The Cretaceous Bashijiqike Formation in the Kelasu structural belt of Kuqa Depression is an ultradeep reservoir with burial depth of more than 6 km. Due to the influence of strong thrust nappe from the South Tianshan in the north, the burial depth gradually deepened from north to south in the Kelasu tectonic belt. With the increase of the burial depth, the influence of tectonic compression relatively weakens, and the formation mechanism that affects the microstructure of ultradeep clastic reservoir is changed. Comprehensive analysis of thin section and quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN), scanning electron microscope (SEM), and micro-CT scanning are used to investigate the characteristics of the Cretaceous Bashijiqike Formation at the depth of 8000 m (TVD) in various stress sections. The result shows that, for the ultradeep reservoir with burial depth up to 8000 m, its reservoir characteristics and interlayer differences are not completely restricted by the neutral surface effect. (1) The porosity and permeability of various stress sections are different, and the reservoir physical properties of the tensile section are higher than that in the compressive section. (2) The pore types, pore radius, and pore throat connectivity of various stress sections are different, and the development of pores and throats in the tensile section is better than that in the compressive section. (3) As an ultradeep clastic rock reservoir, the vertical fracture development characteristics of Well Bozi9 at the depth of 8000 m differ from that in Kela-Keshen area at the depth of 6000 m in the model of strain neutral surface. The interlayer difference in the microstructure of ultradeep clastic rock reservoir is clarified, which has a positive effect to evaluate and predict the distribution of favorable deep reservoir.
The Kelasu tectonic belt is characterized by superior petroleum geological conditions and a complete reservoir forming elements. This tectonic belt is the zone with the richest natural gas accumulation in the Kuqa Depression and the main gas source for the West–East Gas Pipeline. In recent years, Bozi gas reservoir of 900 billion cu. m. and other large-medium size gas reservoirs were discovered in the Bozi–Dabei section of the western part of the tectonic belt, and a second trillion cu. m. of large gas reservoir is being formed [1–4] (Figure 1). Well Bozi9 (TVD) was tested at a depth of 7600-7880 m to obtain high-yield industrial oil and gas flow, that is, the deepest clastic rock reservoir discovered in China. In recent years, significant progress has been made in deep and ultradeep clastic oil and gas exploration around the world. For example, in North Sea basin, a representative basin in Europe, the maximum burial depth of ultradeep clastic reservoir is 5.5 Km with porosity of 5%~35%; in Taranaki Basin, Paleogene Mangahewa Formation burial depth is 4.5~5.0 Km with porosity of 7%~15%; in the Paleogene Wilcox Formation in the deep ocean of Mexico Gulf, America, the burial depth of clastic reservoir is 4.0~7.0 Km with porosity of 2%~22% [5, 6].
Kuqa foreland thrust belt is located in front of the South Tianshan Mountains. Under the influence of strong tectonic compression in the late Cenozoic, contraction structures such as anticlines and faulted anticlines were widely developed . The fracture network system formed by tectonic compression and the phenomenon of dissolution expansion effectively improve the physical properties of deep reservoir and have a significant impact on the distribution and quality of reservoir [8–12]. Theoretically, every tectonic compression movement produces a fold neutral surface, and the folds of the Kelasu tectonic belt have the geological characteristics of the fold neutral surface [13, 14]. With the exploration, more and more wells have confirmed that the deep subsalt of Kela-Keshen area in Kelasu tectonic belt conforms to the classic Ramsay neutral surface model (Figure 2). Previous studies have analyzed the main controlling factors of various stress sections of the Cretaceous Bashijiqike Formation in Kela-Keshen area in combination with the neutral surface model and proposed that the reservoir has vertical stratification [13, 16–20]. However, the target interval of previous studies is mostly concentrated at approximately 6000 m. For the ultradeep reservoir with burial depth up to 8000 m, whether its reservoir characteristics and interlayer differences are still restricted by the neutral surface effect is rarely mentioned in current studies. Taking Well Bozi9 as an example, current study analyzes the response characteristics of different stress sections in ultradeep layers at depth of 7600 m based on micro-CT, SEM, and thin sections and studies the tectonic factors that restrict their differences. Understanding the control of tectonic compression on the vertical distribution of ultradeep clastic reservoir is significant for an in-depth understanding and prediction of the development of ultradeep reservoir.
