higher development degree than others and are the dominant type in these reservoirs.

## 4.1.1. Tectonic Fractures

Tectonic fractures in the outcrops appear in sets, and their occurrences are stable (Figure 4). Statistical analysis of outcrops confirms that tectonic fractures are developed in three major sets of NNE–SSW, NW–SE, and near E–W strikes, while less developed in other directions (Figure 5). On the

cross-section, tectonic fractures may pass through the rock formation interface and have a height of several meters (Figure 4a). Other tectonic fractures within the rock formation with a height of less than a few tens of centimeters were also observed (Figure 4b). On the horizontal plane, tectonic fractures show mutual crosscutting relations, and their lengths vary considerably, from several centimeters to meters (Figure 4c). The dip angles of these fractures are mainly high (>60◦) and near-horizontal (<15◦). Ultimately, the number of tectonic fractures with oblique dip angles (15◦–60◦) was found less than 20% of the total (1326).

Based on core observations, tectonic fractures usually exhibit a fracture plane with steps (Figure 6a). In mudstones and argillaceous carbonate rocks, these fractures demonstrate clear striaes along the direction of fractures propagation, with a very smooth surface (Figure 6b). Borehole image log interpretation indicates that tectonic fractures appear as sinusoidal curves and are randomly distributed (Figure 7a). Some tectonic fractures are intertwined to form a fracture network (Figure 7b). The dip angels of these fractures are concentrated in the range of 60◦ to 85◦, followed by those with angles less than 30◦. In particular, the dip angels of fractures in mudstones are mainly less than 20◦. The linear density of tectonic fractures in a single well varies notably, ranging from 1.2 m<sup>−</sup><sup>1</sup> to 8.6 m<sup>−</sup>1. Furthermore, borehole image logs confirm the existence of fracture sets similar to those in the outcrops, mainly in the NNE–SSW, NW–SE, and near E–W strikes. More than 65% of tectonic fractures are opening-mode ones in which minerals are not filled, while others are entirely or partially filled by calcite, hydrocarbons, and clay minerals (Figures 6c and 7c).

**Figure 4.** Tectonic fractures in outcrops. (**a**) Fractures are developed with a height of several meters on the cross-section. (**b**) Fractures are developed within the rock formation with a height of a few tens of centimeters. (**c**) Fractures show mutual crosscutting relations on the horizontal plane where Set C arrests Set D.

**Figure 5.** Orientations of tectonic fractures in outcrops of carbonate rocks in the Jizhong Sub-Basin (*n* = 1105).

**Figure 6.** Tectonic fractures in cores. (**a**) Fractures in Well R6, depth 5583.18 m (18,317.52 ft). (**b**) Fractures in Well R14, depth 4010.37 m (13,157.38 ft). The dip angle of this fracture is 6◦. (**c**) Fractures in Well R7, depth 4301.19 m (14,111.52 ft). Calcite entirely filled fractures.

The inspection of thin sections reveals that tectonic fractures are widely distributed in these carbonate rocks (Figure 8). Two sets of tectonic fractures can cut through or arrest each other (Figure 8b,c), and multiple sets are interwoven to form a network (Figure 8d). The development of these fractures does not exhibit any relationship with the bedding plane of carbonate rocks. If early developed fractures are filled with minerals such as calcite, pyrite, clay, or hydrocarbons, they will not provide effective storage space and seepage channels for the reservoir and become ineffective (Figure 8d–f) [54,55]. In this regard, the same fracture could be filled multiple times with different minerals (Figure 8e,f). More than 60% of tectonic fractures in thin sections are open and do not show any mineral fillings. The apertures of these fractures vary, while most of them are less than 100 μm and are concentrated below 60 μm.

**Figure 7.** Tectonic fractures in borehole image logs. (**a**) Tectonic fractures are conductive in Well R8. The red sinusoidal curves represent recognized fractures. (**b**) Tectonic fractures that are not filled link together like a network in Well R15. (**c**) Tectonic fractures in Well R3. The yellow sinusoidal curves represent the fractures filled by minerals.

