Abstract
Introduction
The Ordos Basin is located on the North China Craton, and the basin developed during the Mesozoic on an upper basement of Palaeozoic sedimentary rocks, which are underlain by Precambrian crystalline basement rocks. The basin is a major producer of oil, gas, coal, and uranium (Chen et al., 2005; Deng et al., 2005; Wei and Wang, 2004). Subdivisions of the basin include the Western fold–thrust belt, Tianhuan depression, Shanbei slope, Jinxi fold belt, Yimeng uplift, and Weibei uplift, as shown in Figure 1. The Ordos Basin is known for its stability and represents a stable cratonic basin (Liu et al., 2006; Wu et al., 2009; Zhang et al., 2003). The periphery of the basin is surrounded by the Qinling, Helanshan–Liupanshan, Lüliangshan and Yinshan Mountains, which are relatively tectonically active. Therefore, faults and folds are present at the edge of the basin, and these areas have been strongly transformed. Thus, the basin is not a simple stable basin (Chen et al., 2006; Deng and You, 1985; Liu et al., 2006; Ren et al., 2000; Yang et al., 2006, 2008; Zhang et al., 2006, 2007). As exploration and development of oil, gas, coal, uranium and other energy minerals in the basin have intensified, an increasing number of studies have revealed that the basin has experienced obvious structural changes during its evolution–reconstruction process, and its continuous stability has been questioned. Stability and activity are relative, and the stable Ordos Basin undeniably contains relative activity (Di et al., 2003).
Subdivisions of the Ordos basin and location of the Yushuwan fault. Note: the blue rectangle represents the position of Figure 2.
Although a few scholars have studied hydrocarbon dissipation and the uplift and thermal evolution history of the northeastern Ordos Basin (Ding et al., 2011; Ma et al., 2006, 2007), few reports on faults and folds that are extremely close to areas of hydrocarbon exploration have been published. In addition, the boundaries of the Shanbei slope, Jinxi fold belt, and Yimeng uplift have always been unclear. For instance, is the northeastern part of the basin a stable slope? There have been doubts about whether the large number of gas fields discovered in the northern part of the basin can be found in the northeast. Furthermore, is there a structural control on the oil and gas distribution?
Fortunately, during the scientific investigation of the northeastern Ordos basin, the author newly discovered a northwest–southeast-oriented fault and a series of tectonic deformation structures. The fault is located in the Yushuwan area of Zhungeer, and the tectonic deformation zone is located on the southwestern side of the fault. According to the ground investigation, combined with aeromagnetic, gravity, and earthquake data and other indications, the Yushuwan fault may be a deep-cut basement fault or a deep fault, and it may have played an important role in the formation, evolution and transformation of the basin. The study of its structural characteristics, formation and evolution has important structural geological and petroleum geological significance.
Geological background
Characteristics of the Yushuwan fault
The Yushuwan fault is located on the northeastern margin of the Ordos Basin. It is at the junction of the Yimeng uplift, the Jinxi fold belt and the Shanbei slope, and it is mostly in the Zhungeer region of Inner Mongolia. The main strata exposed in the study area are Cambrian, Ordovician, Carboniferous, Permian, Triassic, Jurassic, Neogene and Quaternary strata from east to west, and the strata gradually decrease in age (Figure 2), with large-scale erosion of the eastern strata of the Ordos Basin (Wang, 2011). Silurian and Devonian strata are missing (Bao et al., 2019). On geological maps, this fault represents the linear boundary between the Palaeozoic and Mesozoic strata.
Geological map of the Yushuwan fault. Note: the triangle represents the sampling position, and the letter P and the thick black line represent the location of the strata structural profile of the Yushuwan fault and Figure 4.
