Petroleum Exploration and Development >

2023 , Vol. 50 >Issue 4: 865 - 877

DOI: https://doi.org/10.1016/S1876-3804(23)60434-3

Distribution, evolution and structural properties of Wushenqi paleo-uplift in Ordos Basin, NW China

  • MAO Danfeng 1 ,
  • HE Dengfa , 1, * ,
  • BAO Hongping 2 ,
  • WEI Liubin 2 ,
  • XU Yanhua 1 ,
  • CHENG Xiang 1 ,
  • GOU Junyi 1 ,
  • WU Saijun 3
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  • 1. School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China
  • 2. Research Institute of Exploration and Development under Changqing Oilfield Branch Company, PetroChina, Xi’an, Shaanxi 710018, China
  • 3. Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China
*E-mail:

Received date: 2023-03-06

  Revised date: 2023-05-10

  Online published: 2023-10-25

Supported by

The Major Science and Technology Project of PetroChina Changqing Oilfield Company(ZDZX2021-01)

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Abstract

This paper depicts the distribution of the Wushenqi paleo-uplift in the Ordos Basin by using the latest drilling and seismic data, and analyzes the tectonic evolution of the paleo-uplift with the support of Bischke curve and balanced section. The compressional Wushenqi paleo-uplift which developed in the Early Caledonian orogeny (Huaiyuan orogeny) is approximately a ellipse extending in S-N direction. Its long axis is about 194 km and short axis is about 55-94 km in nearly W-E direction. The denudation thickness and area of the Cambrian in the core are 170-196 m and 11 298 km2, respectively. It was mainly formed during the Huaiyuan orogeny according to the chronostratigraphic framework. It was in the embryonic stage in the Middle-Late Cambrian, denuded after developed obviously at the end of Late Cambrian. The paleo-uplift of the 3rd member of the Ordovician Majiagou Formation was reactivated and reduced in area. In the sedimentary period of the Ma 4 Member-pre-Carboniferous, the paleo-uplift experienced non-uniform uplift and denudation. It entered the stable period of burial and preservation in the Carboniferous and later period. The Wushenqi paleo-uplift was formed on the weak area of the basement and tectonic belts, into an compressional structure with irregular morphology, under the control of the non-coaxial compression in the south and north and the stress transmitted by the uplift in the basin. The Wushenqi paleo-uplift has a controlling effect on the sedimentary reservoirs and hydrocarbon accumulation.

Key words： Ordos Basin; Wushenqi paleo-uplift; Ordovician Majiagou Formation; unconformity; Caledonian orogeny; tectonic evolution

Cite this article

MAO Danfeng , HE Dengfa , BAO Hongping , WEI Liubin , XU Yanhua , CHENG Xiang , GOU Junyi , WU Saijun . Distribution, evolution and structural properties of Wushenqi paleo-uplift in Ordos Basin, NW China[J]. Petroleum Exploration and Development, 2023 , 50(4) : 865 -877 . DOI: 10.1016/S1876-3804(23)60434-3