2. Regional Geology
The Kelasu tectonic belt is located in the northern part of the Kuqa Depression in the Tarim Basin. It is the first row of thrust belts. From west to east, the belt is divided into the Awat, Bozi, Dabei, and Keshen sections, bounded by the Baicheng and Kalabei faults [2, 3]. The tectonic style of the Kelasu deep tectonic belt is mainly expressed as a wedge-shaped fault block clamped by the Kelasu and Baichengbei faults. Affected by the detachment surface, a series of thrust faults with the same tendency are developed in the wedge block, which clamped anticline, faulted anticline, and fault block structure, forming an imbricate thrust structure (Figure 3) and laying a good trap foundation for the reservoir of a large oil and gas province [21, 22]. The target interval of this study is the Cretaceous Bashijiqike Formation in Well Bozi9, Bozi block, and Kelasu tectonic belt, with burial depth of 7664–7880 m (the bottom boundary is not penetrated). Well Bozi9 does not belong to the folded region and has obvious craton properties.
Previous studies found that the tectonic principal stress was closely related to logging resistivity. The greater the principal stress becomes, the higher the corresponding resistivity value [23–25]. Porosity is one of the most important factors affecting resistivity ; it can also be seen from the porosity logging curve that have obvious segmentation from top to bottom. Based on the variation of the resistivity curve and porosity logging curve in the vertical direction (Figure 4), the Cretaceous Bashijiqike Formation of Well Bozi9 is divided into three stress sections from top to down: tensile stress section, transition section, and compressive stress section.
3. Samples and Methods
The anticline above the neutral surface is affected by tensile stress and belongs to the tensile stress section. The anticline below the neutral surface is affected by compression stress and belongs to the compressive stress section [27, 28]. The two core samples collected in this test were selected from the tensile stress section (depth of 7688.95 m) and the compressive stress section (depth of 7793.66 m) of Bashijiqike Formation in Well Bozi9 (the stars are sampling depth in Figure 4). The two representative samples are both brown medium sandstones, which are well sorted and have a low matrix content. Therefore, the influence of sedimentary factors such as particle size, sorting, and matrix content on the reservoir is eliminated or weakened.
The samples were scanned with different scales and different resolutions using QEMSCAN, SEM, and micro-CT scanning analysis methods. First, the high-resolution micron-scale X-CT scanning was used to obtain a 3D grayscale image of a plunger with a diameter of 4 mm (pixel size 2 μm) (Figure 5(a)), and the image was binarized and segmented, with black as pores and white as particles (Figure 5(b)). The binarized image can be extracted from simplified networks of pores and throats with parametrized geometry and interconnectivity from images of the pore space (Figure 5(c)) [29, 30]. Then, SEM samples were prepared for high-resolution nanoscale MAPS (modular automated processing system) scanning (the pixel size was 250 nm and the scanning area was ) to obtain the 2D pore throat distribution characteristics, and the micron-scale QEMSCAN (pixel size 25 μm and scanning area ) was performed to obtain the composition, content, occurrence state, and particle size of minerals. Data processing and calculation of pore throat parameters were performed based on images obtained from different scan. Accordingly, we carried out the comparison and evaluation of microstructure difference in various stress section of ultradeep clastic reservoir.
The whole test analysis was accomplished in the Laboratory of Reservoir Microstructural Evolution and Digital Characterization of Yangtze University. In the test, Helios NanoLab 660 from Thermo Fisher Scientific was used as the SEM device. The voltage, current, and pixel size of recognition image and overlapping rate with adjacent small splicing image were 5~35 KV, 0.01~0.4 nA, 5~500 nm, and 6~8%, respectively; V|Tome|X S180 & 240 was used as micro-CT scanning device. The voltage, power, and pixel size of recognition image and sample size (sample diameter) were 0~240 KV, 1~10 W, 2~122 μm, and 1~260 mm, respectively; QEMSCAN 650F from Thermo Fisher Scientific was used to quantitate mineral composition. The voltage, current, and pixel size of recognition image and overlapping rate with adjacent small splicing image were 1~30 KV, 0.78 pA~26 nA, 0.5~50μm, and 6~8%, respectively.