In the field, the Yushuwan fault is northwest-oriented and is exposed along Hequ in Shanxi and Zhungeer in Inner Mongolia. The southeastern portion originates from Yushuwan in Longkou town, next to the Yellow River. It extends to the Hejialiang area to the northwest, and both ends are covered (Figure 2). The fault is a normal fault that is approximately 15 km in length and 1 km in width, with an extension direction of 320°, and a fault plane orientation of 230°∠50°–60°. The hanging wall strata are composed of P1
Outcrop characteristics of the Yushuwan fault. (a) Yushuwan fault; (b) high-angle strata affected by the Yushuwan fault. Note: O1
Measured strata structural profile of the Yushuwan fault. Note: O1
Peripheral geological structure of the Yushuwan fault
Based on the geological map of the northeastern Ordos Basin (Figure 2), from northeast to southwest and from east to west, the Ordovician, Carboniferous–Permian, Triassic and Jurassic rocks are exposed in turn and gradually become younger, indicating that the northeastern and eastern areas have been uplifted to greater extent than the southwestern and western areas, and that the denudation intensity weakens from east to west. The Mesozoic strata are mainly exposed in the southwestern part of the Yushuwan fault. The residual boundaries of the Lower Triassic Liujiagou Fm., the Heshanggou Fm., the Middle Triassic Zhifang Fm., and the Upper Triassic Yanchang Fm. are mainly exposed in the southwestern and northwestern parts of the Yushuwan fault, and the strata are missing on the northeastern side. The overlying Fuxian Fm. can be superimposed on the different layers of the Yanchang Fm. and the Zhifang Fm. The Yan'an Fm. has a wider sedimentary range. It can be superimposed on different layers, such as the Yanchang Fm., Heshanggou Fm. and Liujiagou Fm. A field geological survey and typical profiles of previous studies (Figure 5) show that the southwestern part of the Yushuwan fault and western part of the Yellow River experienced strong and wide tectonic deformation and transformation during the Indosinian period. These regions mainly underwent fold deformation and strong erosion of the Triassic strata and were covered by Jurassic strata with an unconformity, indicating that structural changes occurred mainly during the Late Triassic–Early Cretaceous period.
Peripheral structural deformations of the Yushuwan fault. The red star symbol represents the section position (A. from Zhao and Liu, 1990; B–G from Sun and Mao, 1978; I from Liu and Zhang, 1989; J–K from Qingshuihe Map, 1972).
In the Wuziwan–Hazhen–Dacha–Qingshui outcrops, the Fuxian Fm. and the Zhifang Fm. are contact across a high-angle unconformity (Figure 5(A) to (D)). Figure 5(A) shows only an unconformity between the Jurassic and Triassic strata; the Triassic strata contain a fold with an axial north‒south direction. In Figure 5(B), the Zhifang Fm. has dip angles of 50–65° and a fault and fold are present. Section C also has a fold structure, the axial direction is northwest‒southeast, and the dip angle is steep. In profile D, the Zhifang Fm. is wrinkled and deformed, and a small fault is present. The fold is oriented NW–SE, which is consistent with the Yushuwan fault, showing that the compressive stress features a NE–SW orientation. The common feature of these sections is that the underlying Triassic strata are deformed and have steep dip angles, while the overlying Jurassic strata have gentle dips.
In Shanjiamao–Gushan–Wenjiapan, the Fuxian Fm. has a low angle of unconformity in relation to the Zhifang Fm. or the Yanchang Fm. The Yan'an Fm. overlaps older strata (Figure 5(E) to (G)). In section E, the Yanchang Fm. contains a fault, which is obviously truncated and overlapped by the Fuxian Fm. In section F, the Yanchang Fm. has obvious erosional characteristics and has a broad synclinal feature.
In summary, the southwestern side of the Yushuwan fault is a tectonic deformation zone, which is approximately 23 km wide from the Yushuwan of Zhungeer to northern Fugu. After the deposition of the Late Triassic Yanchang Fm., tectonic changes triggered faulting, deformation and erosion in the Yanchang Fm. and the Zhifang Fm. The principal stress direction may be NE‒SW, and secondary N‒S stress has been superimposed. The deformation strength weakens from northeast to southwest. In addition, in the Qingshui area of Fugu, several high-angle normal faults with NW–SE strikes and NE or SW dip directions were found, and these faults form a graben and cut Triassic–Jurassic strata, indicating NNE–SSW-directed tensile stress (Zhang et al., 2006, 2007). The stress may be from the slow collision of the North China and Yangtze blocks. The authors also found a northwest-oriented high-angle normal fault cutting the Shanxi Fm. in Fugu (Figure 5(H)). The activity of the fault has a tendency to weaken from the Yellow River to the basin (Zheng et al., 2006).
