The Ordos Basin is a large-scale multi-cycle superimposed basin located in the western part of the North China Craton, stretching from Yinshan in the north to Qinling Mountains in the south, and from Luliang Mountain in the east to Helan Mountain in the west^[1]. Experiencing the Fuping and Wutai multi-stage tectonic movements^[2], the Ordos Basin is rich in oil and gas resources and is an important energy base in Central and Western China. Exploration practices show that natural gas in the Ordos Basin is not only enriched in the Upper Paleozoic clastic rocks^[3], but also in the Lower Paleozoic marine carbonate rocks^[4]. The Lower Paleozoic marine carbonate rocks in the Ordos Basin are widely distributed with large deposition thickness and rich natural gas resources. After years of large-scale exploration, major exploration discoveries have been made in the Ordovician^[5], and two large accumulation systems, the "top of Ordovician" and "inside Ordovician", have been found^[6]. By the end of 2018, the proven natural gas reserves of more than 6.547×10⁸ m³ in the Ordovician had been submitted cumulatively. With the continuous advancement of the theory and technology of onshore deep layer oil and gas exploration and development^[7,8,9], and the continuous advancement of exploration in the Ordos Basin, natural gas explorations have gradually been expanded to deep layers. Presently, some wells have encountered favorable hydrocarbon shows in the Cambrian, revealing a favorable exploration potential in the Cambrian marine carbonate rocks of the Ordos Basin.

Due to the limitation of basic data such as seismic and drilling, the understanding on geological conditions of the Cambrian natural gas is relatively weak, especially the study on the fault characteristics of the Cambrian. At present, the study on the faults of the Cambrian is mainly concentrated on the edge of the basin. Many researchers believe that the southwestern part of the Ordos block was adjacent to the continental margin of the Qinling-Qilian-Helan Mountain trifurcate rift valley system in the early Paleozoic^[10]. Affected by the re-activation of the Qinling-Qilian-Helan Mountain trifurcate rift valley, the western part of the basin was a rift valley in the Early Paleozoic. In the study of the basin's tectonic evolution and lithofacies paleogeography etc., the paleo- structure of Cambrian was somewhat involved. It is believed that due to the influence of the Rodinia super-continental breakup and the expansion of the original Tethys ocean, the basin had slow subsidence inside and relatively rapid sedimentation at the edge in the Cambrian^[11], and the basement structure of the basin affected the Cambrian tectonic pattern^[12]. Some researchers believe that the basin had a pattern of "one uplift and three sags" (i.e., the western margin sag, the southern margin sag, and the eastern sag surrounding the central uplift in a "U" shape) in the Cambrian^[13]. Some other researchers hold that, in the Cambrian, the basin had the structural pattern of “two uplifts, two sags and a saddle" (i.e., the north and south uplifts, the east and west sags, and a saddle in the middle)^[14]. The previous studies only analyzed the paleo-structural characteristics of the Ordos Basin in the Cambrian, but did not cover fault characteristics of the Cambrian systematically. Exploration practices in recent years have proved that faults played important roles in the generation, migration, accumulation and reservoir- forming of oil and gas^[15,16,17]. The improvements in the coverage and quality of seismic data have laid a foundation for the study of the fault characteristics of the Cambrian in the Ordos Basin.

Based on 326 two-dimensional seismic lines with a total length of 30 000 km, using the logging and drilling data of 61 wells encountering the Cambrian, in combined with 10 outcrop profiles in the periphery areas of the basin and the regional geological tectonic background, adopting the means of comprehensive seismic and geology interpretation throughout the whole basin, the characteristics of Cambrian faults in the Ordos Basin are investigated to find out the impact of faults on the Cambrian paleogeographic pattern and their control over the Cambrian sedimentation and reservoirs, and then favorable exploration areas of the Cambrian are predicted.

The Ordos Basin is structurally a part of the North China Plate. As a multi-cycle and large superimposed basin^[18], it has experienced more than 10 tectonic movements such as the Fuping and Wutai movements. Researchers divided the basin into 6 tectonic units, namely Jinxi flexible fold belt, Weibei uplift, western margin thrust belt, Yishan Slope, Tianhuan Depression and Yimeng Uplift (Fig. 1a). The Cambrian is the second set of sedimentary system above the basement. By the time of Cambrian sedimentation, the basin mainly underwent three stages of evolution (Fig. 2).