Introduction

Most of the large-scale oil and gas enrichment areas discovered in craton basins are related to paleo-uplifts. For example, the central and northern Tarim oil and gas enrichment areas in the Tarim Basin, the Luliang uplift oil and gas enrichment area in the Junggar Basin, and the Weiyuan-Anyue large-scale gas enrichment area in the Sichuan Basin are all developed in and around paleo- uplifts [1⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓-14]. In the Ordos Basin, the early Paleozoic has been discovered with Qingyang paleo-uplift, Central paleo-uplift, Wushenqi paleo-uplift and Yimeng paleo-uplift. The discovery and geological knowledge of these paleo-uplifts provide important evidences for understanding the formation and evolution of the Ordos Basin. So far, many discoveries have been achieved through exploration in these paleo-uplifts [15⇓⇓⇓-19]. Typically, the Wushenqi paleo-uplift, was formed during the Huaiyuan Movement and discovered around 2020, is roughly located along Wushenqi-Jingbian. It is similar to the Qingyang paleo-uplift in early formation characteristics, but different in later evolution features [19]. There is no consensus on the spatial distribution and evolution of the Wushenqi paleo-uplift. Wei et al. [18] and Zhang et al. [19] depicted significantly inconsistent planar distribution of the paleo-uplift. Bao at al. [17] speculated that the paleo-uplift was formed in the Neoproterozoic; however, Wei et al. [18] believed that it was developed in the Early Paleozoic. These scholars mainly demonstrated the distribution and evolution of the paleo-uplift from the perspective of sedimentology based on drilling and outcrop data, without detailed structural description. This paper used the latest 2D and 3D seismic data and drilling data, from a structural perspective, to finely delineate the denudation extent of the Wushenqi paleo-uplift and determine the spatial distribution of the paleo-uplift, and analyzed the evolution process of the paleo-uplift by using Bischke curve and equilibrium section restoration methods. Finally, the structural properties of the paleo-uplift were discussed considering the regional geological setting.

1. Geological setting

The study area, located on the Yishaan slope of the Ordos Basin, is adjacent to the Yimeng uplift to the north, the Weibei uplift to the south, the Tianhuan depression to the west, and the Western Shanxi flexure belt to the east (Fig. 1a). The stratigraphic system is incomplete in the study area (Fig. 1b), with the absence of Neoproterozoic, Silurian, Devonian, and Cenozoic, as well as Cambrian locally, with a sedimentary thickness of approximately 4500 m. Upwardly, in the basin, the basement is the Archean metamorphic rock. The Changcheng System of the Middle Proterozoic is continental deposit, which is unconformably overlaid by the Cambrian or Ordovician. The Cambrian-Ordovician is a marine deposit, which is mainly carbonate rock. The Middle Cambrian contains clastic rock in the lower part. The Ordovician Majiagou Formation also contains a small amount of salt rock and gypsum. The Cambrian and Ordovician Majiagou Formation are in unconformable contact. The Carboniferous is a marine-continental transitional facies, which is dominated by coal, sandstone, and mudstone in the lower part, and epeiric carbonate rock in the middle-upper part, with the bottom in unconformable contact with the Ordovician. The Permian-Cretaceous is composed of continental sandstone and mudstone deposits.
Fig. 1. (a) Location of the study area and (b) stratigraphic column of Well CC1.
The Ordos Basin experienced multi-stage tectonic evolution [20⇓⇓⇓-24]: early-middle Mesoproterozoic continental rifting, Cambrian-Middle Ordovician passive continental margin formation, Late Ordovician active continental margin formation and collision orogeny, Late Carboniferous-late Permian splitting around basin, early Mesozoic large-scale intracontinental depression, middle-late Mesozoic intracontinental foreland basin, and Cenozoic peripheral fault depression. During the Early Cambrian-Middle Silurian, the Shangdan Ocean subducted northward [2]. This period is conducive to the development of paleo-uplifts.

2. Identification of the Wushenqi paleo-uplift

The Wushenqi paleo-uplift is distributed in Wushenqi, Jingbian, Hengshan and Yulin (Fig. 1). Due to the limited number and uneven distribution of wells penetrating the Cambrian in the early stage, and the difficulty in distinguishing sandstones of the Middle-Lower Cambrian from that of Changcheng systems, it is highly controversial on the spatiotemporal distribution of the paleo-uplift interpreted from drilling results. Based on the latest 2D and 3D seismic data, this paper identifies the truncation and overlap points of the strata through the phenomena of wave group termination and disappearance, as well as the abrupt changes in amplitude and frequency of seismic events, so as to delineate the distribution of the paleo-uplift. Furthermore, the main development period of the paleo-uplift is analyzed by using a chronostratigraphic framework.