As mentioned above, the southwestern side of the Yushuwan fault is a tectonic deformation zone, while the northeastern side is characterized by structural stability. On the northeastern side of the Yushuwan fault, the exposed strata are relatively old, and Carboniferous–Permian clastic rocks and Ordovician limestone are exposed from southwest to northeast. The formations are nearly flat lying, and the dip angle is generally less than 10°. For example, the Carboniferous–Permian clastic rocks exposed in Heidaigou in the middle of the Zhungeer coalfield, the Lower Ordovician Majiagou Fm. in Pianguan, and the Ordovician and Carboniferous–Permian strata exposed in Huoshaomao–Liujiata are relatively flat lying (Figure 5(1) to (K)), illustrating that the northeastern side of the Yushuwan fault is relatively stable and that tectonic activity is low. The tectonic deformation characteristics on both sides of the Yushuwan fault are completely different, indicating that the fault controls the southwestern tectonic deformation, and its segmentation causes the northeastern side of the fault to have independent tectonic evolution.
Samples and methods
In this paper, six sandstone samples were collected from both sides of the Yushuwan fault in the northeastern Ordos Basin. Fission track (FT) analysis of four apatite samples and four zircon samples was carried out at the Institute of High Energy Physics, Chinese Academy of Sciences. The results of 9 apatite samples were collected, and the sampling positions are shown in Figure 2. Samples beginning with GC were collected in Gucheng town, which is located on the southwestern side of the fault. These samples were collected from the greyish white‒red sandstone of the Lower Triassic Heshanggou Fm., with cross-bedding. GC–01, GC–02, and GC–03 were sampled from top to bottom. Samples beginning with LK were collected in Longkou town, which is located on the northeastern side of the fault. These samples were collected from the grey sandstone of the lower Permian Shanxi Fm., and the samples are LK–01, LK–04, and LK–06 from top to bottom.
Results and discussion
Fission track (FT) analysis
The fission track analytical results are shown in Table 1. Apatite and zircon fission track analyses were carried out on the GC samples from Gucheng (Figure 6).The median age was less than the stratigraphic age, and the apatite fission track length was less than the average length of the initial fission track of 16.3 μm. This result indicates that the AFT has undergone partial annealing or complete annealing after formation. GC–01–AFT has a test probability P(χ2) > 5% and a median age of 167 ± 14 Ma, which is the cooling age of the rebound of the unannealed zone after full annealing. The other GC samples, with test probability P(χ2) = 0 or <5%, are part of the mixed age group. The median age of GC–02–AFT is 154 ± 17 Ma, and the Gaussian fitting ages are 78 Ma and 189 Ma; the median age of GC–03–AFT is 130 ± 10 Ma, and the Gaussian fitting ages are 106 Ma and 156 Ma. The zircon fission track age analysis shows that the median age of GC–01–ZFT is 139 ± 11 Ma, and the Gaussian fitting ages are 118 Ma and 151 Ma; the median age of GC–02–ZFT is 131 ± 9 Ma, and the Gaussian fitting age is 128 Ma. A total of 8 age data points were obtained, including one 190 Ma date, three 150–170 Ma dates, three 110–130 Ma dates, and one 80 Ma date. Based on comprehensive statistics, the authors believe that the early tectonic rise of the southwestern side of the Yushuwan fault may have started at approximately 190 Ma, which is approximately the same as the zircon age of 188 Ma in the northeastern Shaanxi slope (Liu et al., 2006). The later uplift phases occurred at 150–170 Ma, 110–130 Ma, and 80 Ma. This staged tectonic uplift is consistent with the age range derived from the Mesozoic and Cenozoic tectonic thermal evolution history of the Ordos Basin (Chen et al., 2007; Ding et al., 2011; Gao et al., 2000; Liu et al., 2006; Ren et al., 2006a, 2006b; Zhao, 1996).