(1) The crystalline basement formation stage: Studying from different perspectives, researchers considered that the crystalline basement of the Ordos Basin mainly formed from Archean to Early Proterozoic, that is, after the movements of Qianxi, Fuping, Wutai and Lüliang, the crystallization basement finalized at last^[19,20,21]. Some researchers studied the basement characteristics by using the data of underground heat flow values^[22,23], aeromagnetic anomalies^[24], gravity anomalies^[25], seismic profiles^[26], and zircon dating^[27]. The results showed that the Ordos block was not a complete block and had obvious heterogeneity. The basement of the basin was pieced together and solidified by several blocks of different lithologies, different ages and different degrees of metamorphism.

(2) The Meso- and Neoproterozoic continental breakup stage: The Meso- and Neoproterozoic Erathem was the first set of sedimentary caprocks above the crystalline basement. Scholars studied the Meso- and Neoproterozoic structure in the Ordos Basin^[28,29], and believed that as affected by the breakup of Rodinia supercontinent^[30] in the Meso- and Neoproterozoic, the rifting effect was very active^[31], and a series of intracontinental rifts were developed under the background of the strong tensile stress. The structures in these intracontinental rifts were relatively weak, and faults were more easily developed under the influence of the tectonic stress. Therefore, to a certain extent, the rifting action in the Meso- and Neoproterozoic was a precondition for the development of the Cambrian faults.

(3) The Cambrian stretching stage: From the Early Cambrian to the Middle Cambrian, affected by the expansion of the Xingmeng trough in the northern margin and the Qinling trough in the southern margin, the Ordos block was in passive continental marginal tectonic environment and was in the stress background of stretching. In the Late Cambrian, the tectonic stress field in the basin transitioned from tensile to compression, and the basin was in a weak tensile stress background^[31]. In general, the Cambrian was in a tensile stress background.

After the sedimentation of Cambrian, the Ordos Basin underwent a series of tectonic movements such as Caledonian, Hercynian, Indo-Chinese, Yanshan and Himalayan movements in turn^[32]. The Caledonian movement caused the whole Ordos Basin to rise; the Hercynian and Indosinian movements caused the basin to be in the alternation of stretching and compressive stresses, which made the basin evolve into an inland depression basin; the Yanshan movement caused collisional orogeny in the periphery of the basin, thus forming a foreland basin, and the basin transformed into a foreland flexural settlement stage; the Himalayan movement led to fault depression around the basin and uplift within the basin, eventually giving rise to the present structure. In general, the tectonic movements after the Cambrian, except for those in the periphery of the basin, were dominated by vertical ascending and descending movements. The tectonic movements after the Cambrian had a weak effect on the later transformation of the Cambrian faults within the Ordos basin.

In the Early Eopaleozoic period, the Ordos Basin underwent multi-cycles of rapid transgression and slow regression process, leaving a set of relatively stable and traceable sedimentary strata composed of the Cambrian marine carbonate with clastic deposits. In the Ordos Basin, the Lower Cambrian includes the Xinji Formation, Zhushadong Formation and Mantou Formation; the Middle Cambrian the Maozhuang Formation, Xuzhuang Formation and Zhangxia Formation; and the Lower Cambrian the Sanshanzi Formation. The Lower Cambrian is smaller in distribution area, only distributed in the western and southern margins of the basin, and is mainly composed of sandstone, mudstone, phosphorus-bearing sandy conglomerate and limestone. The Middle Cambrian is larger in distribution range, and is mainly made up of dolomite, limestone, mudstone, bioclastic limestone and Oolitic limestone etc. Influenced by later-stage denudation, the Upper Cambrian Sanshanzi Formation is thicker (up to 380 m) in the western margin of the basin and thinner (generally 100 m) inside the basin. It is mainly comprised of gray and light gray dolomite, argillaceous dolomite, and fine-grained dolomite, etc.