2.1. Drilling-constrained identification

On the E-W well-tie section (Fig. 2a), the Cambrian thins gradually from Well TS1 to Well CC1 and pinches out in the Well JT1 area in Jingbian, from west to east, and thins from Well Y9 to Well LT1, from east to west. Generally, it is about 160 m thicker on the west wing than on the east wing of the paleo-uplift. In the core of the paleo-uplift, the carbonate rock of the Ordovician Majiagou Formation is directly in contact with the underlying Mesoproterozoic Changcheng metamorphic quartz sandstone, and displays evident mutations on the acoustic (AC) and gamma ray (GR) logs. On the two wings, there is an unconformable contact between the Majiagou Formation and the underlying Cambrian, and the Ordovician Yeli Formation and Liangjiashan Formation are absent. Above the unconformity, the first to third members of the Majiagou Formation (Ma 1-Ma 3 members, or O1m1-O1m3) and the fifth member of the Majiagou Formation (Ma 5 or O1m5) become thinner towards the west, and the Ma 4 Member thins towards the east. The Ma 6 Member is distributed in the Well Y9 area on the east wing, and the Upper Ordovician in the Well TS1 area on the west wing. The Carboniferous is seen on this section. The thickness changes of the Ordovician reflect the possibility of adjustment and transformation of the paleo-uplift in the late stage, making it migrate westward.
Fig. 2. Well-tie section of Meso-Neoproterozoic-Carboniferous in central Ordos Basin (section location shown in Fig. 1). ∆t—interval transit time; GR—Gamma ray; Ar—Archean; Pt2Ch—Proterozoic Changcheng System; Pt2Jx—Proterozoic Jixian System; —C1m—Lower Cambrian Mantou Formation; —C3s—Upper Cambrian Sanshanzi Formation; O3l—Upper Ordovician Lashizhong Formation; O3w—Upper Ordovician Wulalike Formation; C2b—Upper Carboniferous Benxi Formation.
On the nearly N-S well-tie section (Fig. 2b), the Middle-Upper Cambrian thins gradually from Well S15 to wells JT1 and S476 until pinching-out, from south to north, and also from Well S158 to Well T89, from north to south. Generally, it is about 70 m thicker on the north wing than on the south wing of the paleo-uplift. The strata and contact relationship in the core of the paleo-uplift are consistent with those in Fig. 2a. Above the unconformity, Ma 1-Ma 3 members and Ma 5 Member are relatively thin near Well S476 in the core of the paleo-uplift, and thick on the two wings, reflecting the inherited activity of the paleo-uplift during the deposition of the Majiagou Formation.

