By restoring the point stress state, it is found that the middle principal stress axis σ2 is nearly vertical (σ2 dip angle is 80 ~ 90), and the maximum and minimum principal stress axes σ 1 and σ3 are nearly horizontal (σ 1, σ3 dip angle varies between 0 ~ 12). The horizontal lines σ 1 and σ3 are selected to make the stress trajectory diagram of each period to reflect the stress variation trend of each period in the whole region. According to the complex relationship between structural characteristics and dynamic source, dynamic action mode and its characteristics, movement mechanism, movement nature and dynamic action route, combined with regional structural analysis, the main structural evolution in this area is divided into three stages: Indosinian, Yanshan and Himalayan. On the contrary, three stages of paleotectonic stress field were formed. These studies are the basis for understanding the occurrence and development of gas-controlling structures and their networks, and also the direct basis for determining the current gas-conducting and gas-blocking structures.
The first stage-Indosinian period
As can be seen from Figure 5.38, the maximum principal stress σ 1 acts in the near north-south direction, and the average orientation of σ 1 in the whole area is NE8. The average orientation of the minimum principal stress σ3 in the whole region is N 100. σ 1 is in the north-south direction at the junction of the north and south of the North Zone. In the south and south of the northern region, σ 1 slightly deflects to NNE- south-south-west direction (σ 1 in the southern region is NE 10, and NE8 in the southern region). From the perspective of the whole region, the distribution of stress field in this period is relatively uniform, and there is no obvious phenomenon of stress concentration and dispersion in the whole region.
Fig. 5.38 Principal Stress Trajectory in Indosinian Period
1- mining boundary line; 2— maximum principal stress trace; 3— minimum principal stress trace; 4— Formation compression direction; Five-point stress state
From the dynamic point of view, the above principal stress comes from the strong pushing force of Ordos block from south to north caused by the final closure of Qinling geosyncline and the northward pushing of South China block. The dynamic action mode is mainly squeezing, and the dynamic action route is north-south direction. Therefore, a series of east-west compressive structures and north-south tensile structures have emerged. The nearly east-west fold equilibrium structure in this area may have been formed during this period. Field joint data show that two sets of * * * yoke shear joints were formed during this period. All laid a certain foundation for the later structural development.
5.3.2 The second stage-Yanshanian period
The main characteristics of the tectonic stress field in this period are that the maximum principal stress σ 1 is southeast-northwest, with an average of N307, and the minimum principal stress σ3 is northeast-southwest, with an average of NE38. The stress distribution in the whole region is uniform, the phenomenon of stress concentration and dispersion is not obvious, and the orientation of the principal stress axis is stable. σ 1 in the southern region acts on N295 and σ 1 in the northern region acts on N306. However, in the north-south border area, the stress distribution is obviously uneven From north to south, the stress first dispersed and then concentrated, and the direction of principal stress changed greatly. Generally σ 1 acts on the N320 line (Figure 5.39).
Fig. 5.39 Principal Stress Trajectory of Yanshan Period
1- mining boundary line; 2— maximum principal stress trace; 3— minimum principal stress trace; 4— Formation compression direction; Five-point stress state
The dynamic feature of this period is that with the intensification of the influence of Kula-Pacific plate on Chinese mainland, the Ordos subsidence basin developed since Mesozoic was strongly squeezed and twisted laterally, which led to the uplift and disappearance of the original subsidence basin. The power source comes from the southeast, and the power action mode is still squeezing, but it is limited by the east edge of Ordos near the north-south border, making it turn left. In this way, due to the change of dynamic action route and motion properties, a series of structural characteristics characterized by left-handed compression and torsion have developed in this area. The F 1 normal fault and F2 reverse fault in this area should be compression-torsion reverse faults at this time. Affected by the stress in this period, the east-west and northeast structures developed in the original Indosinian period may be closed to varying degrees due to compression, and only the existing northwest structures will open due to the combination of tensile stress. At the same time, it is also possible to produce new NW-trending tensile cracks and * * * yoke shear joints near the east, west and north.
Tectonics in this period had the greatest influence on this area. In order to determine the mode, nature and route of tectonic dynamic action in coal seam and accurately confirm the existence of left-handed twisting action in this period, the author selected two sampling points in the north and south regions to collect directional coal samples, and measured the reflectivity value on the granular bodies without scoring lines in the normal vitrinite region with a microphotometer. At the same time, directional rock samples were collected at the edge of Xiangshan mine in fault zone F 1 for X-ray petrofabric analysis.
The measurement results of near 1600 reflectivity data show that:
The reflectivity of each sample in 1) region is obviously anisotropic, with the maximum value of 2.4 12, the minimum value of 1.247 and the median value of 1.707. There are both positive light indicator lines and negative light indicator lines, and they also have a good correspondence with the data measured in neighboring areas (Table 5. 12). Obviously, the vitrinite reflectance in this area has both positive and negative optical indexes. The results show that the reflectance of vitrinite in this area is biaxial, which is the result of lateral compressive stress.
