A study on reservoir architecture difference of extra-deep strike-slip fault zone in the Shunbei area, Tarim Basin

Shunbei reservoir is a typical extra-deep carbonate strike-slip fault-controlled reservoir. It has experienced multi-stage tectonic activities, and strong heterogeneity and anisotropy are present in the reservoir. Based on the reservoir cores, imaging logging, pressure build-up curves of typical wells, and dynamic analysis, the reservoir architecture difference between the S-1 and S-5 strike-slip fault zones in the Shunbei area is studied. The results show that the reservoir architecture of the S-5 fault zone is a fault-fracture system formed under the compressive stress environment, with small internal space, poor fluid flow capacity, small reservoir scale, and low energy. However, the reservoir architecture of the S-1 fault zone is the dilational space and caves bounded by fault planes formed under the tensile stress environment. The reservoir space could be categorized as the fault-fracture-cave system, which has large internal spaces and fluid flow capacity. Moreover, the reservoir is substantial in size and is highly energetic. This study has clarified the difference in reservoir architecture between the S-1 and S-5 fault zones and could be used as a classic case to predict the fault-reservoir relationship in the Shunbei area. It is of great significance for the exploration and development of the extra-deep carbonate strike-slip fault-controlled reservoir.


Introduction
Due to the increasing difficulty of exploration and development of conventional oil and gas reservoirs worldwide, unconventional reservoirs have attracted intensive attention (Zou et al. 2014;He et al. 2017).Shunbei reservoir is a typical extra-deep carbonate strike-slip fault-controlled reservoir discovered by Sinopec in 2015.It is located on the northern margin of the Shuntoguole Low Uplift in Tarim Basin, Xinjiang, China (Jiao 2018;Deng et al. 2018Deng et al. , 2022;;Li et al. 2019a, b).The hydrocarbon resources are mainly distributed in the Middle and lower Ordovician Yijianfang Formation and Yingshan Formation, with a burial depth of over 7300 m (Zhao et al. 2019;Wang et al. 2020a;Ru et al. 2022).Strike-slip faults and micro-fractures take up the role of the storage space and migration channels of hydrocarbon resources.With the progress of geophysical technology in a desert area, a total of 18 main strike-slip faults have been identified in the Middle and Lower Ordovician strata in the Shunbei area.The wells in Shunbei Oilfield are distributed along the main strike-slip fault zones (Deng et al. 2019a;Qiu et al. 2019;Zhao et al. 2021).S-1 fault zone and S-5 fault zone are the two typical fault zones for Shunbei Oilfiled.In detail, the S-1 fault zone has been fully developed, while commercial hydrocarbon flow has been obtained in the S-5 fault zone.The development results show that the Shunbei area has great potential.Through the comparative analysis of geological and production data of S-1 and S-5 fault zones, it is found that there are obvious reservoir architecture differences between them, which is of great significance for the efficient development of the Shunbei reservoir.
The theory underlying this work is based on extensive research on the evolution law, internal structure, and reservoir control mechanism of strike-slip faults.(Mitchell et al. 2009) analyzed the fluid properties and flow laws inside the faults by making statistics on the width, length, 57 Page 2 of 11 fault displacement, and other parameters of the fault system development in this fault-controlled reservoir.(Jeanne et al. 2012) qualitatively studied the influence of tectonic activity, rock mechanical properties, and fluid activity on the internal structure of the fault.Their research ideas provide a useful reference for this study.(Jiao et al. 2017) qualitatively studied the evolution law and hydrocarbon distribution characteristics of the strike-slip fault zone in the Shunbei area.They found that the Ordovician reservoirs are deformed in layers in the longitudinal direction, while the deep main slip zone is deformed in segments along the strike-slip fault.(Deng et al. 2018) made a mechanical analysis of the Shunbei S-5 fault zone and proposed that the overlapped pullapart areas within the strike-slip fault are the dominant area for hydrocarbon enrichment.(Li et al. 2019a, b) based on the analysis of reservoir cores and 3D seismic data in the Shunbei area, have divided the types of reservoir space in detail, that is, cave cavities, intergranular fractures, highangle structural fractures, and associated dissolution caves.(Wang et al. 2020a) conducted a permeability evaluation along the fault strike of the Shunbei N-5 fault zone based on reservoir geomechanics research.For the first time, the internal structure of the fault is being studied from the standpoint of the current stress field.(Zhang et al. 2021) studied the productivity differences within wells of the same fault zone in the Shunbei area, and determined that there are differences in the fault internal space structure in different parts of the strike-slip fault zone, which can be divided into fracture-type and cave-type reservoirs.Their work has played a certain role in promoting the research of reservoir architecture in the Shunbei area, but there is a problem that all the basic data are from geophysical data.When the buried depth of the Shunbei reservoir exceeds 7300 m, the accuracy of geophysical data and the reliability of interpretation results are decreased.Therefore, no research has clearly and reasonably explained the reasons for the difference in reservoir architecture and production data between the S-1 and S-5 fault zone now.
Based on the drilling cores, logging, and production data of wells in the S-1 and S-5 fault zones in the Shunbei area, combined with the dynamic characteristics of the fault zone, the reservoir architecture difference and types in the Shunbei area were studied.The research results offer a valuable reference for reservoir evaluation and geological sweet spot prediction of extra-deep strike-slip fault-controlled reservoirs in the Shunbei area.