2.2. High-resolution seismic identification

On the cross-well seismic sections (Figs. 3 and 4), it can be seen that the Middle-Upper Cambrian is relatively thick on the east and west wings of the paleo-uplift, and gradually denuded towards the core. The seismic section of the west wing of the southern part of the paleo-uplift shows that the Middle-Upper Cambrian at the truncation point reflects decreased frequency of the seismic events and weakened amplitude (Fig. 3b). The seismic section of the east wing shows that the Middle-Upper Cambrian at the truncation point exhibits an abrupt weakening and gradual disappearance of strong reflections (Fig. 3c). The seismic section of the west wing of the northern part of the paleo-uplift suggests that the Middle-Upper Cambrian reflects low frequency and weak amplitude at the top, which gradually disappear eastward to medium frequency, similar to truncation, and significantly enhancing amplitude (Fig. 4). It is speculated that the Middle-Upper Cambrian carbonate rock was denuded, with the residual sandstone and mudstone forming strong seismic reflection with the carbonate rock of the Ma 3 Member.
Fig. 3. Seismic geological layer tracing and unconformity characteristics of the AA' section in the study area (section location shown in Fig. 1).
Fig. 4. Seismic geological layer tracing and unconformity characteristics of the BB' section in the study area (section location shown in Fig. 1).
Several seismic sections of the peri-clinal area in the core of the paleo-uplift also reveal the phenomenon of the Middle-Upper Cambrian being truncated by the Ordovician Majiagou Formation (Fig. 5). On the CC' section of the western wing of the paleo-uplift, the Middle-Upper Cambrian thins from west to east and is completely truncated to the east of Well T108. The Middle-Upper Cambrian at the truncation point shows multiphase combination, with diminishing strong wave peak reflection (black event). On the DD' section of the north wing, the Middle-Upper Cambrian thins from north to south, and at the truncation point, displays a gradual disappearance of low-frequency reflections. On the EE' section of the northeast wing, the Middle-Upper Cambrian becomes apparently thinner from east to west, and its strong trough reflection (red event) disappears suddenly near Well D122, suggesting evident truncation characteristics. On the FF' section of the southeast wing, the truncation points at the top of the Middle-Upper Cambrian are more evident and relatively close, indicating that the east wing of the paleo-uplift is steep near this section. In addition to these seismic sections, previous studies also discussed the typical truncation characteristics of the Middle-Upper Cambrian on the southwest wing of the paleo-uplift [19,25], which will not be repeated here.
Fig. 5. Unconformity of CC', DD', EE' and FF' sections near the Wushenqi paleo-uplift (section location shown in Fig. 1).
In addition, multiple truncation unconformities are developed in the strata underlying the Middle-Upper Cambrian near the paleo-uplift. For example, the unconformity between the Archean-Paleoproterozoic and the Changcheng System, and the unconformity between the Changcheng System and the Middle-Upper Cambrian, exhibit evident truncation characteristics of the sudden termination of the seismic event (Figs. 3b, 3c, and 4b), reflecting that the area suffered multiple denudations in the Precambrian. In the Ordovician Majiagou Formation near the paleo-uplift, the Ma 3 Member overlaps the Ma 1 Member-Ma 2 Member (Figs. 3b and 4b), indicating that the Wushenqi paleo-uplift was developed during the deposition of the Ma 3 Member.
To sum up, for the Wushenqi paleo-uplift, the denuded west boundary rests along wells CC1, T59 and T83, and the denuded east boundary along Yulin and Hengshan; the denuded Sanshanzi Formation, Zhangxia Formation, Xuzhuang Formation and Maozhuang Formation cover an area of 23 548 km2, 17 794 km2, 14 172 km2 and 11 298 km2, respectively. The distribution of residual strata is consistent with the thickness distribution of the Middle-Upper Cambrian. This indicates that the depicted spatial distribution of the paleo-uplift is highly reliable. The paleo-uplift spreads in the S-N direction (Fig. 6), as an irregular ellipse, with the long axis of 194 km and the short axis ranging from 55 km (near Hengshan) to 94 km (near Wushenqi), with the wings steep in the east and gentle in the west.
Fig. 6. Thickness map of present-day Cambrian in the Wushenqi paleo-uplift in the Ordos Basin.

