The Ordos Basin is one of the largest petroliferous basins in China with rich oil and gas resources. After more than 50 years of exploration, in the low-permeability reservoirs of the Triassic Yanchang Formation, the Upper Paleozoic tight sandstone gas reservoirs and the Ordovician weathering crust natural gas reservoirs, five large oil-bearing areas with reserves of (15-20)×10⁸ t, one large gas-bearing area with reserves of 3×10¹² m³, and two large gas-bearing areas with reserves of 1×10¹² m³ have been discovered, making it the oil production base with the highest production in China^[1]. With the advancement of oil and gas exploration, researchers are very concerned about the oil and gas exploration potential of the Ordos Basin and the future exploration, especially since the recent discovery of the Neoproterozoic-Cambrian Anyue gas field in the Sichuan Basin^[2]. What is the prospect of the Mesoproterozoic-Cambrian in the Ordos Basin which is also in the Eurasian Plate the same as the Sichuan Basin and whether there are new types of domains in the Lower Paleozoic Ordovician besides the discovered weathering crust gas reservoirs.

Recently, through the study of three major cratons in China, Zhao et al.^[3] concluded that the Mesoproterozoic-Cambrian formations in the North China, Yangtze and Tarim Plates all had large-scale high-quality source kitchens, effective reservoirs and primary and secondary accumulation combinations.

In fact, some exploration discoveries have been obtained in the Ordovician and Cambrian in the Ordos Basin. For example, several wells in the western margin of the basin (such as Well Zhong 4) obtained low-yield flows in the Ordovician. Some wells such as Lintan 1, Xuntan 1, and Yaocan 1 in the southern part of the basin detected gas shows in the Cambrian-Ordovician. Low-yield gas reservoirs have been found in the Cambrian in Wells Long 17 and Long 26 near Qingyang in the central paleo-uplift of the basin. In addition, Well Tao 59 located in the Yimeng Uplift in the northern part of the basin revealed the source rocks of the Cuizhuang Formation of the Mesoproterozoic Qingbaikou System, with a total organic carbon content of up to 5.5%^[1]. Meanwhile, Well Jin 13 in the area showed obvious gas anomaly in the Mesoproterozoic Changcheng System (3 527-3 530 m) and obtained 23 970 m³/d of natural gas flow in the drillstem test^[4]. All these results fully demonstrate that the Mesoproterozoic-Lower Paleozoic in the Ordos Basin is a new field of ​​natural gas exploration that deserves attention.

Since the Ordos Basin is part of the giant craton Basin in North China, its geological conditions are inevitably affected or controlled by the evolution of the North China Craton and the evolution of the ancient trough in the periphery of the basin. In view of this, in this work, from the perspective of the splitting evolution of the North China Craton and the evolution of the Qilian-Qinling oceanic basin, we examine the development characteristics of marine source rocks, distribution of favorable reservoir facies, oil and gas migration direction, and accumulation combination in the Mesoproterozoic-Lower Paleozoic in the Ordos Basin, and on the basis of these, analyze the potential favorable exploration areas and zones, in the hope to provide reference for oil and gas exploration in the ancient strata of the Ordos Basin.

The Ordos Basin is developed on the ancient basement of the North China Craton, and is part of the North China Plate (Fig. 1a). It has experienced the aulacogen evolution in the Middle and Late Proterozoic, sedimentary stage of Paleozoic craton depression (the Early Paleozoic for marine deposition, the Late Paleozoic for marine-continental transitional deposition), and evolution stage of intracontinental basin in the Mesozoic and Cenozoic^[5], forming a relatively complete marine sedimentary sequence of the Mesoproterozoic-Lower Paleozoic (Fig. 1b).

The formation and evolution of the Ordos Basin is closely related to the structural development and evolution of the North China Plate, which is the result of the interaction between the North China Plate and the surrounding ancient Asian Ocean, the ancient Qilian Ocean, the ancient Qinling Ocean and the Tethys Ocean^[6,7,8]. Especially, the geologic events such as the splitting of the North China Craton and the closure of the ancient Qilian-Qinling Oceans to uplift and orogeny since the Paleoproterozoic-Mesoproterozoic have strong influences on the development of marine source rocks and oil and gas accumulation in the Ordos Basin.

In the Middle and Late Proterozoic, the North China Craton, mainly composed of the Ordos, Beijing-Liaoning and Hebei-Shandong blocks, had basically formed. It was next to the vast Mongolian Ocean on the north and the Qinling and Qilian Troughs on the south^[5] (Fig. 2). In the southern margin of the North China Craton, were a series of rift troughs (aulacogen) inserting into the interior of the Craton striking north and northeast direction. It was initially believed that there were three major rift troughs including Helan, Henan-Shaanxi- Shanxi, and Anhui-Jiangsu-Shandong^[5]. Recently, some researchers predicted the spatial distribution of rift troughs in the North China Craton with the latest gravity, seismic and drilling data^{[1, 3, 9]} (Fig. 1a). The results show there are five troughs in the south margin, namely Anhui-Henan, Henan-Shanxi, Shanxi-Shaanxi, Gansu-Helan Rift trough. Most of these rift troughs have a wedge-shaped profile that converges in the north and northeast directions and opens south and southwest (Fig. 1a). The Shanxi-Shaanxi, Gansu-Shaanxi, and Helan rift troughs in the present Ordos Basin extend in a northeast-southwest direction. The Gansu-Shaanxi rift trough in the central part of the basin extends to the northeast and may be connected to the Xing’anling-Mongolia rift trough in the north margin. The Shanxi-Shaanxi rift trough in the southern part of the basin extends eastward, enters the Qinshui Basin and is further connected to the Beijing-Liaoning rift trough. These rift troughs generally experienced three development stages, initial rifting, main fault depression and late depression, and generally control the distribution of the Mesoproterozoic. The corresponding types of formations deposited include the terrestrial volcanic rock-clastic rock, the hugely thick fluvial-littoral shallow sea clastic rock, and late expansive carbonate rock ones^{[1, 10]}.