4.1. Pore Evolution and Burial History of Bashijiqike Formation in Well Bozi9
The fan delta front subfacies and braided river delta front subfacies are developed in the Kuqa Depression Bashijiqike Formation from the bottom to top, and the sedimentary facies have almost no effect on the vertical differences of Well Bozi9 reservoir. In the early diagenesis stage (125-20 Ma) (Figure 6), the initial reservoir porosity ranged 34%~36%, and the reservoir remained in a shallow burial state for a long time corresponding to a relatively weak vertical compaction; in the middle diagenesis stage (20-5 Ma), a set of extremely thick gypsum salt layer was deposited in the overlying Paleogene strata, and the plastic flow of the gypsum salt rock could counteract a part of gravity and further delay vertical compaction; in the late diagenesis stage (5-0 Ma), the reservoir entered the stage of rapid deep burial but for a short period. Compaction and cementation are the main pore reduction effects in the rock reservoir procedure, with the pore reduction ranged from 27%~30%. Dissolution is the main pore enhancement effect, with the pore enhancement of 4.1%. The combination of weak compaction, medium cementation, and medium dissolution results in porosity that averages nearly 10% currently [4, 8].
4.2. Difference in Petrology Characteristic
The QEMSCAN result (Table 1) shows that the mineral composition of the tensile section and compressive section of Bashijiqike Formation in Well Bozi9 block is similar. The skeleton composition is mainly quartz and feldspar, and the difference in mineral composition content is small.
The observation result of thin section shows that differences existed in the contact relationship between the rock clastic particles in various stress sections (Figure 7). Particle contact relationship in the tensile section is point and point-line. Particle shape is complete and no obvious pressure solution phenomenon. Particle contact relationship in the compressive section is line.
4.3. Difference in Physical Property
Micro-CT scanning not only obtains the pore throat size and pore structure characteristics of tight sandstone but also calculates the porosity and permeability. The calculation result shows that the sample porosity of the tensile section is 8% and the permeability is 1.2 mD. The sample porosity of the compressive section is 3% and the permeability is 0.06 mD. According to the result of logging data, the sample porosity of the tensile section is 13.71% and the permeability is 13 mD. The sample porosity of the compressive section is 8.01% and the permeability is 1.59 mD. The reservoir physical properties of the tensile section are more favorable for hydrocarbon production compared to those of the compressive section.
4.4. Difference in Reservoir Space
4.4.1. Difference in Pore Structure
Micro-CT scanning was performed on samples of various stress sections (Table 2 and Figure 8). From the size distribution of pores and throats, we can see that the development of pores and throats in the tensile section is better than that in the compressive section. The distribution frequency of the coordination number of the samples in the compressive section is higher in the interval 1–2 than that in the tensile section, indicating that the pores of the samples in the compressive section are mostly isolated or microconnected. In the range 3-10, the distribution frequency of the coordination number of samples in the tensile section is significantly higher than that in the compressive section, which indicates that the pore throat connectivity of the samples in the tensile section is better than that in the compressive section.
4.4.2. Difference in Pore Type
Thin section and the result of MAPS scanning show that pores developed in the tensile section, with primary intergranular pores dominated (Figures 9(a) and 9(c)). The shape is mostly a triangle or an irregular polygon with smooth and straight edges, and the primary intergranular porosity ratio is 7%, accounting for 85% of the total pore types (it can be obtained by point calculation on thin section). Second, dissolution pores (Figures 10(a) and 10(b)) and a few microfractures exist in particles of brittle minerals such as quartz and feldspar. The pores in the compressive section are not developed, mainly intragranular dissolution pores and microfractures and a small amount of residual intergranular pores (Figures 9(b) and 9(d)). Dissolution pores (Figures 10(c) and 10(d)) and microfractures are developed in quartz, feldspar and other brittle mineral particles.
4.4.3. Difference in Fracture
Tectonic fractures are formed under the action of tectonic stress, so the occurrence and mechanical properties of tectonic fracture vary due to different tectonic stress fields [31, 32]. The thickness of Bashijiqike Formation drilled in Well Bozi9 is 216 m, and the development degree of vertical fracture has obvious segmentation characteristics from top to bottom. According to the interpretation result of image logging, the fracture in the tensile section is developed, mainly high-angle and near-upright fractures (Figure 11(a)), with a dip angle of 70°–90°. The linear intensity of tectonic fracture is 0.61 fractures/m, mainly open fractures and semiexpanded fractures. The open fractures tend to be ESE, and the direction is NNE-SSW. The semiopen fractures tend to be NNE and ESE, and the direction is NNE-SSW and NEE-SEE, arranged in parallel on the FMI imaging logging data; the fracture in the compressive section is not well developed and dominated by medium-high-angle fractures (Figure 11(b)) with a dip angle of generally 50°–70°. The linear intensity of tectonic fractures is 0.16 fractures/m. The fractures are mainly closed, the closed fractures tend to be north and south, and the direction is near the EW direction.