The Cambrian seismic geological horizons were finely calibrated with the well logging and drilling data from 61 Cambrian wells in the basin. The bottom boundary of the Carboniferous System is a strong impedance difference interface, with the seismic reflection characteristics of a single strong wave peak and good continuity, which can be traced and compared within the whole area (Fig. 2). Therefore, this interface was taken as an iconic reflection interface of the whole area in this research. The lithological differences in the Cambrian overlying strata lead to two different seismic reflection characteristics of the Cambrian top interface. One kind is the medium-strong wave trough and the medium-high continuity seismic reflections (Fig. 2a), which is mainly distributed in the northwestern part of the Ordos Basin. And the other kind is the medium-weak trough and medium-low continuity seismic reflection (Fig. 2b), which mainly occurs in the southeastern part of the Basin. The Cambrian bottom boundary also has two different seismic reflection characteristics. When the underlying strata are Sinian, the sandy mudstone at the bottom of the Cambrian and the Sinian conglomerate form a strong positive impedance difference interface, and the bottom boundary of Cambrian takes on the single medium-strong peak and medium-high continuous seismic reflections (Fig. 2a). This case mainly exists in the western Ordos Basin. While in the eastern part of the basin, with the Jixian or the Changcheng system as the underling strata, the Cambrian bottom boundary is a negative impedance difference interface, which has the medium-strong wave troughs and medium- continuity seismic reflection characteristics (Fig. 2b). On the basis of the Cambrian horizon calibration, guided by the tectonic evolution of the Ordos Basin, four kinds of fault identification markers of the Cambrian are established: (1) the dislocation and interruption of the seismic reflection events (Fig. 2a); (2) the obvious fault-plane waves in seismic sections (Fig. 2b); (3) abrupt phase change of the seismic reflection (Fig. 2c); (4) abrupt change in the formation gradient or the formation thickness (Fig. 2d). Guided by these, the tectonic interpretation of the Cambrian was carried out in the whole basin, and the fault characteristics of the Cambrian were studied. Total 93 faults of different types, different occurrences and different scales were identified for the Cambrian.

Affected by the changes of stress field in different tectonic evolutionary stages, faults of different types, different occurrences and different scales develop in the Cambrian. The Cambrian faults in the Ordos Basin are dominated by NE-trending, near-WE trending and NW-trending ones (Fig. 3).

2.2.1. NE-trending faults

The NE-trending faults came into being mainly in the Cambrian and they are the main faults of the Cambrian. On the plane, some of these faults are large in scale, with planar extending distance of hundreds of kilometers. On the cross section, these faults are steeper in occurrence, with a dip angle of greater than 45° in general. The faults are mainly normal ones with fault throws from tens to hundreds of meters. The faults take on the structural style of alternate horst-graben structure or stepped structure, and generally terminate in Cambrian or inside the Ordovician in the upward direction, without cutting the Late Paleozoic strata. Some faults can extend downwards to the Precambrian, and have a certain inheritance from the Meso-and Neoproterozoic extensional faults. The formations on two sides of the faults differ somewhat in thickness or seismic facies, which reveals the NE-trending faults have important control effects on the Cambrian sedimentation. In this research, 50 NE-trending faults were identified in the Cambrian, which are mainly distributed in the southern and central basin (Fig. 3).

Two seismic sections nearly perpendicular to the fault strike clearly shows the typical characteristics of the NE-trending faults (Fig. 4). On cross section EE', the three faults (i.e., No.1, No.2 and No.3 faults), cut downwards the Precambrian strata and disappear upwards inside the Cambrian or Ordovician. From west to east, the three faults show the stepped structure style, with gradually increasing fault throws and gradually thickening Cambrian strata (Fig. 4a). On the cross section FF', No.9 and No.10 faults are normal faults formed in the Cambrian period, with steeper dip angles. Disappearing upwards inside the Cambrian and cutting downwards the Precambrian strata, the two faults present the tectonic style of a graben, and the thickness of the Cambrian in the downthrown wall significantly increases (Fig. 4b).