3. Development, evolution and distribution of the Wushenqi paleo-uplift

3.1. Time limit of unconformity

Through analysis on the regional survey data in western Shandong and the strata in the North China region, Song [26] believed that the Huaiyuan Movement occurred during the late stage of the Middle Cambrian Zhangxia Formation to the Early Ordovician Liangjiashan Formation, and due to the north-south compression, most parts of the North China region experienced "episodic" activities such as seabed uplift, exposed erosion, and oscillatory sedimentation.
The chronostratigraphic framework of the Wushenqi paleo-uplift reveals that the Precambrian experienced the Lüliang Movement, Zhaertai Movement and Yanliao Movement successively, resulting in a time hiatus of 483-1991 Ma between the Middle-Upper Cambrian and the Precambrian due to denudation, and the hiatus gradually increases from west to east (Fig. 7). At the end of Cambrian, the Huaiyuan Movement took place, with the resultant denudation inducing unequal hiatuses in this area from west to east (Fig. 7). The relative sea level declined in this period. The hiatus near Well TS1 and Well Y9 is 7.7 Ma. The hiatus around Well JT1 is 31.3 Ma, where the hiatus of 1091 Ma formed by the Precambrian denudation is superimposed, and the hiatus gradually increases towards Well JT1 on the east and west sides. During the late Middle Ordovician to Late Carboniferous, denudation in this area [24] formed a hiatus of 147-156 Ma (Fig. 7). In the late depositional stage of the Majiagou Formation, there was a hiatus of 0-4.6 Ma, with large values witnessed in the west and middle. From the Upper Ordovician to the Carboniferous Benxi Formation, a hiatus of 147.0-151.4 Ma was formed, and at this time, the hiatus was about 4.4 Ma longer in the middle and east than in the west. The hiatus at the top of Ordovician indicates that "seesaw-like" differential uplift and denudation occurred successively in Wushenqi, resulting in a larger hiatus in the middle than in the east and west.
Fig. 7. Time limit of unconformity near the Wushenqi paleo-uplift (section location in Fig. 1; age cited from References [27⇓-29]).
According to the nature of the Huaiyuan Movement defined in previous studies, it is speculated that the Wushenqi area in the Ordos Basin was significantly impacted by compression at the end of the sedimentation of Sanshanzi Formation during the Huaiyuan Movement. As a result, the paleo-uplift was notably developed, and then suffered denudation and began to recess, which sustained for the longest period of about 7.7 Ma, rather than the continuous and significant development of the paleo-uplift throughout the Huaiyuan Movement. In addition, the decline of relative sea level aggravated the recession of the paleo-uplift, which might be active again before the Carboniferous.

3.2. Bischke curve and paleo-uplift development stages

The Bischke curve, applicable to all sedimentary environments, is mainly used to analyze the thickness changes of formations in different well areas by periods [30⇓⇓-33]. The thicknesses of the Permian and Triassic are laterally stable around the Wushenqi paleo-uplift (Figs. 3 and 4), which indicates that the paleo-uplift was not active evidently in this period, and the eastern part of the basin was uplifted and denuded in the Jurassic and Cretaceous. Therefore, the analysis of the structural evolution of the Wushenqi paleo-uplift should focus on the pre-Carboniferous. In the Gaojiapu 3D area in the east of Wushenqi, Well S100, which displays good well-seismic calibration and complete strata of the Cambrian, is selected as the reference well. The wells in the core and wings of the paleo-uplift are used as analytical wells. If the analytical wells do not penetrate the Cambrian or the Cambrian was eroded, the thickness of the Cambrian is estimated according to the trend of seismic reflection of adjacent wells or nearby the wells.
For the Bischke curve in Fig. 8I, the positive value represents the smaller thickness of a formation in the analytical well relative to the reference well, while the negative value represents the larger thickness; the slope represents the rate of change in formation thickness [30⇓⇓-33]. The curve can tell the changes in the landform relief of a well area relative to the reference well area in different periods, by which the tectonic evolution stages of the paleo-uplift can be roughly judged.
Fig. 8. Bischke curves and equilibrium section restoration maps of the Wushenqi paleo-uplift (section location shown in Fig. 1).
The paleo-uplift evolution in the pre-Carboniferous is divided into three stages: (1) Emergence in the Middle-Late Cambrian. The curves of all analytical wells in this stage almost display positive values, which rise and then fall. This phenomenon indicates that the paleo-uplift area rested as an underwater low uplift relative to the Well S100 area before the deposition of the Zhangxia Formation. The low uplift gradually disappeared during the deposition of the Zhangxia Formation. (2) Development from the end of the Late Cambrian to the Ordovician Ma 3 Member. The curves of multiple wells in this stage tend to ascend at a large slope, and then descends at unequal slopes. Coupling with the truncation characteristics of the Middle-Upper Cambrian on the seismic section, it is believed that the paleo-uplift began to develop significantly at the end of the sedimentary period of the Sanshanzi Formation, and then suffered denudation. In the depositional period of Ma 1-Ma 3, it was an underwater low uplift and gradually evolved into a sag. (3) Transformation from the Ordovician Ma 4 Member to the Carboniferous. The curves of wells CC1, T59 and S476 in the sedimentary period of Ma 4-Ma 5 display a trend of ascending at a large slope, indicating that the area began to rise to the surface during the deposition of the Ma 4 Member [24]. By contrast, the Well JT1, Well Q44, and Well S168 areas were still in recession during the deposition of the Ma 4 Member, and rapidly uplifted to the surface from the depositional period of the Ma 5 Member until the Carboniferous [24], indicating that the recession and transformation in the northwest of the paleo-uplift was earlier than that in the southeast. The Carboniferous Bischke curve keeps stable or ascends slightly. Combined with the previous analysis, it is believed that the paleo-uplift was basically stable in this stage.