Geological background
Shunbei Oilfield is located on the northern margin of Shuntuoguole Low Uplift, Tarim Basin.A secondary tectonic unit in Tarim Basin called Shuntuoguole Low Uplift is a saddle-shaped uplift with the characteristics of alternating uplift and depression.Its east and west sides are Manjiaer Depression and Awati Depression, and the north and south sides are Katak Uplift and Shaya Uplift (Fig. 1).At conjunction of the Awati Depression, Manjiaer Depression, and Shaya Uplift, the Shuntuoguole Low Uplift occupies a promising regional structural location for the accumulation of hydrocarbon resources (Wang et al. 2020b;Sun et al. 2021;Ru et al. 2022).
There are four stages of tectonic evolution: 1.Early Caledonian; 2. Middle and Late Caledonian-Early Hercynian; 3. Late Hercynian-Indosinian; 4. Yanshanian-Himalayan (Wei et al. 2019).The main producing strata are the middle and lower Ordovician Yijianfang Formation and the upper Yingshan Formation, with a thickness of around 160 m for the Yijianfang Formation and a thickness of approximately 250 m for the upper Yingshan Formation.The lithology is mainly limestone, and the dolomite content gradually increases from shallow to deep (Lan et al. 2015;Qi. 2021).There is a high-steep vertical strike-slip fault system developed in the middle and lower Ordovician strata, which breaks down to the Cambrian Yuertusi Formation (the source rock strata).It is a favourable area for long-term migration and accumulation of hydrocarbon resources and has great exploration and development potential (Deng et al. 2018;Ru et al. 2020).The S-1 and S-5 fault zones are the main research objects in this paper.To be more specific, the longest strike-slip fault zone in the Shunbei and its adjacent areas is the S-5 fault zone, which has evolved in the Tabei Uplift, Shuntuoguole Low Uplift, and Tazhong Uplift.The recognizable length of the S-5 fault is about 270 km, and the strike of the S-5 fault zone has a deflection of around 40° from north to south.It is approximately NW20° in the Tabei Uplift, NE10° in the Shuntuogole Low Uplift, and NE20° in the Tazhong Uplift.According to the difference of fault strike, the S-5 fault zone can be divided into three sections: the north section (strike-NW20°), the middle section (strike-NS-NE10°), and the south section (strike-NE20°) (Deng et al. 2019b;Hu et al. 2021).The S-1 fault zone is a branch fault situated on the east side of the middle section of the S-5 fault zone, the strike is roughly NE40°.There are differences between the deformation characteristics of deep and shallow strata.Deep strata create high-angle strike-slip faults, while shallow strata develop flower structures and en-echelon normal faults (Fig. 1).
At the T 7 4 interface (the top interface of the Middle-Lower Ordovician Yijianfang Formation), the middle and north sections of the S-5 fault zone are under the background of compression torsion tectonic stress.Therefore, the faults are right-lateral left stepover obliquely distributed, and compressional uplifts are mainly developed in the overlapping section of faults.While, at the T 7 4 interface, the S-1 fault zone is under the background of tension torsion tectonic stress, the faults are left-lateral left stepover obliquely distributed as the consequence.In the faults overlapping section, pull-apart and drop-offs are mainly developed.There are pairs of echelon normal faults in the S-1 fault zone at the T 7 0 interface (the top interface of Ordovician strata).

Reservoir architecture analysis
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. 2019a, b).
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.

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 rightlateral left stepover strike-slip fault zone was developed

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. 6).The interlaminar fractures are developed along the micro-bedding plane, primarily halffilled, with certain conductivity, and can be used as the flow channel of hydrocarbon resources.
The structural fractures are generally shear fractures caused by the shear stress under the action of structural stress.The unfilled, and half-filled structural fractures developed in the reservoir are the main flow channel of the hydrocarbon resources, and asphaltene can be seen on the fracture wall (Fig. 7a).In addition, the structural fracture calcite filled does not have conductivity (Fig. 7b).The structural fractures 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.
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.

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. 8), 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).

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. 9) 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.

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 T 7 4 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 (Figs. 10 and 11).

Core observation
According to the observation of reservoir cores in the S-1 strike-slip fault zone, natural fractures and dissolution caves are developed, and the dissolution caves are filled with asphaltene (Fig. 12a1), and calcite (Fig. 12a2).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 certain conductivity (Fig. 12c).The dissolution caves are mostly distributed along large natural fractures, and evident dilatation can 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 (Fig. 12b).