2.2.2. Near EW-trending faults

The near EW-trending faults are also main faults of the Cambrian. On the plane, they occur mainly in the northern part of the basin, and are 50 to125 km in extension distance. On cross sections, they are mainly normal faults, steeper in dip angle, and generally smaller than 100 m in fault throw. The faults terminate upwards in the Ordovician, not penetrating the Carboniferous, and extend downwards to the Precambrian strata. The Cambrian strata in the downthrown wall of the faults increase in thickness. Mainly affected by the near EW-trending basement faults in the northern basin, the near EW-trending faults were formed by the reactivation of the basement faults during the Cambrian period and have a certain control effect on the Cambrian sedimentation in the northern part of the basin. A two-dimensional seismic section in the northern part of the basin shows that the Precambrian faults are abundant, and the Cambrian faults in nearly EW-trend are formed by the re-activation of the Precambrian faults (Fig. 5). The faults are steep in dip angles and not large in fault throw. But the Cambrian strata on the two sides of the faults differ obviously in thickness. A total of 10 near EW- trending faults were identified in the Cambrian in this study.

2.2.3. NW-trending faults

The NW-trending faults were mainly formed in the sedimentary period of Cambrian. They are adjustment faults generated by the extension vector under the Early Paleozoic tensile stress. The faults generally have small extension ranges, striking NW on the plane, and small in fault throw, so this type of faults has hardly any control on deposition.

A flattened seismic section of the Ordovician top in the west of the basin (Fig. 6) shows that No.26 and No.27 faults are NE-trending faults, which cut the Cambrian strata and have relatively large fault throws. Whereas the NF1 fault is a NW-trending normal fault, which is an adjustment fault generated by the extensional vector under the tensile stress background. The NW-trending faults such as NF1 have a small fault throw and do not affect Cambrian sedimentation.

The 3 groups of faults in NE-trending, near EW-trending and NW-trending in the Cambrian differ widely in fault throw and planer extending distance etc. Some large faults extend several hundred kilometers but some minor faults extend only several kilometers. Therefore, in order to better analyze the fault characteristics, on the basis of the fault classification rules^[33], according to the roles of the faults in the structural units and the scales of the faults, the faults of the Cambrian are divided into 3 orders, namely second-order, third-order and fourth-order faults.

The second-order faults control the secondary tectonic units of the Cambrian in the Ordos Basin, namely, the boundary of the Cambrian sags. The second-order faults are large in scale, with the planar extension distances of more than 95 km and fault throw of greater than 85 m. The second-order faults are mainly NE trending faults (Fig. 3) and there are a total of 22 second-order faults in Cambrian. On the cross sections, the second-order faults are normal faults generated under the background of tensile stress. The faults extend upwards to the inside of the Cambrian or Ordovician and have the dip angles of more than 45°. The second-order faults are synsedimentary faults, which apparently control the changes in the Cambrian deposition thickness on two sides of the faults (Fig. 7). Seismic reflection characteristics on the two sides of the faults have obvious differences. The Cambrian strata in the fault downthrow walls mainly have the seismic reflection characteristics of medium- strong amplitude and medium-high continuity, while in the fault upthrow walls, the seismic reflection features medium-low amplitude and medium-low continuity. The second-order faults extend downwards into the Precambrian strata, and have a certain inheritance from the Precambrian faults.

The third-order faults refer to the faults developing inside the secondary tectonic units, which mainly control the high and low undulations of the local structures. The three-order faults are smaller than the second-order faults, with the planar extending distances of 20 to 95 km and fault throws of 25 to 85 m. On the plane, the third-order faults are dominated by the NE-trending faults, mainly distributed along both sides of the second-order faults or in parallel with the second-order faults (Fig. 3). There are a total of 38 three-order faults in Cambrian. On the section, the third-order faults are the synsedimentary faults generated under the tensional stress background, belonging to the derivation or associated faults of the second-order faults. The third-order faults have smaller scale than the second order faults and mainly cut the Cambrian to Ordovician strata (Fig. 8). The influence of the third order faults on the thickness of the Cambrian strata is not as obvious as that of the second-order faults. The third-order faults control the distribution characteristics of the local structures of the Cambrian, and have some influence on the Cambrian seismic reflection characteristics of the two sides of the faults such as seismic amplitude, frequency, while the seismic reflection characteristics such as amplitude and frequency are the direct reactions of the sedimentary characteristics, therefore the third order faults affected the sedimentation of the Cambrian.