3.3. Formation and evolution of the paleo-uplift

The Wushenqi paleo-uplift is located in the craton basin, with laterally stable strata and small faults. The time-depth relationship of Well CC1 with good well-seismic calibration can be used to convert the seismic horizon of the AA' section into geological horizon. Then, the denuded stratum can be restored and the thickness be calculated by the trend method. Fig. 8II shows that, during the Huaiyuan Movement, the Middle-Upper Cambrian near the core of the paleo-uplift was eroded by 170-196 m. The maximum erosion (about 270 m, including the Changcheng System) in this period was witnessed near Well JT1, and the erosion at the wings was 0-60 m.
The deformation and displacement characteristics of the pre-Carboniferous were restored by using the equilibrium section (Fig. 8II). The structure at the end of the Middle-Upper Cambrian is shortened by 4.1 km or 1.8% as compared to that before the Middle-Upper Cambrian (Fig. 8a, 8b). The restored AA' equilibrium section at the end of the Middle-Upper Cambrian shows no uplift shape, which indicates that the Wushenqi paleo-uplift was not remarkably developed at the position of AA’ in the same sedimentary period of the Middle-Upper Cambrian. This finding is consistent with the conclusion that the paleo-uplift emerged in the Middle-Late Cambrian, according to the analysis of Bischke curve. In addition, since the vertical resolution of drilling is higher than that of seismic section, Well JT1 and Well Q44 reflect small thickness difference (about 15 m) of the Upper-Middle Cambrian Xuzhuang Formation and Maozhuang Formation, which does not show evident response on the seismic section nearby (Fig. 8b), but has the advantage of lateral seismic continuity. Therefore, the analysis by using the Bischke curve of well and the equilibrium section restored from seismic data can better reflect the evolution of the paleo-uplift and provide more reliable results.
The shortening in Fig. 8c relative to Fig. 8b is 8.1 km, with a shortening rate of 3.6%, indicating that the area was significantly compressed before the deposition of the Ordovician Majiagou Formation, forming a buckle fold. That is, the Wushenqi paleo-uplift was formed in this period, and then began to suffer erosion and gradually declined. This is consistent with the results of the Bischke curve analysis. The section before the deposition of Ma 3 Member shows a slightly thicker Majiagou Formation in the east (Fig. 8d), which is consistent with the result of the Bischke curve analysis that the Ma 1 and Ma 2 thickness of Well JT1 is about 35 m smaller than that of Well Q44.
Fig. 8e-8g shows the equilibrium sections restored for the Ma 3-Ma 5 period. They exhibit the shortening of 3.5 km and 2.2 km in the Ma 3 and Ma 5 members, respectively, and also reflect that the Well CC1 area was a sag in the Ma 3-Ma 4 period and gradually uplifted after the sedimentation of the Ma 5 Member, which is consistent with the analysis results of the Bischke curve of Well CC1 in this period. Moreover, the restored equilibrium section of the Well JT1 area in this period agrees with the Bischke curve analysis results. Fig. 8e-8g reveals the "seesaw-like" evolution of formation thickness from west to east, due to the fact that the northwest of the paleo-uplift declined and underwent transformation earlier than the southeast.
By using Bischke curve and equilibrium section, the formation deformation characteristics of the Wushenqi area in the pre-Carboniferous were analyzed, showing consistent results. It is found that the intense deformation corresponds to notable shortening. The restored equilibrium section confirms that a structural compression occurred in the central Ordos Basin in the period from the end of Late Cambrian to the Early Ordovician Majiagou Formtion (Huaiyuan Movement), forming the Wushenqi paleo-uplift. In the period of structural compression, faults or fractures might develop in and around the Wushenqi paleo-uplift. Later in the period of weathering denudation, good karst reservoirs were formed near the surface along the early faults or fractures. Such reservoirs were also found on the outcrop sections of the Middle-Upper Cambrian in the southern and eastern parts of the basin [34].

