Ordos basin is sandwiched between Qinling and Yinshan belt structure system in the north and south, with Baiyun Obo Mesoproterozoic aulacogen in the north and Qinling belt structure in the south. Early Paleozoic had a great influence on Ordos basin. This area has been in a relatively stable state since it was closed in the late Mesoproterozoic, and it exists in the form of ancient land. After Caledonian movement in Ordos Basin, the opening and closing center of Qinling banded structural belt moved southward, which weakened the direct influence on Ordos Basin and only rose during the formation of Weihe Basin in Late Cenozoic (Deng Naigong et al., 1996). The east side of Ordos Basin is the north-south structural belt of Lvliang uplift, which is the direct cause of passive uplift in the east side of Ordos Basin (Fan Tailiang et al., 1998), and the west side is the north-south structural belt of Liupanshan and Helanshan, with obviously stronger structural deformation than the east.
Ordos basin is located in the inner side of the "mountain" front arc of Qilu Helan, and is the backbone of the eastern shield structure. At the same time, it overlapped with a folded basin of Cathaysian system and was swept by Neocathaysian system. The Neocathaysian is distributed in the western margin of the basin, dominated by NNE faults, which control the development of Yinchuan fault depression. This is a superimposed structure. The Neocathaysian system is stably distributed in the NNE structural block of the basin, and two groups of checkerboard faults are developed. Ordos basin is a compound basin controlled by many systems.
20 1 1 There are 7 survey routes for field geological structural deformation (Figure 2- 13), namely: ① Baiyunebo-Baotou-Ordos; ② Siziwangqi-Hohhot-Yin Shan; ③ Jiexiu-Lishi-Suide-Yulin; ④ Chenggu-Foping-Zhouzhi; ⑤ Tianshui-Qingshui-Ankou; ⑥ Xiji-Guyuan; ⑦ Shitanjing-Shizuishan-Tao Le route. There are 70 observation points of structural deformation, and the structural elements such as fold deformation pattern, slip vector of fault scratches, vein intersection and conjugate joints are collected in the field. Through field survey, indoor preliminary data processing and inversion of tectonic stress field, combined with regional tectonic evolution, the following understandings are obtained:
1) Late Paleozoic and Mesozoic strata are exposed in Shikanjing-Shizuishan-Tao Le area on the northwest edge of Ordos Basin (Figure 2- 13). The outcrop reveals the combination and superposition of the banded tectonic belt and the NE-trending Cathaysian tectonic belt, and a series of near east-west folds were formed in the early stage (Figure 2- 14). After NW-SE compression, the late NE-trending folds were superimposed (Figure 2), and the tectonic stress field of NE-SW compression can also be identified.
Fig. 2- 14a is the observation point of EP00 1, with the coordinate position of N39°05' 26 "and e106 25' 28". Gray-black shale and purplish red mudstone are exposed in the timely sandstone of Jingyuan Formation of Middle Carboniferous. Due to the difference of rock properties, shale and mudstone constitute structural detachment layer, sandstone and shale. Figure 2- 14a shows the state of tectonic stress field obtained by inversion of slip vector of fault scratches. The three stress axes are σ 1 18/40, σ 2274/ 16, σ 3 167/45 respectively, indicating near-SN extrusion. Fig. 2- 14b is the observation point of EP007, and the coordinate positions are N39 16' 14 "and E 106 18' 59". Black carbonaceous shale and coal seam are exposed in thick timely sandstone of Jingyuan Formation of Middle Carboniferous, and the development hub strikes nearly east-west.
Conjugate joints inversion of EP002, EP004, EP008, EP0 1 and EP 01outcrops shows that the paleotectonic stress field in this period is NW-SE compression (or NWW- SW compression) (Figure 2- 16), and the latest stratum involved in this period of fold deformation is Middle Jurassic.