The fourth-order faults are the faults developing inside the local tectonic units in the Cambrian, which are mainly in NW-trend (Fig. 3). The fourth-order faults have very small fault scales, usually with the planar extending distances of smaller than 20 km and fault throws of less than 25 m. On the seismic section, they appear as small seismic reflection events interruption (NF1 fault in Fig. 6). The fourth-order faults are mainly adjustment faults generated by the stretching vector under the Eopaleozoic tensile stress background. There are no obvious stratum thickness and seismic reflection differences on two sides of the fourth-order faults in the Cambrian. The fourth-order faults do not control the Cambrian sedimentation.

The second-order and the third-order faults in Cambrian are both synsedimentary faults, which control the paleo-tectonic pattern of the alternate sag and salient, high-low differentiation of the Cambrian.

4.1.1. The second-order faults control on macroscopic paleo-tectonic pattern of alternate sag and salient

A NW-trending seismic section perpendicular to the fault strikes in the western margin of the basin (Fig. 9) shows that the Cambrian strata present the structural pattern of alternate horsts and grabens under the control of the second-order faults. In Fig. 9, there is a horst between No. 24 fault and No. 25 fault, and a graben between No. 25 fault and No. 27 fault. The thickness of the Cambrian strata is relatively thin in the horst but obviously increases in the graben, showing a tectonic pattern of alternate sag and salient on the plane.

4.1.2. The third-order faults control on microscopic paleo-geomorphic differentiation

The third-order faults control the high and low fluctuations of the local structures inside the macroscopic structural units of salients or sags, which further form the micro geomorphological characteristic of the high and low differentiation of the Cambrian. Therefore the third-order faults control the differentiation of the micro-geomorphology in the Cambrian. A seismic section located in the salient (Fig. 10) shows that No.46, No.47, and No.48 faults are third-order synsedimentary faults. On cross section PP', segment A-B and segment C-D are local geomorphological high positions, while segment B-C and segment D-E are local geomorphological low positions, which shows that the third-order faults control the high-low fluctuations of the local geomorphology on the background of the salients or the sags, forming the high and low differentiated micro-geomorphic characteristic.

Controlled by the second-order and third-order faults, the Cambrian system in the Ordos basin presented the micro- paleo-geomorphologic characteristics of sag-salient alternation and high-low differentiation. Such micro paleo-geomorphology has controlled the Cambrian sedimentation and made an obvious sedimentary facies differentiation in the Cambrian. The north-south sedimentary facies correlation section (Fig. 11) shows that in the micro-paleo-geomorphologic high parts on the salient, with the relatively shallow water body and strong hydrodynamic force, some relatively high-energy sedimentary facies such as the platform margin shoals and the intra-platform shoals developed, and the main lithologies are oolitic limestone, oolitic dolomite and dolomitic limestone. For instance, Well Bu1, Well Tianshen1, Well Qingshen2 and Well Zhentan2 are all located in the high parts of the micro paleo-geomorphology on the salient,and they all have well developed oolitic shoal facies. Fig. 11 shows that the larger the scale of adjacent sag is, the larger the scale of the oolitic beach in the margin of the sag is. With the relatively weak hydrodynamic force, the sedimentary facies of relatively low energy developed in the sag. For example, the Morgou outcrop and the Tongxinqinglongshan outcrop are located in sags and the basin facies is well developed in these locations. In the sag, the Cambrian strata thicken obviously and are dominated by limestone, argillaceous limestone, mudstone and shale.

It can be seen that the Cambrian in the Ordos basin presents the paleo-geographical pattern of sag-salient alternation and high-low differentiation under the control of faults. This paleo-geographic pattern further controls the development of the Cambrian shoals. The shoals mainly occur in the high positions of the micro-paleo-geomorphology in the margin of the sag. The distribution direction of the Cambrian shoals is mainly NE-trending, which is basically consistent with the extending direction of the second-order and third-order faults (Fig. 12). At the same time, the larger the scale of the sag is, the larger the scale of the shoal developed in the margin of the sag is.