The Ordos Basin developed and evolved just under the holding of these rifts or clamped basement faults. Although the abovementioned rift troughs were closed after the Jinning Movement, a unified North China Plate was formed, and the North China Plate including the Ordos Basin entered a new sedimentary period, the long-term recessive activities of these ancient rift troughs or basement faults directly or indirectly affect the development of overlying sedimentary caprock and even oil and gas distribution^[11]. For the Cambrian sediments, its sedimentary pattern inherited some of the Mesoproterozoic^[12]. In the early period of Early Cambrian (equivalent to the Meishucun and Qiongzhusi Stages), affected by the Mesoproterozoic rift trough or basement fault, a paleo-geomorphic pattern of alternate uplifts and sags came up along the southwestern margin of the North China Plate, with a number of deepwater bays (Fig. 2). The Helan Bay and Luochuan Bay located on the western and southern margins of the present Ordos Basin were in “L” shape around the periphery of the Qingyang Paleo-uplift, where a set of Early Cambrian signature deposits-phosphorus-bearing fine clastic sediments, which can be correlated globally, were developed. In the late period of Early Cambrian, the transgression expanded somewhat, and thus offshore sediments such as mud and sand flat, mud dolomitic flat developed in the southwestern margin of the basin. In the Middle Cambrian, the transgression expanded continuously, and the vast central and eastern parts of Ordos gradually turned from the sand mud flat to a constrained-open platform, and the southwestern margin of the basin evolved into a platform margin-deepwater trough. In the Late Cambrian, regression started, and the central and eastern parts evolved into a constrained platform mud flat, while the deepwater slope-trough in the southwestern margin of the basin remained^[13]. In the Early Ordovician, the Ordos was mainly a paleo-continent, and only in southeastern part of the basin emerged the mud dolomitic, and dolomitic and lime flats. In the Middle Ordovician, the North China Sea transgressed in large scale, the gypsum salt lake sediments were formed in the central and eastern parts of the basin, and constrained platform and open platform deposits developed outwardly. In the late period of Middle Ordovician, the southwestern margin of the basin subsided again, turning into platform front slope-deepwater trough. The Caledonian Movement in the Late Ordovician caused the Ordos to uplift overall, but the southern margin and the western margin subsided continuously, forming an “L”-type Qinling-Qilian Trough (hereinafter referred to as the “L”-type trough), where deepwater slope-trough sediments and eperic rimed platform deposits developed^[13,14].

The characteristics of the abovementioned tectonic sedimentary evolution indicate that the Mesoproterozoic have certain inheritance from the Lower Paleozoic in the Ordos Basin. In the early period of Mesoproterozoic, there was rift trough sedimentation, and in the late period of Early Paleozoic the rift trough was closed, and depression or deepwater bay deposits developed along the “L”-type trough margin and the original “wedge” rift trough.

The main body of the “L”-type trough (Qilian-Qinling orogenic belt) located in the southern and western margins of the Ordos Basin belonged to the Tethys tectonic domain. For a long time, the closure time of the Tethys oceanic basin (that is, the splicing time of the North China and Yangtze plates) has two different views, one is in the Late Caledonian-Early Hercynian period^[15,16] and the other is in the Indosinian period^[17,18]. Most researchers supported the understanding of the Indosinian collisional orogeny, that is in the Late Caledonian-Early Hercynian period, the Tethys Ocean between the two ancient plates of North China and Yangtze were belonged to the multi-island ocean system, and there were soft collisions between massifs and no strong orogenic activities. For example, Yin^[19] pointed out that “although the North China Plate and the Yangtze Plate began to unite in the Late Caledonian, it did not lead to an immediate full collision orogeny; instead, there were multiple opening and pulling-apart episodes; from the Late Devonian to the early period of Middle Triassic, the Qinling Mountains became the deep sea basin of the eastern Pacific Ocean with a westward trumpet-like opening to the ancient Tethys Ocean^[19]. Moreover, a large number of marine lamellibranchia and brittlestars fossils were found in the Lower Triassic Liujiagou Formation in the northern part of the Ordos Basin, stretching hundreds of kilometers from Lushan to Tongchuan^[20]. It is speculated that at least in the Early Triassic, the northern part of the area was the marginal bay of the Qinling-Tethys Ocean. At that time, the vast area from the southern part of Weibei to the north Qinling Mountains was low in terrain, and the Qinling Mountains had not yet formed.