Observation point EP0 10, coordinate position: N39 03'14 ",E106 05' 21",exposed lower Jurassic sandstone, located in the southeast wing of middle Jurassic syncline in Rujigou coal mine area. The three stress axes of fault scratch sliding vector inversion are σ. σ 3 1 15/ 1 indicates that the paleotectonic stress field is NE-SW compression (Figure 2- 17), which is later than NW-SE compression tectonic stress field. The outcrop observation points EP00 1, EP004, EP008 and EP0 10 have obvious structural responses to the stress field in this period (Figure 2- 16).
Figure 2- 13 Field Geological Route and Observation Point Bitmap of Ordos Basin
2) On the routes of Baiyunebo-Baotou-Ordos and Siziwangqi-Hohhot-Yin Shan in the north of Ordos Basin, there are mainly Yinshan banded structural belts and structural deformation in the northern margin of the basin. The adjacent orogenic belts in the north are dominated by pre-Sinian, Cambrian-Lower Silurian, Jurassic and Hercynian intrusive rocks, and Yanshanian intrusive rocks are developed near Baotou and Hohhot in the south. Jurassic and Cretaceous strata are exposed in the northern margin of the basin (Figure 2- 13). On the observation route, the general feature is that the early northern part is dominated by strong NE-SW ductile shear deformation (Figure 2- 18), and the foliation and foliation tend to NW with an inclination of 50 ~ 80. The fold deformation into the basin is not strong. At observation point EP0 15, Proterozoic Zaertai schist is exposed, and the strata are subjected to strong shear deformation and felsic veins are subjected to strong shear tension, resulting in NE-SW fold in hinge direction. On the foliation surface, NW-trending biotite tensile lineation develops, and the lateral dip angle is about 10, which is the same as EP0 12.
Fig. 2- 14 nearly east-west folds are formed in the latitudinal structure of Tao Le-Shizuishan-Carboniferous well route.
Fig. 2- 15 NNE fold deformation of Tao Le-Shizuishan-Carboniferous well route.
Inversion of scratches along the line can identify the four-stage tectonic stress field (Figure 2- 19). Sandstone and mudstone of Upper Jurassic Daqingshan Formation are exposed at observation point EP023, and the coordinate positions are n411'39 "and e 1 1 42' 44", with two overlapping scratches, NW-SE-SS in the early stage. Observation point EP0 18, coordinate position: N40 52' 22 ",E10 05'10", exposed Archaean strata, and the structural stress field reversed by scratches is early NEE—SWW compression and late NE-SW extension; Observation point EP03 1, coordinate position: N39 37 ′ 59 ″, E112 38 ′18 ″, located in the northeast of Ordos Basin, where the upper Carboniferous limestone is exposed, and the tectonic stress field is squeezed in NW-SE direction. The difference of three-point tectonic stress field is the result of tectonic action in different areas around the basin, and it is the result of composite superposition of banded structural belt, north-south structural belt and Cathaysian structure.
This route is EP02 1 in Ordos Basin, and its coordinate positions are N39 47' 01"and E10/KLOC-0' 37". Typical burnt rocks are developed in the top stratum of Zhiluo Formation in Middle Jurassic, and Cretaceous argillaceous siltstone is parallel and unconformity with Jurassic stratum. Among them, Jurassic siltstone is white weathered, with interlayer coal lines developed, and Jurassic ancient oil reservoirs are exposed in gullies striking SN (Figure 2-20). A large amount of black asphalt is produced between sandstone layers, and the asphalt migrates along interlayer cracks and upward along a set of fault systems controlled by conjugate joints. The occurrence of conjugate joints is 174/54 and 10. The conjugate joints occurrence of overlying Cretaceous sandstone mudstone is 180/88 and 98/82, and the calculated tectonic stress field is near north-south compression (stress field at point EP02 1-2 in Figure 2-20). This shows that the ancient oil reservoir experienced the process that oil and gas migrated along the east-west direction and volatilized upward from the river channel driven by the compressive tectonic stress field in the near north-south direction.