Table 5. 12 List of Primary Reflectance and Dual Reflectance Values
2) The maximum value of photometric long axis orientation is 54, and the minimum value is 35. The average value is 44.5, which is basically consistent with the strike of NE-trending folds and faults in the area. The compressive stress direction of the paleostructure thus recovered is N 140, which is basically consistent with the compressive stress direction of the regional stress field in the late Yanshan period (Table 5. 13, Figure 5.40).
Table 5. 13 Comparison Table of Photometric Long Axis Orientation and Main Structural Strike
Fig. 5.40 Horizontal profile distribution of vitrinite, the optical index of coal seam in this area and adjacent mining areas.
3) The change of average reflectivity and maximum reflectivity corresponds to the difference of structural complexity in this area. The closer it is to the main fold axis, the greater its value, and the structural changes suffered here are relatively intense. The minimum value is located in the area where Sangshuping Mine is located, and the structure of this area is relatively simple. The total average reflectivity gradually increases from west to east. However, the influence of NE-trending structures generally weakens from east to west.
The above three points show that the dynamic action of Yanshan period was obviously reflected in the coal seam, so the coal seam structure was mainly formed in this period.
X-ray petrofabric analysis is a new method to study mineral lattice deformation on the basis of macro-structural deformation research. The principle can be simply described as taking a certain amount of rocks in the same direction to make powder crystals, then measuring the diffraction intensity of each surface network of minerals, and drawing the powder crystal curve directly by computer, which shows that the surface network of minerals has not changed, that is, minerals have no directionality. Secondly, directional slices were cut on the same sample for X-ray diffraction analysis. The diffraction intensity of the corresponding surface network on the obtained diagram is compared with that on the powder crystal diagram. Its value is close to 1 (generally, the difference is not more than 5%), indicating that minerals have no orientation, and vice versa. This directionality is generally caused by structural compression, shearing or compression and torsion. By comparing Figure 5.4 1 with Figure 5.42, it can be seen that there are obvious differences in the peak intensities of calcite and dolomite minerals in the F 1 fault zone, and the difference in the diffraction intensity ratio of the corresponding surface network is more than 5% (Table 5. 14). This shows that these two minerals have obvious or obvious directionality. Combined with the characteristics of a large number of scratches and joint structures developed on the fault plane, this directionality is the product of compression and torsion.
Fig. 5.4 1 Xiangshan gully F 1 X-ray diffraction curve of directional rock block on fault plane
Fig. 5.42 X-ray diffraction curve of directional rock slice at F 1 fault plane in Xiangshan gully mouth.
Table 5. 14 X-ray diffraction data table
Note: A is powder crystal; B is directional rock block; ① It has obvious directionality; ② It has obvious directionality.
The above analysis further shows that the dynamic action modes of tectonic deformation in this period are mainly compression and compression-torsion action.
5.3.3 The third stage-Himalayan period
It can be seen from the Himalayan principal stress trajectory reflected in Figure 5.43 that the main characteristics of the stress field in this period are: the maximum principal stress σ 1 turns to NNE-SW direction, with an average direction of NE38, and the minimum principal stress σ3 turns to N 130. During this period, the distribution of stress field in the whole region was uneven, mainly manifested in the obvious concentration of stress field in the north-south boundary and the obvious accumulation of σ 1 traces, which led to the complex structure here. As far as the whole region is concerned, the direction of σ 1 has little change, and the dominant direction is NE40, but it slightly deflects in the direction of NNE-South-South-West at the northern end of the north area, and the dominant direction is NE23 —N203. From the perspective of dynamics and kinematics, Himalayan tectonic evolution can be divided into Paleogene-Neogene and Quaternary.
Fig. 5.43 Himalayan stress trajectory map
1- mining boundary line; 2— maximum principal stress trace; 3— minimum principal stress trace; 4— Formation compression direction; Five-point stress state
Paleogene-Neogene
During this period, two major tectonic events occurred almost simultaneously between Eurasia plate, Pacific plate and India plate. One is that the Kula-Pacific plate, which initially moved to the northwest, turned to the northwest after the Kula plate disappeared to the north. The other is the collision between the Australia-India plate and the Eurasian plate, which prevented the counterclockwise rotation of the Eurasian plate. At the same time, the Indian plate continued to push northward after the collision, which produced a strong right-lateral compression and torsion on Chinese mainland. Huang and others also believe that the occurrence and development of a series of dextral extensional fault basins in North China and other places since Cenozoic and the appearance of modern earthquakes are closely related to this role. Other structural features also show that the Asian continent is moving from south to north relative to the Pacific plate.
The dynamic action mode of the first event is mainly compression, and the dynamic action route is from east to west. Weihe fault depression is the result of this dynamic action.