A large amount of data showed that^{[19, 21-22]}, the Qinling oceanic basin finally closed together in a “scissor-style” way from east to west, that is, the Dabieshan-Hefei area was roughly connected at the end of the Early Permian-Middle Triassic, the Sanmenxia area in the west spliced ​​at the end of the Middle Triassic to the Late Triassic, and the West Qinling area didn’t completely collide until the late period of the Late Triassic- Early Jurassic. The paleomagnetic data further confirms that^[21], the “scissors-style” collision and closure of the Qinling-Dabieshan Oceans also caused the counterclockwise rotation of the North China Plate, which in turn resulted in the alternate collision orogeny and rifting subsidence in the southern margin of the North China Plate. Until the late period of the Late Triassic, the Qinling Ocean finally closed, the North China and the Yangtze Craton were spliced, and the Qinling area was fully collided.

As the closure of the Qinling oceanic basin (especially during the formation of the Northern Qinling orogenic belt) is related to the accumulation of Lower Paleozoic oil and gas in the southern margin of the Ordos Basin, the tectonic-sedimentary history of the Proterozoic-Early Paleozoic sediments in the southern margin of the North Qinling-North China Plate has been systematically studied in the work. Firstly, the contact relationship between the Upper and Lower Paleozoic in the study area was analyzed, and the contact relationship map of the strata was compiled (Fig. 3)^{[23,24,25,26,27,28,29]}. Fig. 3 shows that except the individual outcrop sections including Fanghougou in western Qishan and Jiuqinyuan of Cuoeshan in Jingyang, in the vast areas along the southern edge of the North Qinling-North China Plate (Xinyang, Lushan in He’nan to the east, and the Tongchuan in Shaanxi, and Pingliang in Gansu to the west), the Lower and Upper Paleozoic are in parallel unconformable contact, which fully indicates that the Caledonian Movement in the Late Ordovician period was mainly characterized by the overall structural uplift and subsidence rather than the folding orogeny. This is completely consistent with the abovementioned understanding that there was a “soft collision” between the Qinling-Tethys oceanic block and orogeny in the Qinling area happened in Indosinian. As for the angle unconformity contact between the upper and lower Paleozoic in the Qishan-Jingyang area, it can be explained as follows: because the Qishan-Jingyang area is located at the southwest corner of the Ordos block to the Qilian-Qinling Trough, a tectonic stress concentration zone was likely to occur during the closure of the oceanic basin, and the folding orogeny there started early. But this is only a local geological phenomenon.

Based on the above analysis and related data from predecessors^{[10, 30-31]}, the “L”-type trough-the tectonic sedimentary evolution profile of the Ordos Basin was compiled (Fig. 4). The section revealed that the western and southern sections of the L”-type trough in the Cambrian-Ordovician were tectonic slopes dipping toward the deep waters of trough, and there developed continental shelf-platform margin slope facies in the western marginal thrust belt and the southern margin of the Weibei-Qinling area. In the Silurian-Devonian, the Caledonian Movement caused the Cambrian-Ordovician formations of the Ordos Basin to be uplifted and denuded, and the paleo-uplift was gradually flattened, but the stratigraphic occurrence of the Cambrian-Ordovician in the south of the Qingyang Paleo-uplift had not changed, and remained tilting to the south. In the Carboniferous-Early Middle Triassic, the southern and western parts of the Ordos Basin were steadily subsided and accepted the transitional marine-continental sediments. At this time, although a certain amount of detrital material was supplied to the northern Longdong area due to the local uplift in the Qinling area, the strata generally maintained “overall ups and downs” and were tilting southward (Fig. 4a). This may be the reason for the parallel unconformity between the Upper and Lower Paleozoic in most areas of the study area. After the Late Triassic, the Qinling Trough completely closed in the southern margin of the basin, and the North Qinling was uplifted (Fig. 4a). The western margin of the basin was thrusted and the Tianhuan Depression was gradually formed (Fig. 4b). The paleo-uplift striking NS within the basin completely disappeared and the whole basin entered in the stage of continental lacustrine basin.

The understanding of the closure of orogenic mountains after the Late Triassic in the North Qinling area can explain the oil and gas generated by the Lower Paleozoic source rock in the “L”-type trough in the early stage (End of Permian-Early Middle Triassic) migrating toward the Qingyang Paleo-uplift, and the details are described as below.

The opening and closure of Mesoproterozoic rift trough and the “L”-type trough not only controlled the type and distribution of MesoProterozoic-Lower Paleozoic sediments, but also controlled the development of source rock, distribution of favorable reservoir facies belt, oil and gas migration direction and reservoir combination type in the Ordos Basin.

Controlled by the abovementioned regional tectonic-sedimentary environment, the Ordos Basin and the periphery have three sets of source rock, the Mesoproterozoic, Cambrian and Upper-Middle Ordovician.

2.1.1. Mesoproterozoic source rocks

The available data indicates that^{[1, 3, 32]}, the Mesoproterozoic source rocks in the North China Craton are mainly distributed in the Beijing-Liaoning rift trough, the northern marginal rift trough, the Shanxi-Shaanxi rift trough, and the He’nan-Shanxi rift trough (Table 1, Fig. 1a). For example, many wells in the Beijing-Liaoning rift trough encountered high-quality marine source rocks. Among them, the argillaceous source rocks in the Xiamaling Formation and the Hongshuizhuang Formation have an average organic carbon content of 5.2% and 4.1%, respectively. The average organic carbon content of marine mudstone source rocks in the Chuanlinggou Formation of Changcheng System was 2.0%^[3]. Moreover, a number of ancient oil and gas reservoirs supplied by the source rocks in the Hongshuizhuang Formation and the Tieling Formation have been discovered in the rift^[32]. In addition, in the northern marginal rift trough (Guangyang in Inner Mongolia), the Shanxi-Shaanxi rift trough (Luonan in Shaanxi, Yongji in Shanxi), and the He’nan-Anhui rift trough in the southern edge of the North China Craton, Mesoproterozoic marine argillaceous source rock was found^[3] (Table 1, Fig. 1a).