3.4. Distribution of the paleo-uplift in different periods

Based on the identification of truncation and overlap points near the paleo-uplift, the planar distribution of the paleo-uplift in different periods was depicted (Fig. 9). At the end of Early Cambrian, the basin was generally a denudated ancient land. At the end of Late Cambrian, the paleo-uplift was much smaller than the ancient land, indicating that the Wushenqi paleo-uplift was not formed in the Early Cambrian. In the depositional period of the Early Ordovician Ma 3 Member, the paleo-uplift shrank from 2.4×104 km2 at the end of Late Cambrian to 9260 km2, and the long axis of this elliptical uplift stayed in the NE-SW direction. At present, the Wushenqi paleo-uplift developed during the Huaiyuan Movement mainly distributes in S-N direction (5° north by east), being irregularly elliptical, with the long axis of 194 km and the short axis ranging from 55 km near Hengshan to 94 km near Wushenqi, and with the wings steep in the east and gentle in the west (Fig. 6).
Fig. 9. Distribution of the Wushenqi paleo-uplift in different periods of the Ordos Basin.

4. Dynamic mechanism of the formation of Wushenqi paleo-uplift

The Wushenqi area is a weak zone of basement and structure. During the Huaiyuan Movement, the non-coaxial compression from the north and south of the basin, as well as the stress transmitted by the uplift and depression of the basin, enabled an irregular compression type paleo-uplift in this area.
According to the gravity and magnetic research results of the basin [35], the basement in the Wushenqi paleo-uplift reflects generally moderate gravity and magnetic anomalies, but strong heterogeneity. Specifically, relatively high value areas and low value areas alternate frequently in the SW-NE direction. The northern and southern parts of the periphery of the paleo-uplift are high value areas of gravity anomaly. It can be inferred that the Wushenqi paleo-uplift was developed on a basement as a weak area of gravity and magnetism. The seismic section reveals many truncations in the Archean-Ordovician Majiagou Formation in Wushenqi, which reflects that the area was exposed and denuded for many times before the deposition of the Majiagou Formation, with frequent tectonic activities. The overlap unconformity of Majiagou Formation records the reactivation of the Wushenqi paleo-uplift in this period. The analysis results of equilibrium section show that the Wushenqi paleo-uplift was mainly formed by tectonic compression during the Early Caledonian (Huaiyuan Movement). From the perspective of evolution around the basin, southward and northward subductions occurred respectively along with the formation of the Paleo-Asian Ocean in the north margin and the Shangdan Ocean in the southwest margin during the Cambrian-Middle Carboniferous, probably forming the non-coaxial compression in the central part of the basin. In this period, the Helan rift in the west of the basin and the eastern depression in North China continued to develop, and the Lüliang uplift to the east of the Wushenqi paleo-uplift was also active evidently. Clearly, the compression around the basin and the uplift and depression evolution of the basin are the stress sources for the formation of the Wushenqi paleo-uplift.
There are evident differences between the denuded areas of the Changcheng System and the Middle-Upper Cambrian identified by using seismic data. The former is widely distributed in the basin, while the latter is only limited to the Wushenqi area (Fig. 9), indicating that the Wushenqi paleo-uplift was formed on the ancient land extensively denuded in the Precambrian, but not completely inherited. This understanding is consistent with the previous research results of sedimentology [19]. Furthermore, based on the trend of Cambrian thickness and the identification of overlap points in seismic sections at the basin margin, it is speculated that the Lower Cambrian overlies the underlying strata at the basin margin (Fig. 10a). Coupling with the above analysis, the evolution process of the Wushenqi paleo-uplift and peripheral rifts and depressions is divided into four stages (Fig. 10).