Study on oil control function of tectonic system in Ordos basin
Figure 2- 17 NE-SW compressive stress field of middle Jurassic sandstone
3) The deformation of fold structure in the east of Ordos Basin is weaker than that in the north, which is mainly characterized by SN- trending structural belt formed by near east-west extrusion, and most of them are characterized by wide and gentle folds, including: ① Southern anticline belt, namely Daning-Jixian anticline belt and Shilou-Daning anticline belt, with the fold axis of NE 10 ~ 40, and the strata involved are Triassic sandstone; ② The monoclinal structure in the north is most developed in Baode-Haizemiao area, with the structural strike of 5 ~ 30 northeast, involved in Triassic strata, extending to the west or northwest, with an inclination of several degrees to more than ten degrees. From south to north, the intensity and scale of fold deformation in the eastern margin of the basin gradually weakened, but the structural characteristics remained at Ne 10 ~ 30 (Wang Xiyong et al., 20 10).
Fig. 2- 18 shear folds in gneiss of zhaertai group in northern Ordos basin
Fig. 2- 19 inversion of tectonic stress field data in the northern and eastern margins of Ordos basin
The solid black arrow indicates the extrusion stress; Arrows that are not filled indicate tensile stress.
4) The main fault in the eastern margin of the basin is Lishi fault zone, which is located between Ordos Basin and Shanxi Uplift, with nearly N-S and N-NE strike, with obvious segmentation characteristics, and develops from north to south in turn: Fault section I is arranged in a wild line; Tracing the fault section (Ⅱ); Buried fault zone (Ⅲ); Dense thrust fault section (Ⅳ); Crushing section (V). Among them, the activity in the northern segment began at the end of late Archean, characterized by the fracture of Si-Mg layer, and the activity in the southern segment began in the Middle Triassic, which was the fracture of Si-Al layer. In the Late Triassic, the north and south were connected as a whole, and this fault was not a lithospheric fault, but a crustal fault (Bai Yubao et al.,1996; Figure 2-2 1).
Fig. 2-20 Burnt rocks, ancient reservoirs and oil and gas migration in sandstone of Zhiluo Formation of Middle-Upper Jurassic.
Fig. 2-2 1 Lishi fault distribution map
(According to Bai Yubao et al., 1996)
Point EP047 of Jiexiu-Lishi-Yulin line, with the coordinate position of N37 26 ′ 40 ″ and E10 43 ′ 09 ″. Wide and gentle east-west compressional folds are developed in the interbedded sandstone and shale of Lower Triassic Liujiagou Formation, indicating the near east-west compressive stress field, while the inversion tectonic stress field in conjugate joints is northeast. EP044 point, coordinate position: N3714 ′ 59 ″, E112 ′12 ″, a series of knee-folded structures are developed in the interbedded limestone and shale of the Zhangxia Formation of the Middle Cambrian, showing a "B" shape. The observation point EP033-EP042 is located in the Paleozoic-Mesozoic strata of Luliang Mountain in the eastern margin of the basin. Inversion of conjugate joints and fault scratches shows that the near east-west compressive tectonic stress field mainly develops along this line, and near east-west extension develops in the later period (Figure 2-23), which may be the product of late Mesozoic tectonic extension.
Wang Xiyong et al. (20 10) suggested that the Indosinian movement in the eastern margin of the basin had a relatively weak influence on the eastern structure, and was influenced by the collision between the Yangtze plate and the North China plate, forming a nearly north-south compressive stress field in this area; Affected by the remote tectonic effect caused by the subduction of the ancient Pacific plate and the Asian continent, the tectonic stress field in Yanshan period showed NW-SE or near EW compression (Liao Changzhen et al., 2007). During the Himalayan movement, the compression direction of the eastern margin of the basin changed to NE-SW direction, which was mainly driven by the collision between the Indian plate and the Eurasian plate and the remote action of intracontinental subduction after the collision. Liao Changzhen et al. (2007) obtained the NW-SE extensional tectonic stress field.