Recently, Well 59 in the basin (belonging to the Gansu- Shaanxi rift trough) and Well Jitan 1 drilled in the Weihe Basin in the southern margin of the basin (belonging to the Shanxi-Shaanxi rift trough) revealed source rock of the Changcheng System (Table. 1, Fig. 1a). The argillaceous source rock encountered by Well Jitan 1 has a thickness of 22 m and an organic carbon content of up to 0.98%. The argillaceous source rock in Well Tao 59 has an organic carbon content of generally 3.0 % to 5.0 %, on average 3.6%, representing medium-good source rock^[1].

The argillaceous source rocks in the Changcheng Formation revealed by Well Tao 59 correspond to a set of reflection with strong amplitude on the seismic section^[3]. Based on this information and previous data, the mudstone (source rock) in the Changcheng System in the Shanxi-Shaanxi rift trough and Gansu-Shaanxi rift trough within the Ordos Basin was predicted with two-dimensional seismic data at 50-100 m thick generally, and thicker at the western and southern margins, with a maximum thickness of 100 m (Fig. 5a). In summary, it is highly likely that large-scale source rocks exist in the internal rift trough of the basin, but the result needs further verification.

2.1.2. Cambrian source rocks

The Cambrian is an important period of source rock development in the world. Both the Yangtze and Tarim cratons in China have Cambrian source rock^[33,34]. It has always been considered the Cambrian of the North China Craton was dominated by oxidizing environment so there was no source rock developed. But researchers discovered black mudstone in the Lower Cambrian Madian Formation in the Hefei Basin on the southern margin of the North China Craton, with a thickness of 20-40 m and an average TOC value of 6.13 %, representing good source rock^[33]. Controlled by fault depression in distribution, it deposited in reduction environment of starving continental shelf. Its lithological characteristics, stratigraphic sequence and sedimentary tectonic setting are very similar to those of the source rock in the Qiongzhusi Formation in the Sichuan Basin (Yangtze Block) and those of source rock in the Yu’ertusi Formation in the Tarim Basin^[33]. The source rock of the Madian Formation is equivalent to the depositional period of the Lower Cambrian Dongpo Formation in the western He’nan-Luonan area^[33]. Recently, the authors also found a set of lightly metamorphic gray-black argillaceous slate in multiple outcrops in the north Qinling and the south slopes of the southern margin of the Ordos Basin. The slate layer is 2 to 80 m thick in the bottom of Cambrian^[37], and in foliage shape after weathering (Figs. 2b and 6). Analysis shows it has a TOC value of 0.19%-11.18%, on average 3.14% (45 samples), Type I organic matter, and high thermal evolution degree, belonging to high-abundance high-quality source rock (Table 1).

Considering that the Lower Cambrian source rocks are mainly controlled by deepwater shelf environment and paleo-geomorphic rift trough (bay)^[33], in order to further predict the distribution of source rocks, the authors analyzed the paleo-structural characteristics of the North China Plate before the deposition of Lower Cambrian, and based on outcrops and previous data, the distribution map of the residual strata of the Lower Cambrian in the southern margin of the North China Plate (Fig. 5b) was compiled. It can be seen from Fig. 5b that the Cambrian is thickest in the southern part of the Ordos Basin-North Qinling area, which was probably the hydrocarbon generation center of the Lower Cambrian. It is speculated that the Cambrian source rock may still exist in the Fuping-Luochuan area now.

In addition, Well Tao 59 in the northern part of the basin revealed dark-colored mudstone of 30.5 m thick in the Xuzhuang Formation in the central part of the Cambrian. Pyrolysis analysis show the mudstone has a TOC value from 0.3% to 0.4% (Table 1, Fig. 2b). Core analysis shows that the mudstone has a TOC value of up to 0.67 % locally and Type I-II₁ organic matter. At the same time, the authors also found a set of dark gray-black fine-grained clastic sediments with a continuous thickness of more than 20 m on the outcrop of the Xuzhuang Formation in the Shanghan Section of the Liquan area in the southern part of the basin (location in Fig. 2b). In it, the dark gray shale is 6 m thick, the gray-black mudstone is 1.5 m thick, and its lateral extension straight line is more than 5 km from outcrop tracking (covered by thick vegetarian). Although the outcrop samples have low organic carbon content (as ancient source rock, it experienced long-term physical weathering, and all samples have TOC value of less than 0.3%). But the possibility that the mudstone turns better in hydrocarbon generation indexes in the basin can’t be ruled out.

The discovery of the high-abundance source rock in the North Qinling-Luonan and the “dark-colored clastic rock series” of the Xuzhuang Formation in the basin changed the previous view that the Cambrian in the Ordos Basin was dominated by carbonate platform sediments, and there were no paleo-structural-geographical conditions for source rock development. The exploration significance of this discovery deserves attention.