Apatite and zircon fission track mixed age analysis results. (a)-(c), (g)-(h) radial distribution plots of detrital zircon or apatite fission-track ages, X-axis denotes standard error; (d)-(f), (i)-(j), (l)-(q) composite probability density distribution and grain-age histograms for the detrital zircon or apatite fission-track (FT) ages; (k) locations of the collected samples. Note: “★” represents the sample locations according to Ding et al., 2011. See location of the FT sample in Figure 2 with the symbol “⊿”.
Data of apatite and zircon FT analysis from the northeastern ordos basin.
n = number of grains counted; Ns, Ni, Nd, number of tracks counted to determine the reported track densities; ρs, spontaneous track density; ρi, induced track density; ρd, dosimeter track density; P(χ2), chi–square probability that the single grain ages represent one population; L ± σ=mean confined horizontal track length ± standard deviation; age is given as median age. Data with “*” from Ding et al., 2011.
Similarly, the fission track analysis of the LK samples in Longkou town showed (Figure 6) that the median age of LK–06–AFT is 54 ± 17 Ma, with test probability P(χ2) = 0, and the Gaussian fitting ages are 20 Ma, 62 Ma and 111 Ma; LK–01–ZFT and LK–04–ZFT have median ages of 170 ± 26 Ma and 162 ± 11 Ma under P(χ2) > 5%, respectively.
Studies have shown that the northeastern side of the Yushuwan fault also experienced uplift phases at 160–170 Ma, 110 Ma, 60 Ma, and 20 Ma. Compared with the southwestern side of the fault, the northeastern side near the Lüliang Mountains experienced later uplift at of 60 Ma and 20 Ma, and research has shown that the Lüliang Mountains experienced at least four intermittent uplift phases at 58 Ma, 49–53 Ma, 38–43 Ma, and 26 Ma during the Cenozoic (Li, 2009); some researchers believe that a period of rapid uplift at 95 m/Ma and denudation cooling occurred in the eastern Ordos Basin, causing denudation of approximately 2000 m of strata (Ren, 1995; Zhao, 1996). Therefore, the authors believe that the last stage of uplift in the northeastern part of the fault may be related to the uplift of Lüliang Mountain.
In addition, a total of 28 apatite and zircon fission track data points were collected from the northeastern Ordos Basin, and these samples yielded one 190 Ma date, five 150–170 Ma dates, four 110–130 Ma dates, eleven 60–80 Ma dates, and seven 20–40 Ma dates. According these data, the northeastern Ordos Basin may have experienced uplift phases at 190 Ma, 150–170 Ma, 110–130 Ma, 60–80 Ma, and 20–40 Ma. The data measured by the authors are compared with Ding et al. (2011) fission track data. The uplift in the northeastern part of the Ordos basin may have occurred earlier than that in the eastern margin of the basin.
Geophysics
Based on direct observations and tracking along outcrops, the Yushuwan fault extends approximately 15 km. According to the trend of the fault from southeast to northwest, the fault extends 50 km. Whether the fault continues to extend farther can be determined only through the use of geophysical exploration data. The regional geophysical coverage is includes mostly gravity and magnetic exploration data.