5) Cenozoic structural deformation in the central Ordos Basin, starting from point EP058 of Jingbian-Dingbian line, sandstone and mudstone of Lower Cretaceous He Huan Formation-Huachi Formation developed east-west folds (Figure 2-24), and sandstone-mudstone fold detachment layer was formed in the lower gypsum stratum, with the fold hinge strike of 95, lateral dip E and lateral dip less than10. It and observation points such as EP049, EP050, EP052, EP054, EP056, EP059 indicate near-North-South compression (Figure 2-25), which is the product of Himalayan structural revival deformation of the banded structural belt with 38 north latitude in the basin.
Figure 2-22 Wide and gentle fold deformation and knee fold structure in eastern Ordos.
Figure 2-23 Structural Stress Field Data of Jiexiu-Lishi-Yulin
6) On the Xiji-Guyuan section, the lower Cretaceous sandstone in the western margin of Ordos Basin is in angular unconformity contact with the Paleogene mudstone (Figure 2-26), and the Paleogene-Lower Cretaceous strata mainly develop two stages of tectonic stress fields, namely the early NW-SE compression and the late NE-SW compression (Figure 2-27), and the latest stratum involved in this stage is Paleogene sandstone. It shows that the tectonic activity in the western margin has been strong and obvious since Himalayan period, and the compressive activity has continued since Paleogene, which is the product of the structural deformation of Liupanshan-Helanshan SN- trending structural belt and NE-NNE-trending Neocathaysian period. On the whole, it is characterized by the development of large-scale west-dipping and east-dipping thrust faults, which are arranged in strips (Figure 2-28; Li, 2006).
Figure 2-24 Schematic Diagram of East-West Folds in Sandstone and Mudstone of Lower Cretaceous
Study on oil control function of tectonic system in Ordos basin
7) The structural deformation of the southern margin of Ordos Basin is controlled by the "mountain" structure of Qilu Helan and the banded structural belt of Qinling Mountains. The gneiss thrust shear fault of Lower Paleozoic Niutouhe Group is developed near the basin side, and the pivot direction of gneiss and compression-shear fold is NW (W)-SE (E) (Figure 2-29); Observation point EP067, coordinate position: N3441'06 "'06", E10558' 36 ",exposed Sinian phyllite and schist, with" B "fold, dextrally sheared to form a felsic pudding-like structure, with stretching lineation along the schist plane. In the later stage, the tectonic inversion occurred, the axial plane NEE dip developed on the section, and a series of knee folds with the pivot direction of 135 ~ 150 were the products of regional extension structure.
8) The western fold-thrust belt starts from Dengkou, Inner Mongolia in the north, passes through Table Mountain, north-central Helan Mountain, Yinchuan Plain, Hengshanbao-Majiatan area, Qinglong Mountain and Pingliang, and reaches Qianyang area in Shaanxi in the south. It is bounded on the west by the western foothills of Helan Mountain and Qingtongxia-Guyuan, and on the east by the eastern foothills of Zhuoshan Mountain, Majiatan-Liutiaojing and Huianbao-Shajingzi faults. It is about 640km long from north to south and 50 ~ 120 km wide from east to west. It spans four provinces (regions) including Inner Mongolia, Ningxia, Gansu and Shaanxi. The famous Wuhai Coalfield, Helanshan Coalfield, Rujigou Coalfield, Hengcheng Coalfield, Lingyan Coalfield, Weizhou Coalfield, tan shan Coalfield, Wangwa Coalfield, and some parts of Longdong Coalfield and Huanglong Coalfield are all located in this structural zone.
Figure 2-26 Structural deformation of Paleogene-Lower Cretaceous strata along Guyuan-Xiji in Ordos.
Study on oil control function of tectonic system in Ordos basin
Figure 2-28 Structural Profile of the Western Edge of Ordos Basin
(According to Li, 2006)
① Bayin Aobao fault; ②-Wuhushan fault; ③-Gandre East Foot Fault; ④ —— Fault on the east side of abutment.