2.1.3. Middle-Upper Ordovician source rocks

The Middle-Upper Ordovician source rocks were developed in the platform margin slope and the intra-platform depression^[1]. The Upper Ordovician marine source rocks are mainly developed in the deepwater shelf-slope environment of the “L”-type trough in the western and southern parts of the basin. The source rocks of the Qilian Trough in the west were developed in the Middle and Upper Ordovician Wulalike Formation-Lashizhong Formation/Pingliang Formation, in which the argillaceous source rocks contained a cumulative thickness of 100-200 m (Fig. 5c) and high organic matter abundance (Table 1), representing good source rock. The source rocks in the southern Qinling Trough mainly include marlite in the Middle-Upper Ordovician Beiguoshan Formation, the graptolite in the Pingliang Formation, and the limestone in the sixth member of the Majiagou Formation. Among them, the Upper Ordovician marlite and mudstone have a thickness of 220-360 m and high abundance of organic matter, with the highest TOC value of 5.64%, representing good source rock^[1] with high hydrocarbon generation potential.

The Middle Ordovician marine source rocks were developed around the intra-platform salt depressions in the North China Sea in the eastern part of the basin (Fig. 5d). They were dominated by marl, and the single layer was thin and multifarious. The source rocks contained a TOC average of 0.2% (Table 1), which had certain hydrocarbon generation potential and had an important contribution to the self-generation and self-storage gas reservoirs of the Ordovician salt^{[1, 35-36]}.

It should be emphasized that the latest research shows that the Middle-Upper Ordovician source rock located in the marginal slope area generally contain flaky micropores and micro-fractures with certain storage capacity. Because the source rocks are dominated by lime mudstone (shale) (in the western margin) or argillaceous limestone, pure limestone intercalated with shale (in the southern margin), and contain relatively high content of brittle minerals which are conducive to the construction and transformation of shale gas. Therefore, the Middle-Upper Ordovician source rocks (especially the “L”- type trough shelf-slope environment) are important for both conventional and unconventional gas accumulation.

Controlled by the coastal environment, paleo-uplift and platform margin, the Mesoproterozoic-Lower Paleozoic in the Ordos Basin mainly has four types of reservoirs, namely the Lower Cambrian (Xinji-Zhushadong Formations, Mantou- Maozhuang Formations) and Mesoproterozoic littoral shallow sea quartz sandstone, Middle and Upper Cambrian-Ordovician weathering crust and dolomitized reservoir and Ordovician “L”-type marginal reef shoal body. In contrast, the Mesoproterozoic carbonate rock experienced weak weathering and leaching effects due to the low oxygen content in the atmosphere at that time, and strong recrystallization due to multi-stages of tectonic thermal events, so it has poor reservoir quality on the whole.

The Lower Cambrian (Xinji-Zhushadong Formations, Shantou-Maozhuang Formations) littoral shallow sea quartz sandstone reservoirs are intermittently distributed along the coastline in the western and southern margins of the basin. According to literature^[13], the set of reservoirs had good physical properties and a total thickness of 30-100 m. Through field survey in the Pengyang ancient town in the southern part of the basin and the Mantou Formation in the Luonan area recently, the authors found that this set of sandstone was mainly medium feldspar quartz sandstone with a thickness of about 30 m and a stable distribution. The sandstone has good roundness and a porosity of up to 11.6%, representing good reservoir. In addition, sandstone reservoirs are also common in the Mesoproterozoic Changcheng System in the basin^[37,38], which is mainly quartz sandstone. Although old in age and experiencing strong diagenesis, this sandstone still has some storage space, largely residual intergranular pores and secondary dissolved pores.

The Middle and Upper Cambrian-Ordovician weathering crust and the dolomitized reservoirs are controlled by the paleo-uplift striking NS. The weathering crust reservoir is the most important reservoir type in the Lower Paleozoic of the basin. It was previously thought that this type of reservoir mainly developed on the top assemblage of the Ordovician Majiagou Formation (Ma 5¹-Ma 5⁴ Submember), and the Jingbian gas field with reserves of hundreds of billion cubic meters was discovered following this idea^[39]. But with the deepening of exploration in recent years, it has been found that karst or weathering crust reservoirs existed in both the deep lower assemblage of the Ordovician Majiagou Formation (Ma 4 Member and below) and the Cambrian (Zhangxia - Sanshanzi Formations). In addition, the dolomitized reservoirs are commonly seen in the abovementioned strata^[39,40]. Geographically, the weathering crust reservoir is not limited to the central and eastern parts of the basin where the Jingbian gas field is located, but also occurs in the paleocontinent striking NS, especially in the Qingyang Paleo-uplift and its eastern flank. The dolomitized reservoir (dolomite body) is also controlled by the paleo-uplift striking NS and is distributed on a large scale in the paleo-uplift and the east and west slopes^[41,42].