On an aeromagnetic map, the boundaries of different magnetic field regions, such as a linear gradient zone, a beaded anomaly zone or a linear anomaly zone, are often reflections of a large basement fault (Zheng et al., 2006). The aeromagnetic anomalies in the northern Ordos Basin are complex. In the east‒west direction, northeast and northwest-trending basement faults were identified based on aeromagnetic anomalies. These faults were formed in different geologic periods (Ding, 2000; Jia et al., 1997). In the Zhungeer-Hequ-Wuzhai area, there is an obvious northwestward strip-like distribution (Figure 7), with a linear gradient zone. The aeromagnetic extent is 5 km (Figure 8), and the linear gradient zone is obvious. The Yushuwan fault passes through this zone, and the characteristics of the aeromagnetic anomalies on both sides of the fault are obviously different. The aeromagnetic anomalies on the southwestern side are clearly northeast-oriented strips, the magnetic anomaly contours are dense, and the values are high. On the northeastern side, the magnetic anomaly strips are not obvious, the magnetic anomaly contours are relatively smooth and sparse, and the values are low. These results show that the fault bounds the magnetic anomaly features on both sides, which may be because of the action of a deep fault. The obvious magnetic anomaly partitioning on both sides of the fault indicates that differences in the composition of the basement may be present, and the fault formed along a weak surface in the material. In the past, the eastern boundary of the Yimeng uplift was unclear. The different aeromagnetic features on both sides of the Yushuwan fault limit the extension of the high Yimeng uplift anomaly to the east, indicating that the fault was the boundary between different basement compositions, and likely represents the eastern boundary of the Yimeng uplift. On the gravity-normalized total gradient contour map (Figure 9), there is also a distinct linear gradient zone in Zhungeer–Hequ–Wuzhai, illustrating that the Yushuwan fault is a deep basement fault.
Aeromagnetic anomaly map of the northeastern Ordos Basin (By Ding, 2000, Revised).
Aeromagnetic extent in 5 km isoline map (From Li et al., 1998).
Normalized total gradient isoline map (From Li et al., 1998).
The aeromagnetic and gravity data indicate that the Yushuwan fault zone is a northwest-oriented deep fault, and its southeastern extent may reach Wuzhai. The northwestern part extends to the northwest of Zhungeer, resulting in an extent of approximately 150 km (Figure 5).
Since 2000, the Institute of Geology and Geophysics of the Chinese Academy of Sciences has implemented the “North China Internal Structure Plan (NCISP)”, which has deployed more than 200 broadband mobile seismic stations, one of which is located in the northern Ordos Basin. Based on a comprehensive study of the geology, geomorphology and pre-Palaeozoic basement depth through which the section passes, detailed deep structural information and crustal thickness variations were obtained from dense seismic arrays and fixed-network records (Chen et al., 2010). A comprehensive analysis of the corresponding cross-section (Figure 10) shows that the surface elevation of the Yimeng uplift is high in the west and low in the east, and the basement depth of the pre-Palaeozoic strata is characterized by a single oblique trend from east to west. The Moho is deep in the east and shallow in the west and is consistent with the topography. Notably, there is a sudden change in the thickness of the crust near Zhungeer. The location of the change coincides with the location of the northwestern Yushuwan fault at the surface, indicating that the fault is associated with an abrupt change in the thickness of the crust and the deep structure on the east and west sides. In the narrow section of the Yushuwan fault, the thickness of the crust increases by approximately 3.7 km.
Cross-section of the tectonic and geologic structure of the Yimeng uplift in northern Ordos basin (Moho depth data from Chen et al., 2010).
Our research suggests that there may be two reasons for the formation of this phenomenon: one reason is that there is a sudden change in the thickness of the crust, which should be caused by a deep fault, indicating that the fault has cut through the crust; second, there is no change in crustal thickness at this location. The obvious change in the thickness of the crust is due only to the relatively small seismic velocity in the fracture zone and the long propagation time. If a normal seismic velocity is used, the depth of the Moho surface is deeper. The two possible reasons, both indicate that the fracture is deep and cuts through the Earth's crust.
Estimation of the fault distances
Based on the pre-Palaeocene geological map of the area adjacent to the Yushuwan fault (Figure 11), the Carboniferous–Permian strata that are exposed on both sides of the fault strike to the northeast and dip to the northwest. The contemporaneous strata in the fault zone dip to the southwest, indicating that the fault affects the spatial variation in the strata. Field observations show that the strata in the nearby fault zone are steeply dipping and that the fractured zone is narrow, but tectonically generated rocks are not very developed. Horizontal or low-angle scratches are visible, and the fault extension along the strike of the fault zone is relatively straight, which is common for strike-slip faults. Points A and B on the geological map are the corresponding boundary points between the Liujiagou Fm. and the Shiqianfeng Fm., and points C and D represent the Taiyuan Fm. and the Shanxi Fm. on both sides of the fault, respectively. The distances between A and B and between C and D are approximately 10 km. According to the abovementioned structural characteristics and strata distribution, we believe that the fault has the characteristics of strike-slip motion, and the slip distance may be approximately 10 km.