The result of SEM images shows that the brittle mineral particles in the tensile section develop particle-edge fractures and particle-penetration fractures that cut through particles (Figures 12(a) and 12(b)); the brittle mineral particles in the compressive section develop particle-edge fractures and intragranular crushing fractures (Figures 12(c) and 12(d)), with tensile lengths of micrometers and millimeters. The microfractures in the compressive section are often distributed around particles or the weak points of the intragranular stress, linear, and more common in geese pattern. As the opening is usually small, the contribution of microfractures to improve the permeability of the reservoir is limited, and the microfractures mainly connect small and medium size pores to expand the formation space [8, 33].
As an ultradeep reservoir, the vertical fracture development characteristics of Kela-Keshen area at depth of 6000 m differ from that in Well Bozi9 at depth of 8000 m in the model of strain neutral surface [16–20]. Take the fracture development of Bashijiqike Formation in Well Keshen208 as an example (Figure 13), the fracture in the upper tensile section (6563~6703 m) is well developed, mainly open, and semifilled isolated fractures. The linear intensity of fracture is mainly 2~10 fractures/m, the dip angle is mainly greater than 70°, and mainly high-angle fractures, and low- and medium-angle fractures are not developed; the fracture in the lower compressive section (6809~6871 m) is abnormally developed, and mainly filled network fractures. The linear intensity of fracture is mainly 5~20 fractures/m, and the dip angle is mainly 30°~50°, with medium-low angle fractures, and high-angle fractures are not developed .
The Kuqa foreland thrust belt developed an anticline structure under the influence of strong tectonic compression in the late Cenozoic. The reservoir of an anticline structure caused changes in the local stress field. There are different stress characteristics, reservoir physical properties, and reservoir space differences in various vertical stress sections.
5.1. Genesis of Difference in Pore Structure
The pore throat radius in the tensile section is larger than that in the compressive section, and the pore throat connection in the tensile section is well. For tight sandstone reservoir, the size and connectivity of pore throat are important factors that affect the physical properties of the reservoir [35–39]. The microscopic pore throat characteristics of the reservoir indicate that the local tensile stress above the neutral surface can offset or weaken the effect of tectonic compaction and protect the intergranular pores. At the same time, the tensile stress increases the distance between particles and increases the intergranular pores. The compressive stress below the neutral surface can further enhance the effect of tectonic compaction. The particles are strongly deformed by compression and present a mosaic contact relationship, which makes particles more compact and increases the degree of sandstone densification. Plastic deformation occurred in the cement, and at the same time, many areas in the SEM image show that the mica was plastically deformed by bending, and the pore space was greatly reduced [19, 20]. The overall compaction effect is different due to the different degrees of stress compensation, which makes the existing pore throat size and the connectivity of the pore throat in the tensile section better than that in the compressive section. The development characteristics of vertical pore throat in Well Bozi9 area are similar to that of Kela-Keshen area studied by previous researchers [16–20].
5.2. Genesis of Difference in Pore Type
The tensile section is dominated by primary intergranular pores, and the compressive section is dominated by intragranular dissolution pores and microfractures, leaving a small amount of residual intergranular pores. A small amount of tectonic stress in the compressive section does not directly act on the pore fluid of the reservoir but on the sediment particles. Its influence on the reservoir is manifested by increasing the compaction of the reservoir in the horizontal direction [40, 41]. In the compressive section, the total volume of the rock is reduced due to the continuous compression stress, and the clastic particles undergo rearrangement, plastic deformation, pressure solution, and brittle deformation processes, to achieve the tightly packed state with the lowest potential energy [41, 42]. In this process, the primary intergranular pores shrink and deform, and the porosity decreases. In the vertical direction, both the tensile section and compressive section are subjected to dissolution, but due to the relatively developed microfractures in the compressive section, unstable minerals such as albite and potash feldspar react along with the microfracture network under the action of acid fluids [43–45]. As a result, a large number of dissolution pores emerge, making the proportion of dissolution pores and microfractures in the total pores of the compressive section higher than that of the tensile section. The development characteristics of vertical pore types in Well Bozi9 are similar to that of Kela-Keshen area studied by previous researchers [16–20].