The second-order and third-order faults jointly controlled the paleo-geographical pattern of the sag-salient alternation and high-low differentiation in the Cambrian. This paleo- geographic pattern further controlled the development of the high-quality reservoirs in the Cambrian, mainly in the following two aspects. (1) On the one hand, this paleo-geographic pattern controls the development of primary pores in Cambrian reservoirs. Under the joint control of the second-order and the third-order faults, the primary pores are well developed in the reservoirs located at the high positions of the micro-paleo-geomorphology in the margin of the Cambrian sags (See the II, III, and IV locations in Fig. 13). In these locations, with the relatively shallow water body and strong hydrodynamic force, the oolitic beach facies well developed and the primary pores of the reservoirs were very good. For example, the Zhangxia Formation of Well Xuntan1 is platform margin oolitic shoals, composed of mainly oolitic dolomite (Fig. 14a). (2) On the other hand, this paleo-geographic pattern controlled the development of the secondary pore in the Cambrian reservoirs. The micro-paleo-geomorphologic high positions in the margins of the sags were beneficial to the development of the secondary pores in the Cambrian reservoirs (See the II, III and IV locations in Fig. 13). Being on the local high parts of the salient background, under the influence of the periodic changes of sea level, these positions were likely to be exposed to the surface and subject to the leaching of atmospheric fresh water. This was not only conducive to the formation of dissolved pores by dissolution in the quasi-contemporaneous period, but also beneficial to the formation of the secondary pores by dolomitization in the quasi-contemporaneous period and in the shallow burial period. For example, the Zhangxia Formation in Well Ling1 is bright crystalline oolitic dolomite, with high degree of dolomitization and abundant intergranular and intragranular dissolved pores (Fig. 14b); and the Zhangxia Formation in Well Ningtan1 has rich intergranular dissolved pores and intercrystalline solution pores (Fig. 14c). It can be seen that the high positions of the micro-paleo-geomorphology in the margin of the sags are not only conductive to the development of shoals, but also beneficial to formation of favorable reservoirs, so these positions are the development areas of high-quality reservoirs of the Cambrian.

Through comprehensive analysis of the hydrocarbon accumulation and exploration effectiveness, it is considered that in the high parts of the micro-paleo-geomorphology adjacent to the margin of the sags, not only the oolitic shoal facies usually with larger scales turns up, but also the primary and secondary pores of the reservoirs are well developed. Moreover, the hydrocarbon source rocks in the sags are relatively well developed and the source-reservoir allocation is in good combination. Therefore, the high parts of the micro-paleo-geomorphology adjacent to the margin of the sags are the favorable exploration areas for the Cambrian.

The Cambrian system in the Ordos basin mainly has three groups of faults, namely the NE-trending faults, the near EW-trending faults and the NW-trending faults, and the NE-trending and near EW-trending faults are the main faults of the Cambrian. According to the roles of the faults in the tectonic units and the scales of the faults, the faults of the Cambrian are divided into three orders: namely the second-order faults, the third-order faults and the fourth-order faults.

The second-order and third-order faults control the paleo- tectonic pattern of the Cambrian. The second-order faults control the macroscopic paleo-tectonic pattern of the alternate salient and sag in the Cambrian. The third-order faults control the micro-paleo-geomorphological characteristics of the high- low differentiation in the Cambrian. Under the joint control of the second-order and third-order faults, the Cambrian system presents the combining paleo-structural features of the macroscopic paleo-tectonic pattern of salient-sag alternation and the micro-paleo-geomorphology of high-low differentiation.

The paleo-tectonic pattern of the Cambrian controls not only the development of the favorable shoal facies, but also the distribution of the Cambrian high-quality reservoirs. The high positions of the micro-paleo-geomorphology adjacent to the margins of the Cambrian sags are the superimposed development zones of the favorable shoal facies and the favorable reservoirs, and therefore are the favorable exploration areas of the Cambrian.