Pre-Palaeogene geological map of the area adjacent to the Yushuwan fault. Points A and B on the geological map are the corresponding boundary points between the Liujiagou Fm. and the Shiqianfeng Fm., and points C and D represent the boundary points between the Taiyuan Fm. and the Shanxi Fm.
In summary, the data from aeromagnetic, gravity, and seismic array sounding analyses as well as from geological structures in outcrop, indicate that the Yushuwan fault is a northwest-striking crustal-scale deep basement fault, that experienced strike-slip motion during the main activity period.
The foregoing outline discusses the possible slip distance of the Yushuwan fault, and the vertical rupture that may occur along the fault is discussed. Currently, no other geophysical profiles pass through the Yushuwan fault. The authors attempt to estimate the vertical offset along the Yushuwan fault by comparing the existing strata thicknesses on both sides of the fault. Because the study area was part of a cratonic basin or residual cratonic basin during the Cambrian–Ordovician, Carboniferous–Permian and Triassic–Jurassic sedimentary periods, the sedimentary environment was relatively stable (Bai and Dai, 1996; Li et al., 1998), and the stratum thickness does not change significantly in the area.
On the northeastern side of the Yushuwan fault in Longkou town (Figure 12, Column B), the elevation of the top surface of the Lower Ordovician Majiagou Fm. limestone is approximately 1100 m, the overlying Carboniferous–Permian strata are basically eroded, and the residual strata thickness is approximately 0–10 m. The Benxi and Taiyuan Fms. have a cumulative residual thickness of approximately 50 m in the Yushuwan and Fangtagou areas of the Zhungeer coalfield, and these rocks are overlain by Pliocene strata.
Strata comparisons on both sides of the Yushuwan fault. The blue stars represent the locations of columns A, B and C.
On the southwestern side of the Yushuwan fault (Figure 12, Column A), the elevation of the deep trench corresponding to the boundary between the Liujiagou Fm. (T1
In the Heidaigou area of the Zhungeer coalfield (Figure 12, Column C), the elevation of the boundary between the Xiashihezi and Shanxi Fms. is 1070 m. The coal field data show that the Benxi–Shanxi Fms. have a total thickness of approximately 134 m. The unconformity between the lower and upper Palaeozoic strata may be 936 m above sea level, and decrease in the same unconformity surface at point B is approximately 124 m. However, the distance between points B and C is approximately 40 km, whereas point A is only 2 km away from point B, indicating that the drop in the upper and lower Palaeozoic unconformity between points A and B basically represents the displacement of the Yushuwan fault. In addition, it indicates a step exists in the northeastern Ordos basin due to the segmentation of the Yushuwan fault.
Significance for basin tectonics and hydrocarbon exploration
The Yushuwan deep fault zone is located in the northeastern Ordos Basin at the junction of the Yimeng uplift, the Shanbei slope, and the Jinxi fold belt. Its special location has important tectonic and petroleum geological significance.
Currently, the Yimeng uplift is spreading in the east‒west direction, and the tectonic pattern is characterized by high values in the north and east and low values in the south and west. Overall, it is a large southwestern monoclinic structure. It is connected to the Hetao grabens in the north, the Tianhuan depression in the west, the Shanbei slope in the south and the Jinxi fold belt in the east, and is bounded by the Zhuozishan fault in the west. Before Palaeozoic sedimentation, the Yimeng uplift was a southern convex arc-shaped uplift belt. The Palaeozoic strata in the basin exhibit thinning, overlap, and pinching out over the Yimeng uplift from south to north, and lower Palaeozoic strata are missing in the north. However, the eastern boundary of the Yimeng uplift is ambiguous and has been inconclusive for a long time. However, on the aeromagnetic anomaly map, the Yushuwan fault zone has an obvious aeromagnetic anomaly gradient zone, allowing the aeromagnetic anomaly to be clearly divided: the aeromagnetic anomaly on the southwestern side of the fault is a distinct northeastward strip, and the magnetic anomaly contours are dense, indicating a high-value area; the characteristics of the aeromagnetic anomaly strips on the northeastern side are not obvious, and the contours of the magnetic anomalies are relatively smooth and sparse, indicating a low-value area. The fault represents the eastern extent of the high magnetic anomaly zone in the Yimeng uplift area. This fault represents the eastern boundary of the Yimeng uplift and controlled the late Triassic structural deformation strength of the sides.