5.3. Genesis of Difference in Fracture
The fracture is not developed in Well Bozi9. In the vertical direction, the fracture in the tensile section is more developed than that in the compressive section. As an ultradeep reservoir, the vertical fracture development characteristics of Well Bozi9 at depth of 8000 m differ from that in Kela-Keshen area at depth of 6000 m in the model of strain neutral surface. The burial depth of the Kelasu tectonic belt in the Kuqa Depression gradually increases from north to south and can be divided into six rows of secondary tectonic belts, namely, Kelasu anticline, Keshen North zone, Keshen zone, Keshen South zone, Baicheng North district, and Baicheng district . The maximum horizontal principal stress in Bozi well area is mainly derived from the south Tianshan in the north. Well Bozi9 is located in the Baicheng district; compared with Kela-Keshen region, it is farther from the orogenic belts, and the intended burial depth is deeper, the tectonic activity is shorter, and the formation time is later. Strong compressive tectonic stress was offset and weakened by thrust anticlines and fault block structures formed in rows, and the structural deformation and lateral compression are weaker. The lateral tectonic compression stress of Well Bozi9 is 25 MPa, much lower than the 70 MPa of Kela-Keshen area in the northern strong stress zone [2, 47], so the structural fracture of Well Bozi9 is relatively undeveloped compared to Kela-Keshen area.
The difference between the microfractures in the tensile section and compressive sections of Well Bozi9 is the result of various stress properties. Under the condition of ductile deformation of the rock layer, the outside of the fold rock mass is stretched laterally and becomes thinner in the vertical direction. The distance between particles is opened up to form a large number of particle-edge fractures. As the tectonic compression continues, when the tensile stress exceeds the toughness of the rock, the outside of the neutral surface is stretched and fractured to form high-angle near-upright opening and semiopening fractures. Then, particle-penetration fractures cut through particles in some brittle mineral particles. The inside of the fold rock mass is thickened due to the vertical flattening to form medium-high-angle and closed fractures, and the inside of the brittle mineral particles is easily crushed to form crushing fractures [19, 20].
The microstructure of ultradeep clastic reservoir is different in various stress sections. Particle contact relationship in the tensile stress section is point and point-line. Particle shape is complete and no obvious pressure-dissolving phenomenon. Particle contact relationship in the compressive section is line; the porosity and permeability of the tensile section are higher than that of the compressive section, and the reservoir physical properties of the tensile section are significantly better than that of the compressive section; compared with the compressive section, the tensile stress section has larger pore throat radius and better pore throat connectivity; the fracture in various stress sections is also different. Macroscopically, fracture in the tensile section is more developed than that of the compressive section. Microscopically, brittle mineral particles in the tensile stress section develop particle-edge fractures and particle-penetration fractures that cut through particles. Brittle mineral particles in the compressive stress section develop particle-edge fractures and intragranular crushing fractures. As an ultradeep reservoir, the vertical fracture development characteristics of Well Bozi9 at depth of 8000 m differ from that in Kela-Keshen area at depth of 6000 m in the model of strain neutral surface, and Well Bozi9 reservoir characteristics and interlayer differences are not completely restricted by the neutral surface effect.
The difference in the stress section is the main factor that causes the difference between layers. In the tensile stress section, the tensile stress can offset or weaken the compaction effect and protect the intergranular pore space, making the pore structure well and the reservoir developed. In the compressive stress section, the compressive stress further enhances the tectonic compaction effect, which makes particles tightly arranged with poor reservoir performance. Various tectonic stress environments cause significant reservoir quality differences. Compared with the compressive stress section, the tensile stress section is a high-quality reservoir.
The stratification effect of tectonic stress in ultradeep clastic rock reservoir can still trigger the reservoir difference mechanism, which is useful for understanding the formation mechanism of deep clastic rock reservoir in the central and western superimposed basins developed under an active and strong tectonic background. The mechanism provides a significant informative reference.
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work is funded by the National Natural Science Foundation of China (42072121).