The strike-slip fault in the Shunbei region controls the formation of Ordovician reservoirs, and hydrocarbon resources are primarily spread along this fault zone. The previous exploration work has confirmed that the favorable reservoirs in the Shunbei area are mainly distributed in large strike-slip fault zones, associated secondary fault zones, fracture-cavity connected with faults, and the distribution of reservoirs is directly affected by fault activities. The reservoir rocks are damaged and fault-fracture systems are created by the action of the tectonic stress field. A reservoir eventually arises in the fault zone as the reservoir fluid migrates along the fault-fracture systems. The fault-fracture system serves as both the diversion channel between adjacent fracture-cavity units and the reservoir space for hydrocarbon resources. (Zhang et al., 2021).
The reservoir space of the Middle and Lower Ordovician in the Shunbei area includes 3 types: pore, cave, and fracture. According to the origin, shape, size, and combination of the reservoir space, the type of reservoir architecture can be divided into: 1. fractured reservoirs; 2. pore-type reservoirs; 3. fracture-cave reservoirs; and 4. cave reservoirs (Li et al., 2019).
Because of the difference in the local stress field between the tension torsion segment and the compression torsion segment of the strike-slip fault, the associated structures are obviously different. Under the background of compression torsion stress, the strike-slip fault will derive reverse fault, strike-slip fault, normal fault and fold; Under the background of tension torsion stress, only normal faults will be derived or associated, and no compressional or shear structures will occur (Fig. 2). Therefore, the internal structure of the compressional uplifts segment is frequently more complex.
Twelve wells in the S-1 fault zone and the S-5 fault zone area were provided as the research materials. (Fig. 3). Based on core, imaging logging, and pressure build-up well test curves, the reservoir architecture differences between the S-1 and S-5 fault zones are studied and analyzed, providing a reference for the efficient development of the Shunbei reservoir in the future.
2.1 The S-5 strike-slip fault zone
The S-5 strike-slip fault zone is located in the transition between the conjugate strike-slip fault system of the Tabei Uplift and the single-shear strike-slip fault system of the Tazhong Uplift. At the early stage of the evolution of the S-5 strike-slip fault zone, the structural characteristics of a single strike-slip fault match to the Riedel shear model. In episode Ⅲ in the middle Caledonian period, a right-lateral left stepover strike-slip fault zone was developed on the T74 interface, indicating that it was affected by NE direction compressive stress at the stage. The segmentation deformation characteristics of the S-5 strike-slip fault zone are caused by the local stress differences near the fault and the superposition of multi-stage faults. The S-5 strike-slip fault zone is controlled by compression, therefore, the phenomenon of compressional uplift is more pronounced on the profile.
2.1.1 Core observation
It is authentic to confirm that obvious natural fractures have formed in the S-5 strike-slip fault zone by observing reservoir cores. These fractures can be further classified into structural and non-structural fractures based on their geological origin. The reservoir rocks in the Shunbei area are brittle, and the overlying pressure is huge. Therefore, the non-structural fractures developed in reservoir rocks are mainly interlaminar fractures and suture lines. Among them, the suture lines are formed by pressure dissolution under strong compaction background, with serrated development, mostly filled, and no conductivity (Fig. 4). The interlaminar fractures are developed along the micro-bedding plane, primarily half-filled, with certain conductivity, and can be used as the flow channel of hydrocarbon resources. Structural fractures are generally shear fractures caused by the shear stress under the action of structural stress. They are mainly high-angle fractures with a long extension distance. They appear in groups, with an opening ranging from 5 mm to 30 cm. There is an obvious cutting relationship between the structural fractures of different groups. In addition, the structural fractures are mostly filled or half-filled with calcite (Fig. 5).
High-angle structural fractures, horizontal interlaminar fractures, and almost 90° vertical high steep structural fractures can all be detected in the reservoir cores of the S-5 strike-slip fault zone. No obvious dilatation phenomenon is found along the structural fractures, indicating that the reservoir in the S-5 strike-slip fault zone has undergone weak corrosion transformation (Wu et al., 2019). There are oil traces in vertical high-angle fractures, while no filling is found in horizontal interlaminar fractures.
2.1.2 Imaging logging analysis
According to the imaging logging interpretation results of wells in the S-5 strike-slip fault zone, it can be observed that high-angle fractures are developed (Fig. 6), with small fracture openings and smooth fracture walls. In addition, natural fractures are relatively developed in the main production layer (Yijianfang Formation and upper Yingshan Formation).
2.1.3 Pressure build-up curve analysis
It can be found from the pressure build-up well test curves of four wells in the S-5 strike-slip fault zone (Fig. 7) that only a few fractures or faults are the main oil and gas flow channels in these wells, and there may be a few small holes with low fluid supply capacity in the reservoir. Typically, the far-end pressure is low. For example, wells S5-9 and S5-10 only supply fluid from fractures during pressure propagation. The double logarithmic curve dropped slightly in the later period, indicating that there may be edge and bottom water with low energy at the far end.
2.2 The S-1 strike-slip fault zone
The S-1 strike-slip fault zone is located in the east of the middle section of the S-5 fault zone, which is a NE trending branch fault of the S-5 fault zone. The S-1 fault zone is a tension torsion stress fault active zone under the background of compression torsion stress. Under the action of local tensile stress, there are numerous visible pull-apart and drop-off segments at the T74 interface. The stress state in this area is a strike-slip stress state. It is a left-lateral left stepover strike-slip fault zone, which is rhombic in shape. Meanwhile, diagonal R faults are developed at both ends of the fault zone, and on the profile, it is shown as a falling graben or negative flower structure style.
2.2.1 Core observation
According to the observation of reservoir cores in the S-1 strike-slip fault zone, natural fractures and dissolution caves are developed (Fig. 8). Among them, natural fractures can be divided into non-structural fractures and structural fractures due to geological origin. It is apparent that the S-1 fault zone's reservoir rocks have a higher density of natural structural fractures than the S-5 fault zone does. High angle structural fractures and vertical high steep structural fractures of about 90 degrees are created. The fracture width is large, and most of them are calcite filled or half-filled, with good conductivity. The dissolution caves are mostly distributed along large natural fractures, and evident dilatation cab be seen along structural fractures, indicating that the reservoir in the S-1 strike-slip fault zone has undergone dramatic corrosion transformation(Wu et al., 2019), and the dissolution caves are filled or not filled with asphaltene .
2.2.2 Pressure build-up curve analysis
It can be found from the pressure build-up curves of four typical wells in the S-1 strike-slip fault zone (Fig. 9) that the reservoir targets of these wells are primarily dissolution caves and fault-fracture systems with huge internal space, with large reservoir scale and adequate fluid supply capacities. The wells S1-14 and S1-12's logarithmic curves show that they have extended radial flow stages that are indicative of quasi-radial flow. The single well has a large controlled reserves scale, a relatively far boundary, and sufficient reservoir fluid supply capacity. It demonstrates that there are multiple fluid supply channels when the well encounters fault-fracture systems or dissolution caves with large internal space. It can be seen from the double logarithmic curves of wells S1-11 and S1-13 that there are multiple sets of fracture-cave structures in the reservoirs, with the radial flow in the early stage and spherical flow in the later stage. Hence, these wells have sufficient fluid supply capacity.
The analysis results of pressure build-up curves of the four wells show that the internal architecture of the reservoirs in the S-1 fault zone is basically consistent, and they are all reservoirs of inner caves and outer fractures, and the reservoir architecture and seepage characteristics are basically consistent.