In fact, from the end of the Ordovician to Early Carboniferous, due to the Caledonian tectonic uplift and the existence of paleo-uplift striking NS, on one hand the Upper Cambrian-Ordovician in the study area suffered long-term weathering and karstification; on the other hand, the paleo-uplift area was often exposed intermittently, thus the limestone in the shallow burial stage was likely to dolomitize under the effect of atmospheric fresh water^[40]. These two factors may mainly responsible for the development of large-scale weathering crust dissolved pore reservoirs (resulted from the dissolution of gypsum-bearing rock or limestone) in the Ordovician Majiagou Formation and the dolomitization reservoirs (the original rock is the shoal facies limestone) in the Cambrian Zhangxia-Sanshanzi Formations in the central paleo-uplift area and its periphery.

The Ordovician “L”-type platform margin reefs are mainly distributed in the transitional parts between the trough and the Ordos platform, including the current western marginal thrust belt and the Weibei area in the southern margin. On horizon, they are mainly distributed in Middle Ordovician Kelimoli Formation and the Upper Ordovician, with a total area of ​​about 3×10⁴ km². After late reformation, the reef-shoal facies mainly has two types of effective reservoirs, limestone and dolomite. So far, several wells in the area have encountered caverns and caves and obtained good gas shows, confirming the effectiveness of such storage space^{[39, 43]}.

According to source rock, reservoir, and exploration practice in recent years, the accumulation combinations in the Mesoproterozoic of the basin can be divided into two categories: the intra-platform and the platform margin (Fig. 7).

The intra-platform category consists of two sub-categories, “source-above-reservoir” and “source-reservoir-in-one”. The source-above-reservoir one is the dominant type. Specifically, it refers to the combination of the overlying Carboniferous-Permian coal measure source rock and the Cambrian- Ordovician reservoirs. In this combination, the gas generated by the source rock migrated through the unconformity down to the reservoirs to accumulate. The Jingbian large gas field in the central east of the basin and a series of important discoveries (the Well Chengtan 1 obtained a weathering crust gas layer of 1.2 m in the Ma 2 Member, and low-yield natural gas flow of 1.26×10⁴ m³/d in gas test) in the pre-Carboniferous weathering crust in the Longdong area are all belonged to this type. The source-reservoir-in-one refers to the marine marl source rock under the salt sag in the eastern basin and the dolomite reservoir within salt depression. Currently, some reservoirs of this type have been found (such as Well 74 drilled a gas layer of 5.5 m in the subsalt of Ma 5 Member, which was tested a high-yield industrial gas flow of 127.98×10⁴ m³/d), but the scale of source rock and the resource potential need to be further confirmed.

In the platform margin zone, as there are Cambrian and Ordovician etc. several sets of source rocks and multiple types of reservoirs, the source reservoir combinations are plentiful, including both marine shale gas reservoirs and conventional gas reservoirs. The former are mainly distributed in the Middle and Upper Ordovician source rocks in the marginal slope area (described earlier), the latter generally have two subtypes, “source-above-reservoir” and “source-reservoir-in-one”. The source-above-reservoir type is similar to that within the platform, referring to the overlying Carboniferous-Permian coal measure source rock and the Cambrian-Ordovician reservoir. The source-reservoir-in-one type refers to the Cambrian- Ordovician source rock (such as the source rock layers in the Wulalike-Lashizhong Formations) and the Cambrian-Ordovician reservoir (Fig. 7).

In addition, regardless of the internal platform or the platform margin, there may be source-below-reservoir type combination (Fig. 7) in the case of deep and large faults connecting the Changcheng System rift trough, that is, the source rock in the Changcheng System and the upper Cambrian-Ordovician reservoir. Although this type of combination has not been proved by exploration in the Ordos Basin, it is worth of attention.

According to the above analysis of source rock, reservoir distribution and source-reservoir combination characteristics, it is considered that there are four new fields of natural gas exploration in the Mesoproterozoic-Paleozoic in the Ordos Basin (Fig. 8), the pre-Carboniferous in Qingyang Paleo-uplift, the “L”-type marginal belt in the southwestern margin of the basin, the Ordovician subsalt combination and the Mesoproterozoic-Cambrian System in the central and eastern parts of the basin. Among them, the pre-Carboniferous weathering crust and the “L”-type marginal facies belt are more realistic replacement areas. The Ordovician subsalt combination and the Mesoproterozoic-Cambrian have good exploration potential and deserve further prospecting.

The paleo-uplift is one of the important geological factors controlling oil and gas accumulation in sedimentary basins^[34]. The Qingyang Paleo-uplift is located in the southwestern part of the basin, covering an area of ​​over 10 000 square kilometers (Fig. 8). It is generally believed that it was formed in the Early Cambrian, and evolved into the “L”-type central paleo-uplift^[30,31] separating the Qinling-Qilian Seas and the North China Sea in Ordovician. In Carboniferous-Permian, it was adjusted to a low uplift, and eventually disappeared in Middle-Late Cretaceous (Fig. 4).

It is well known that the tectonic uplift at the end of the Ordovician period caused long-term weathering and erosion of the Lower Paleozoic sediments in the Qingyang Paleo-uplift and its periphery, where Ma 5-Ma 1 Members, Sanshanzi Formation, Zhangxia Formation, and the Xuzhuang Formation etc. sequentially expose to the surface from east to west (Fig. 8). At the same time, due to the paleo-geomorphology high in west and low in the east, karst highland, karst terrace, karst slope and karst depression came up in sequence from west to east^[39]. Because the Ordovician Majiagou Formation and the Cambrian Sanshanzi-Zhangxia Formations around the paleo-uplift were mainly composed of gypsum- bearing dolomite or shoal grainstone, the weathering crust and dolomitization reservoir were likely to be created after long-term exposure and weathering leaching. The weathering crust dissolved vug or shoal dolomite reservoirs combining with the overlying Carboniferous-Permian coaliferous source rock would form effective natural gas accumulation.