Figure 2-29 Early Paleozoic structural deformation characteristics in the southern margin of Ordos Basin
The crystalline basement of this zone is basically consistent with the North China Platform, belonging to Archean and Early Proterozoic medium-deep metamorphic rock series. After Mesoproterozoic, it was in a depressed state for a long time, and its stratigraphic sequence was basically the same as that of Ordos Basin, but the types of sedimentary formations and subsidence amplitude were different. Tectonic activity and deformation are very strong, and magma intrusion can be seen in many places. It can be called the boundary of stratum, structure and landform between the east and the west of northern China, and it is also the abrupt change zone of geophysical field and crustal thickness. Tectonic movement is multi-stage, and the deformation modes are complex and diverse. Yanshan movement shaped the basic structural pattern characterized by compression and compression and torsion in this area. During the Himalayan movement, the Yinchuan fault depression and the Liupanshan arc structural belt were strongly reformed and superimposed, which made the structural image in this area more complicated.
This belt is composed of 10 large thrust faults with west dip and south thrust extending in the north-south direction, several large normal faults in the same direction and some large translational faults in the east-west direction (Figure 2-30). Among them, local structures are also quite developed, mainly anticlines, synclines, flexures, fault blocks and various faults.
The location, distribution and general trend of this belt are mainly controlled by Helan Mountain Trough and passive continental margin. Its main body is basically located in the steep slope area inclined to the west in the early Paleozoic or the east wing of the aulacogen. The total force source for the formation of this tectonic belt comes from the pushing of the Indian plate in the NE direction, but the specific force acting on this tectonic belt is different in the north and south. Under the action of general force source, the wedge-shaped Alashan block moved eastward under the compression of Qilian fold and Inner Mongolia-Xing 'an fold, thus causing nappe deformation in this belt. The area south of Qingtongxia is directly affected by the Qilian Mountain fold belt under the action of the total force source. The deformation degree and characteristics of different parts of the belt depend not only on the size and mode of action, but also on the diversity, complexity and distribution of strain strata.
Figure 2-30 Pattern Diagram of Western Edge Thrust Nappe Structural Belt
(According to Guo, 1990)
Although there are many main thrust faults in this belt, none of them runs through the whole area. They are mainly in groups of three or five, parallel to each other, and appear at the same position at roughly equal distance. Segments are often separated by large-scale east-west translation faults, which generally have the nature of thrusting southward and sliding rightward, so that the difference of eastward movement speed of segments is adjusted. According to the data of ground exploration and seismic exploration, the profile of main faults tends to be steep on the top and gentle on the bottom, and the underground usually slides horizontally along the Carboniferous-Permian coal measures strata for a long distance, and the sliding distance is generally 2 ~ 5 km, and the maximum can be about 15 km. Accordingly, many researchers (Guo et al.,1986; Yang Junjie et al.,1987; Chen Fajing et al.,1987; Tang Xiyuan et al., 1988) identified it as thrust nappe structure or thrust structure. Its structural modes are shown in Figure 2-30, Figure 2-3 1 and Figure 2-32. According to its wide distribution and many nappes, it can be considered that it is not only a thrust nappe belt or fold thrust belt, but also a giant belt. The appearance of this belt has made the Carboniferous-Permian and Jurassic coal measures in the western margin of Ordos Basin undergo a strong transformation, which is unique in the whole basin. First of all, it destroyed the integrity of coal-bearing strata, so that the coal-bearing strata in a large range were completely weathered and denuded, and the complete coal-bearing basin was decomposed into many discontinuous isolated bodies; Secondly, the original occurrence state of coal-bearing rock series and coal seam has been changed, and its occurrence and buried depth have changed significantly; Thirdly, due to the abnormal development of faults in this area, the coal seam is cut into many blocks of different sizes and shapes, which is not conducive to coal seam mining. Although due to the uplift of folds and faults, local coal seams have changed from unilateral exposure to multilateral exposure, resulting in shallow burial and increasing the scope of development and utilization, it is insignificant compared with the damage caused to coalfields. According to the characteristics of structural development and its influence on coalfield, the structural belt is divided into five sections.
Figure 2-3 1 Bayin Aobao-Table Mountain Structural Profile
(According to Tang Xiyuan 1992)