In the northeastern Ordos Basin, the exposed strata from the northeastern margin of the basin to the interior of the basin become younger, and these strata include Cambrian, Ordovician, Carboniferous, Permian, Triassic, Jurassic, and Cretaceous strata. The basin is known for its stability, and the strata in the basin are gently dipping. Therefore, the northeastern part of the Ordos Basin has long been considered a monoclinic structure that is inclined to the southwest. The deep basement-cutting Yushuwan fault has a vertical drop of at least 900 m, indicating that the northeastern part of the basin is not a simple monoclinic structure but rather a stepped structure.
The Yimeng uplift in the northern Ordos Basin has many oil seeps. Bleached sandstone is widely distributed in the Dongsheng area approximately 24 km southwest of the Yushuwan fault, and the large Dongsheng sandstone-type uranium deposits are closely spatially related. Most of the oil and gas in the Yimeng uplift is distributed in an interval of 60 m from the bottom of the Cretaceous strata, and the underlying strata are Carboniferous–Permian; there is no oil and gas display in the contact between the Cretaceous and the Jurassic or Triassic strata. The Cretaceous oil seeps and Permian crude oils are similar, and the oil is sourced from the Carboniferous–Permian system and is not related to the Triassic and Jurassic crude oils in the Ordos Basin and the Tertiary oils in the Hetao Depression (Liu, 1982). The existence of the Cretaceous oil seeps confirms that the natural gas in the upper Palaeozoic strata has been transported to the northern edge of the basin and lost. Bleached sandstone is widely distributed at the top of the Yan'an Fm. in the northeastern basin. The thickness of the bleached sandstone ranges from less than 1 m to more than 10 m, and the distribution area is more than 100 km2. The bleached sandstone is observed in outcrops and boreholes over a wider area than the area of the outcrops examined in this study (Ma et al., 2006). The Dongsheng sandstone-type uranium deposit is located in the eastern Yimeng uplift and is distributed in a discontinuous band in the east–west direction.
The spatial distribution of bleached sandstone and Cretaceous oil seeps has a good correspondence (Figure 13). Gas-producing wells contain bleached sandstone, and the gas comes from Carboniferous–lower Permian coal-bearing strata, which suggests that the bleached sandstone is closely related to gas derived from the upper Palaeozoic coal. The top bleached sandstone of the Yan'an Fm. is located in the lower part of the sandstone of the uranium ore body in the Zhiluo Fm. The two are in pseudointegrated contact. The fluid inclusion and calcite cement
Distribution of bleached sandstone, oil seeps and uranium deposits in the northern Ordos basin (modified after Liu et al., 2006).
The sandstone bleaching phenomenon was accompanied by the migration and escape of oil and gas during the late stage of basin reconstruction. The tectonic movement provides the force and pathways for the migration and escape of oil and gas. The oil and gas interacted with the sandstone layers they came into contact with during the migration process, and the sandstones were bleached to varying degrees. During the later stage of basin reconstruction, due to structural and other factors, the oil and gas migrated, and some natural gas escaped, which led to the bleaching of some sandstones. The existence of sandstone bleaching not only indicates that hydrocarbons were transported and dispersed but may also indicate the presence of oil and gas reservoirs in the vicinity (Ma et al., 2006).