At the end of the Indosinian-Yanshan Period, the paleo-uplift disappeared with the uplifting of the eastern part of the basin. The paleo-geomorphology became high in the east and low in the west. The gas from the Upper Paleozoic migrated along the weathering crust window to the updip direction in the east, when blocked by the paleo-geomorphology and lithologically tight belt, the stratigraphic-lithologic trap gas reservoir was created (Fig. 9a). At present, several exploration wells (such as the Well 17) have obtained natural gas flow from the Lower Paleozoic, revealing that the pre-Carboniferous weathering crust on the eastern side of the paleo- uplift has potential conditions to form oil and gas reservoir.

This domain refers to the transitional area between the Ordos platform and its western marginal Helan Trough and the southern margin Qinling Trough (Fig. 8). In the Cambrian- Ordovician, the area was located in the platform margin-continental shelf-slope environment, where multiple types of source-reservoir-caprock combinations developed, providing good geological conditions for conventional and unconventional natural gas accumulation.

As far as shale gas is concerned, large-scale effective marine source rock has been developed on the southern and western margins of the “L”-type trough, which has certain storage capacity as mentioned above. Recently, quite a few exploration wells in the Majiatan Area, the Yindongzi Area in the southern margin of the basin and the Linyou Area in the western margin of the basin detected abnormal gas shows in the glutenite in the Wulalike Formation/Pingliang Formation, and the Beiguoshan Formation. Especially in the western margin of the basin, Well Zhong 4 tested an industrial gas flow of 4.14×10⁴ m³/d in the mudstone interval, indicating that the source rock contains abundant shale gas resources, therefore deserves attention.

In terms of conventional natural gas, in the western margin of the basin there developed high-energy shoal, lagoon, platform margins and open sea continental shelf facies from east to west in the Ordovician Kelimoli Formation. Among them, reef shoal deposits such as pelleted limestone, grainstone and biogenic limestone are widespread in the high-energy shoal and platform margin uplift, which evolved into dolomite and karst fracture-vug reservoirs due to dolomitization and karstification in the later period^[1]. Meanwhile, this domain has multiple sources to supply hydrocarbon. The oil and gas generated by the Carboniferous-Permian coal measure source rock and the source rock of Ordovician Wulalike Formation can migrate to the paleo-uplift and accumulate in the Ordovician reef body.

It should be noted that as the structures in this area mainly belong to the western thrust belt and the east and west flanks of the Tianhuan Depression, with many faults, the accumulation conditions are complicated here. Influenced by the activities of the western marginal thrust belt and the migration and evolution of the Tianhuan Depression, the oil and gas reservoirs formed in the early stage could adjust somewhat. But as there are mudstone or weakly dolomitized marlite in the lagoon area or the fault in the updipping direction with sealing capacity, complex structural-lithologic and stratigraphic-lithologic gas reservoir are likely to come up (Fig. 9b). Wells Zhongtan 1 and Li 16 are two typical examples, which both obtained low-yield gas flow from the Ordovician. Moreover, recent studies have suggested that footwall of major thrust and large wide gentle syncline belt have weaker structural deformation, especially the Lower Paleozoic has better preservation conditions^[40].

The southern margin of the “L”-type marginal belt mainly refers to the Weibei Uplift belt in the southern part of the basin. In the Cambrian-Ordovician, this area was next to the Qinling Trough in the south and the Qingyang Paleo-uplift in the north. This area has Ordovician platform margin reef shoal, dolomitized reservoirs in the Middle Cambrian Zhangxia- Sanshanzi Formations, the Lower Cambrian (Xinji-Zhushadong Formations, Shantou-Maozhuang Formations) trans-continental littoral shallow sea quartz sandstone and multiple sets of reservoirs. Also this area has thicker Pingliang Formation marine mudstone, and is adjacent to the source rock areas in Mesoproterozoic Shanxi-Shaanxi rift trough and Cambrian bay (Fig. 8), so it has the advantage of multiple sources of hydrocarbon supply. As the Qingyang paleo-uplift was long-term uplift, oil and gas from various sources may migrate to the paleo-uplift and eventually accumulate in the favorable Cambrian-Ordovician reservoirs. Despite the uplift of late tectonic activity (from the end of the Indosinian-Yanshan Movement), the area has tilted and turned from the south-dipping trough to an uplifting area^[10], because of the larger burial depth of the Cambrian-Ordovician, the structurally stable zone still has fairly good preservation conditions, and it is speculated that structural-lithologic composite gas reservoirs related to the reef body and dolomite body can be formed (Fig. 9d). Although no discovery of this kind has been found yet, as this area has always been the pointing area (details as described below) of oil and gas migration in the deepwater-estuary environment in the North Qingling area before the Late Triassic (Late Indosinian Movement), it is the most favorable exploration area of ancient marine source rock in the basin currently, and is expected to become a new replacement zone for natural gas exploration.

In general, compared with the internal platform, although the platform edge belt has multiple generation-reservoir- caprock combinations, conventional gas reservoirs are more difficult to preserve due to strong tectonic activities. The effectiveness of structures and traps needs to be examined closely next to sort out zones relatively stable in structure to search for oil and gas.