The second event has a far-reaching impact than the first. At this time, the compressive force parallel to the direction of Longmenshan is decomposed into a pair of left-handed torsional forces when it meets the Qinling fold belt, and a pair of right-handed torsional couples are formed at the southeast edge of Ordos. Under the action of this couple, the NE-trending structure is tensile, the NW-trending structure is compressive (the formation of coal seam folds is related to this period), while the EW-trending and NNE-trending structures are either compressive or tensile. However, considering that the stress direction in this area is NW-SE, the torsional compression effect in this area should not be the main one (Figure 5.44), but the extension along the near NW direction and the extension along the NW-SE direction begin to play a leading role. During this period, Fenhe Graben was formed in the adjacent area and connected with Wei Fen Graben System.
Fig. 5.44 Schematic diagram of regional stress field and local stress field in Neogene.
Quaternary in 5.3.3.2
Since the Quaternary, with the extensive extension of Wei Fen fault basin, some fault activities in this area are obvious, and seismic activities, landslides, water system changes and other phenomena are very common. It affects various structural features that have been formed in the early stage and causes them to crack to varying degrees. For example, F 1 fault, as a representative of active faults in Northeast China, cuts through and controls Quaternary deposits to varying degrees, and its upper wall Quaternary thickness is only 100m in Yumenkou area, but it can reach 400 m in Yingshan area; Satellite photos also clearly show that many water systems affected by F 1 have obvious dextral dislocation, and the NW-trending active faults on the upper wall of Hancheng County and F 1 fault are relatively developed, with a relative dislocation distance of about 2m in recent 3000 years and an annual activity rate of 0.
Geological conditions and occurrence regularity of coalbed methane in Hancheng mining area
This is basically consistent with the movement rate of the North China plate measured by Ma Xingyuan et al. From a regional perspective, the activity rate of NE and EW active faults may be higher than that of NW active faults. In this way, if the lower limit of Quaternary is put before 2Ma, the F 1 fault has been extending horizontally along the dip since Quaternary, and this distance is roughly equivalent to the plane combination width of the stepped fault group above the F 1 main section.
According to the occurrence of Shiniupo fault, the plane extension distance of Quaternary strata is about 0.55 meters, which is also very close to the average width of fractured cracks seen in limestone exposed areas such as Shangyukou, Xiyuanshan, Huazishan and Xibeizhuang in hancheng city.
There are few examples where the activity of east-west faults directly controls the Quaternary sedimentation in this area, but referring to the activity characteristics of east-west faults in the southwest neighborhood of this area, the activity amplitude of extensional faults in this area at this time can still be explained. Such as the Luqiao-Guanshan fault in the adjacent area, the thickness of Quaternary system in the upper wall is over 1, 200 m, and the footwall is about 600m m.. If compared with the activity range of NE active faults in the area, the activity range of EW structure may be larger than that of NE or NW.
Fig. 5.45 vertical crustal deformation rate map of Ordos and its surrounding areas (1955 ~ 1986)
Isokinetic unit: mm/year
(Research Group of Active Fault System around Ordos, State Seismological Bureau, 1988)
It can be seen that the main dynamic action mode in this period is tension, and the dynamic action route is northwest-southeast, followed by north-south direction and northeast-southwest direction derived from it. As a result, a series of nearly east-west and northeast normal faults and northwest transport faults were formed. Normal faults all descend in a step-like manner along the extension direction, and the transmission faults are caused by the difference in the expansion speed of normal faults in NE direction and EW direction, but they are mainly tensile or torsional. The multi-directional extension of this aspect continued until modern times. According to the recent geodetic survey results, the vertical crustal deformation rate in this area is 2 ~ 7 mm/a (Figure 5.45), and the geological profile of vertical crustal deformation in the adjacent area also shows great settlement (Figure 5.46), which are the direct basis of plane extension effect, and they also have a good corresponding relationship.
Fig. 5.46 Geological Profile of Suide -Xi 'an Vertical Deformation (1976 ~ 1986)
(Research Group of Active Fault System around Ordos, State Seismological Bureau, 1988)
To sum up, there are many groups of structural traces produced by multi-stage and multi-directional structural stress in the geological history of this area. According to their ultimate mechanical properties, NE-NE fracture structures should have tensile or torsional properties, near EW structures have tensile and torsional properties, NW structures have tensile and torsional properties, and other directions have different degrees of compression or compression-torsion properties. Since the Himalayan period, although a series of folds have been formed or inherited in the near east and northwest, they are all relatively wide and gentle, and tension-crack compound is an important structural feature along the axis and wings.
With the extensive activities of extensional structures in Wei Fen graben system, various structural features in the whole region have been affected and opened to varying degrees, especially the fault structures in the above groups. As far as the north-south direction is concerned, the south area is closer to the intersection of the extension system of Fenhe Graben and Weihe Graben, and the extension and opening function of fault structures is stronger than that of the north area.