The fault structure may have played an important role in the mineralization process of sandstone-type uranium deposits (He et al., 2003). Fault activity not only affects the sedimentary facies but also affects the deep to shallow migration of material and groundwater discharge, which have important impacts on the formation and location of sandstone-type uranium deposits. The late Palaeozoic terrain in the Ordos Basin was high in the north and low in the south, and the natural gas migrated from south to north. At the peak of gas generation during the Early Cretaceous (He et al., 2003; Ren et al., 2006a, 2007), the basin gradually transitioned from high in the east to low in the west, so natural gas migration occurred from west to east and from south to north (Jia et al., 2005; Wang et al., 1998; Wei and Wang, 2004). Teng et al. (2008) proposed a sedimentary depression controlled by the Daotu–Dongsheng fault, with a width of approximately 180 km and a sedimentary thickness of 4–6 km. The deep metamorphic rocks in the crystalline basement have a gentle change and are separated from surrounding blocks by two basement faults in the north and south. Therefore, this region may be a promising prospect for oil and gas exploration due to its good preservation and superior conditions for accumulation. Zhang et al. (2006, 2009) also proposed that there was strong mantle fluid activity in the Ordos Basin. The middle crust of the Dongsheng area in the Yimeng uplift has a low-velocity and high-conductivity layer, and the lower part of the uranium-bearing Dongsheng sandstone may have large oil and gas reservoirs. The exploration for natural gas in the middle Proterozoic strata indicates that the northern basin has good prospects for oil and gas exploration.
Past researchers thought that the northeastern part of the Ordos Basin was a monoclinic tectonic belt with a southwestern tilt, but the discovery of the Yushuwan fault indicates that the northeastern part of the basin is not a simple monoclinic belt but rather a step with a large drop. The formation of this step and relatively deep burial in the southwestern part resulted in the transport of natural gas from the basin's interior and its accumulation and enrichment in the northeastern part of the basin. Therefore, in the wider area to the southwest of the Yushuwan fault, there should be good natural gas resource potential and exploration prospects. Currently, in the shallow Jurassic Yan'an Fm. strata in the basin, sandstone “bleaching” caused by the migration of coal-derived natural gas with a high maturity is widely present, which proves that coal-derived gas in the upper Palaeozoic strata of the basin migrated to the area. Less natural gas has migrated to the northeastern part of the Yushuwan fault due to the blocking effect of the fault. In addition, this area is high in elevation, and the erosion is strong, as evidenced by the complete erosion of the Mesozoic strata and the lack of good preservation, so the basic conditions for natural gas occurrence and possible exploration potential are lacking.
There are abundant natural gas resources in the northern part of the basin, such as the Mizhi, Jingbian, Wushenqi, Yulin, and Sulige gas fields (He et al., 2003). It has long been believed that the northern basin is a slope from west to east, and the burial depth in the northeastern basin is generally shallow, which is not conducive to gas preservation, promoting instead gas escape (Ma Yanping et al., 2006). The Yushuwan fault has a vertical throw of at least 900 m, so the western side of the fault may be buried deeper, with the potential to preserve natural gas.
Conclusions
The Yushuwan fault in the northeastern Ordos Basin is a deep northwest-trending strike-slip basement fault (crustal scale) with length of approximately 150 km, a vertical fault displacement of approximately 900 m, a horizontal slip distance of approximately 10 km, and a deformation width on the southwestern side of the fault of approximately 23 km.
The apatite and zircon fission track analysis shows that the early uplift of the southwestern side of the Yushuwan fault may have started at approximately 190 Ma, and later tectonic uplift occurred at 150–170 Ma, 110–130 Ma, and 80 Ma. The northeastern side also experienced phases of uplift at 160–170 Ma, 110 Ma, 60 Ma, and 20 Ma. This uplift controlled the structural deformation strength on both sides of the fault.
The discovery of the Yushuwan fault indicates that the northeastern part of the basin is not a simple slope but rather a step with a large drop. In the area to the west of the Yushuwan fault, there should be good natural gas resource potential and exploration prospects.
On the aeromagnetic anomaly map, the contours of the magnetic anomalies on the southwestern side of the fault are high in value and dense in spacing, and the contours of the northeast magnetic anomalies are low in value and broad in spacing. The aeromagnetic characteristics on both sides of the Yushuwan fault are completely different, which determines the eastern boundary of the Yimeng uplift.
Footnotes
Acknowledgements
Author contributions
Declaration of conflicting interests
Funding
ORCID iD