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. 13) 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 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.

Results and discussion
Based on dynamic analysis, core observation, imaging logging, and pressure build-up test curves of typical wells, the architecture difference of Lower Ordovician reservoirs in the S-1 and S-5 strike-slip fault zones in the Shunbei area is analyzed.
In the S-5 fault zone, a right-lateral left stepover strikeslip fault zone was developed on the T 7 4 interface, indicating that it was affected by NE direction compressive stress in episode III in the middle Caledonian period, because of the overall compression stress background, the compressional uplift phenomenon is more obvious on the profile.Although the S-1 fault zone is under the influence of local tensile stress overall, a left-lateral left stepover strike-slip fault zone has developed on the T 7 4 interface, indicating that the S-1 fault zone is a tension torsion stress fault active zone under the background of compression torsion stress.On the profile, it is shown as a falling graben or negative flower structure style.
Depand on the cores and imaging logging interpretation results, it can be seen that suture lines, interlaminar fractures, and high-angle structural fractures are developed in the reservoir rocks of the S-5 fault zone, and fewer dissolution caves are developed.The fault-fracture system with a small internal space is the major space for reservoir fluid storage and migration.A large number of suture lines, interlaminar fractures, and high-angle structural fractures are also developed in the reservoir rocks of the S-1 fault zone.Numerous of dissolution caves will asphaltene filled, indicating that the fault-fracture-cave system with large internal space is the central reservoir space and fluid flow channel in the reservoir of the S-1 fault zone.
In terms of the analysis of typical pressure build-up well test curves, there are mainly a few fractures or faults as the major fluid flow channels in the wells of the S-5 fault zone, and there may be a few tiny caves with meager fluid supply capacities in the reservoir.There are multiple oil supply channels in the wells of the S-1 fault zone, and the reservoir space is primarily fault-fracture-cave systems with large internal space.The reservoir is large in scale and has sufficient fluid supply capacity.It reveals that the reservoir architecture in the S-1 fault zone is basically consistent, and they are all reservoirs of inner caves and outer fractures.
The strata deposited in the Ordovician era in the Shunbei region were dislocated by strike-slip faults in the later period.It is inferred that the strata were still exposed on the surface or located on the shallow surface at that time.The fault zone is reformed by water-rock interaction, but this kind of transformation is uneven, the influence is small in some parts, and the original fault plane and fault core are retained.When the water-rock interaction is further strengthened, the underground fluid interacts with the soluble carbonate rock along the fault zone, and dissolution caves are developed along the high permeability fault fracture.Tensile stress affects the S-1 fault zone, while compressive stress impacts the S-5 fault zone.Therefore, the dissolution caves along the fault-fracture system in the S-1 fault zone are more developed, providing good conditions for later hydrocarbon resource filling.With the decline of the crust, compaction and cementation become the main functions in the process of diagenesis and transformation.Some large karst caves are crushed under the compaction, and the broken rock mass is reconnected under the cementation.Ultimately, the preserved karst caves and fractures become  (3) The S-1 strike-slip fault zone's reservoir architecture is a fault-fracture-cave system with a large reservoir scale and powerful energy, whereas the S-5 strike-slip fault zone's reservoir architecture is a fault-fracture system with a limited reservoir scale and weak energy.( 4) During the development of extra-deep strike-slip faultcontrolled reservoirs, the strike-slip fault zone under the background of tensile stress, such as the S-1 strikeslip fault zone, should be selected as the drilling target.It is of great significance for the efficient development of such reservoirs.

Fig. 2
Fig. 2 Schematic diagram of strike-slip fault stress analysis and secondary structure

Fig. 9
Fig. 9 Well test curve of wells in the S-5 strike-slip fault zone.a Well test curve of well S5-15.b Well test curve of well S5-9.c Well test curve of well S5-10.d Well test curve of well S5-1x

Fig. 10
Fig. 10 The seismic Profile of Well S1

Fig. 13
Fig. 13 Well test curve of wells in the S-1 strike-slip fault zone.a Well test curve of well S1-11.b Well test curve of well S1-12.c Well test curve of well S1-13.d Well test curve of well S1-14

Conclusions ( 1 )
There are numerous strike-slip faults, natural fractures, and dissolution caves developed in the Middle and Lower Ordovician reservoirs in the Shunbei area, which are the main storage spaces and flow channels of reservoir fluids.(2)The S-5 strike-slip fault zone was affected by NE direction compressive stress in episode III in the middle Caledonian period, and a right-lateral left stepover strikeslip fault zone was developed on the T 7 4 interface, the compressional uplifts are obvious on the profile.To the contrary, the S-1 strike-slip fault zone is a tension torsion stress fault active zone under the background of compression torsion stress.As a result, the profile clearly displays the falling graben or negative flower structural style.

Fig. 14
Fig. 14 Reservoir architecture Pattern of S-5 Fault Zone