## 4.1.2. Pressure-Solution Fractures

Pressure-solution fractures are formed during structural and diagenetic processes due to pressure solution [56]. The pressure-solution fractures in these reservoirs are composed of bed-parallel fractures and stylolites. The bed-parallel fractures are formed along the depositional interface under various geological conditions, with distinguishable characteristics such as bending, discontinuity, and branching (Figures 9a–c and 10a) [57]. These fractures exhibit small dip angles and are nearly parallel to the depositional interface. There are normally insoluble mineral residues recognized in them, such as clay minerals, while they can also be filled with hydrocarbons or other minerals. Stylolites are generally irregularly wavy or serrated, parallel or sub-parallel to the horizontal plane, with a small number intersecting the horizontal plane at a small angle (Figures 9c,d and 10b). Iron argillaceous minerals and hydrocarbons can fill some of these fractures. These pressure-solution fractures are poor in lateral continuity, and their apertures in the thin sections are commonly less than 35 μm.

## 4.1.3. Dissolution Fractures

Dissolution fractures are formed through long-term underground fluid, including the new fracture formed after the dissolution transformed the earlier fracture and the fracture formed when the dissolution connected a lot of pores [58,59]. When dissolution fractures are formed from earlier ones, their fracture walls are rough and uneven, and their apertures are larger than the previous stage fractures (Figure 11a). Although newly dissolved pores are preserved inside or at the edges

of the original fractures and the shape of the initial sets are changed after the dissolution process, the original distribution of these fractures can still be discerned. Dissolution fractures that are formed when multiple pores are connected like a string of beads will become an effective storage space for hydrocarbons in the reservoir (Figure 11b). Besides, fractures filled with minerals can also become dissolution fractures when unstable filling minerals like calcite entirely or partially are dissolved via acidic water leaching or groundwater scouring (Figure 11c,d). Overall, dissolution fractures are irregular in shape, often in the pattern of snake-like and anastomosing (Figures 10c and 11) [60]. The apertures of dissolution fractures measured in thin sections vary significantly and are between 40 μm and 80 μm and sometimes become relatively large (up to 200 μm).

**Figure 8.** Tectonic fractures in thin sections. (**a**) Fractures in Well R2, depth 5039.60 m (16,534.12 ft). (**b**) Fractures in Well R2, depth 5039.35 m (16,533.30 ft). Group B terminated Group A. (**c**) Fractures in Well R5, depth 5916.02 m (19,409.51 ft). Group D terminated Group C. (**d**) Fractures are filled with calcite in Well R9, depth 4548.10 m (14,921.59 ft). (**e**) Fractures in Well R5, depth 5728.31 m (18,793.67 ft). E shows hydrocarbons, and F is calcite. (**f**) Fractures in Well R8, depth 4703.26 m (15,430.64 ft). G is dolomite, and H is calcite. The directions of these thin sections are vertical to the wellbore.

**Figure 9.** Pressure-solution fractures in thin sections. (**a**) Bed-parallel fractures inWell R2, depth 5041.27 m (16,539.60 ft). (**b**) Bed-parallel fractures in Well R8, depth 4861.80 m (15,950.79 ft). (**c**) Fractures in Well R8, depth 4862.12 m (15,951.84 ft). A is the bed-parallel fracture. B is the stylolite. (**d**) Stylolite in Well R9, depth 4548.53 m (14,923.00 ft). The directions of these thin sections are parallel to the wellbore.

**Figure 10.** Fractures in borehole image logs. (**a**) Bed-parallel fractures in Well R15. (**b**) Stylolites in Well R1. (**c**) Dissolution fractures in Well R16. The arrows mark the identified fractures.

**Figure 11.** Dissolution fractures in thin sections. (**a**) Fractures in Well R12, depth 3105.18 m (10,187.60 ft). (**b**) Fractures in Well R12, depth 3112.35 m (10,211.12 ft). (**c**) Fractures in Well R17, depth 2764.60 m (9070.21 ft). (**d**) Fractures in Well R17, depth 2764.00 m (9068.24 ft). The directions of these thin sections are vertical to the wellbore. The minerals dissolved in these filled fractures are calcite.

#### *4.2. Factors Influencing Fracture Development*