Fig. 10. Schematic evolution of the Wushenqi paleo-uplift and its periphery in E-W direction.
(1) Emergence in the Middle-Late Cambrian (Fig. 10b-10e). During the Middle Cambrian, seawater invaded from the southwest of the basin [17], and submerged the corresponding denuded area within the basin, forming underwater low uplift. The depression and rift in the east and west continued to subside, giving rise to the Shenmu depression and Lüliang uplift in the east of the basin. The continuous development of the uplift and depression pattern caused the underwater low uplift to deform and shrink. The low uplift in the Wushenqi area basically disappeared in the sedimentary period of the Zhangxia Formation, and the Sanshanzi Formation was deposited in the Late Cambrian.
(2) Development from the end of the Late Cambrian to the Ordovician Ma 3 Member (Fig. 10f). The Lüliang uplift to the east of the Wushenqi paleo-uplift became intensely active again in the Early Caledonian (Huaiyuan Movement), which may be the main stress source in the east of the Wushenqi paleo-uplift, thus shortening the east-west structure of the Wushenqi paleo-uplift. Before the deposition of the Ordovician Majiagou Formation, the paleo-uplift was denudated and then submerged under water. During the deposition of the Ma 3 Member, the paleo-uplift also became active again, but its coverage reduced and its extension changed to the NE-SW direction. The high part of the ancient landform in the Ma 1 Member to Ma 3 Member migrated westward as a whole, and the thickness of the strata gradually decreased westward. In the western part of the paleo-uplift, granular shoals and microbial mounds were developed as the primary dolomite reservoirs.
(3) Transformation from the Ma 4 Member till the Carboniferous (Fig. 10g-10h). The eastern and western parts of the basin were still relatively receded. The western part was thick in the sedimentary period of the Ma 4 Member. During the deposition of the Ma 6 Member until the Carboniferous, affected by the Alxa North China uplift[21,36], the western part of the Wushenqi paleo-uplift was uplifted and eroded, making the Ma 5 Member thin in the west and thick in the east.
(4) Stabilization in the Carboniferous-later. The Wushenqi paleo-uplift was not active, but deeply buried along with the rising in the east and the falling in the west at the periphery. In general, the paleo-uplift was relatively stable in this stage.

5. Conclusions

On plane, the Wushenqi paleo-uplift formed in the Early Caledonian (Huaiyuan Movement) is in irregularly elliptical shape distributed in the S-N direction, with a certain angle to the distribution direction of the Changcheng System rift, rather than the other scholars’ idea that the Wushenqi paleo-uplift and the Changcheng System rift are distributed in the same direction. The paleo-uplift covered an area of about 2.4×104 km2 in that period. It was mainly formed by tectonic compression, and sustained for as long as 7.7 Ma before being denudated. During the deposition of the Ordovician Ma 3 Member, the paleo-uplift experienced an inherited activity, and distributed in NW-SE direction, with the area shrinking to 9260 km2. The paleo-uplift development was affected by the strong heterogeneity of gravity and magnetism of the basement and the frequent uplift and denudation of the area in the pre-Paleozoic. Its evolution can be divided into four stages: emergence in the Middle-Late Cambrian; development from the end of the Late Cambrian to the Ordovician Ma 3 Member; transformation from the Ma 4 Member till the Carboniferous; and stabilization in the Carboniferous-later. The Wushenqi paleo-uplift was a compressional paleo-uplift when its main part was formed, characterized by buckle fold, with a structural shortening rate of 3.6%. In this period, due to weathering denudation, the paleo-uplift and the faults or fractures nearby could form karst reservoirs, providing migration pathways or storage space for oil and gas. In addition, the west wing of the paleo-uplift remained a control over the reef beach reservoirs of the Ordovician Majiagou Formation in the development and transformation stages.

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