The Ordovician in the central and eastern parts of the basin can be divided into two combinations: “supersalt” and “subsalt”. In the supersalt combination, the large Jingbian gas field has been discovered (Ma 5^1-5 Submember)^{[1, 35]}, so it will not be discussed herein. The subsalt combination discussed in this paper refers to the Ordovician Ma 5⁷ submember-Ma 1 Member, which geographically includes the deep layers of the eastern Yancheng and Jingbian weathering crust gas reservoirs, with an exploration area of ​​about 10.0×10⁴ km². The Ma 5⁶ gypsum salt layer overlying the combination has a large thickness, large distribution area, and strong sealing ability, so it can act as a regional cap layer for the subsalt gas reservoir. The Ma 5⁸ Submember, Ma 5¹⁰ Submember, and the deeper Ma 1 and Ma 3 Members also have gypsum layers, which can act as direct covers for the subsalt gas reservoirs. Previous studies show that^{[1, 35-36]} the algae-containing limestone in the intra-platform salt sag has certain hydrocarbon generation capacity, and the inter-salt Ma 5⁷ Submember, Ma 5⁹ Submember, and Ma 4 Member have dolomite reservoirs, which is favorable for the formation of natural gas reservoirs (Fig. 9c). In fact, as mentioned above, Well 74 in the Yancheng area has obtained high-yield industrial gas flow from the Ma 5⁷ Submember, which proves that the subsalt is the main field for looking for “inner” Ordovician natural gas reservoir.

From a global perspective, high-quality source rock often occurs before salinization of seawater in the occluded salt basins or evaporative lagoon environment, while the salt rock after salinization provides good sealing conditions for subsalt oil and gas accumulation^{[1, 44]}. Therefore, the next step is to confirm the main source rock and resource potential in this field and sort out the main controlling factors of gas reservoir development. On this basis, reliable traps can be selected to drill in priority, to seek new discoveries.

Large-scale effective source rocks haven’t been found in the Mesoproterozoic-Cambrian System in the Ordos Basin. High-abundance source rock has only been discovered in the Luonan area of ​​the northern Qinling Mountains, and only a few wells in the basin encountered “dark mudstone” with certain hydrocarbon generation potential in the Xuzhuang Formation (Well Tao 59). Therefore, the main exploration work around this layer system has two aspects: (1) to continue to find large-scale effective source rock within the basin; (2) to explore the contribution of Cambrian high-abundance source rock in the Luonan area of ​​the northern Qinling Mountains to the oil and gas accumulation in the basin itself, especially in the southern margin of the Qingyang Paleo-uplift. Below, the authors focus on the analysis of the latter.

The contribution of the Cambrian high-abundance source rock in the Luonan area of ​​the North Qinling to the internal reservoir formation involves the issue if the hydrocarbon generation and expulsion periods of the Lower Paleozoic source rocks matched with the uplift of the Qinling Orogenic belt. For the rise of the Qinling orogenic belt, as mentioned above, the authors believed that the period of the full rise of the North Qinling Mountains should be from the Late Triassic. For the hydrocarbon generation and expulsion periods of the Lower Paleozoic source rock, a lot of research has been done before. He^[45] believed that the Lower Paleozoic source rocks entered high maturity at 240 Ma and its thermal evolution stopped after the uplifting in the Indosinian Movement. Dai^[33] suggested that the Madian Formation source rock started to generate hydrocarbons in the Late Caledonian. Zhu et al.^[46] considered that at the end of the Permian, most of the source rock ​​exceeded the R_o of 1.0% and entered the peak of hydrocarbon generation. Wang et al.^[47] concluded that the large- scale hydrocarbon generation period of the Lower Paleozoic source rocks in the southern North China region was at the end of Permian, and that of the deep Lower Cambrian source rocks might be earlier. Obviously, it is widely acknowledged that the first large-scale hydrocarbon generation and expulsion periods of the Lower Paleozoic source rocks in the study area was at the end of the Permian to Early-Middle Triassic. That is, the Qinling Mountains had not yet risen before the Late Triassic. From this point of view, the Qingyang Paleo-uplift was the pointing direction of oil and gas migration. The oil and gas generated by the Cambrian high-abundance source rock and the Mesoproterozoic source rock in the North Qinling-Luonan area are completely possible to migrate to and accumulate in the northern paleo-uplift (Fig. 10). At present, the ancient asphalt or gas shows discovered in the Zhangxia Formation of the Well Lintan 1, Xuntan 1, and Yaocan 1 in the southern flank of Qingyang Paleo-uplift may be the embodiment of this process.

It should be noted that due to the late structural transformation, the Qingyang Paleo-uplift disappeared and the northern Weibei Uplift was formed, the oil and gas reservoirs formed earlier would be adjusted. The influence of structural change on paleo-reservoirs needs to be investigated to find out favorable exploration targets.

At present, in addition to the study on the weathering crust of the Ordovician top in the Ordos Basin, the Mesoproterozoic- Lower Paleozoic is generally low in exploration degree, there are still many new areas in new strata or old strata, and many basic geological problems haven’t been resolved yet. The pre-Carboniferous weathering crust reservoirs which are expected to be a replacement domain have high heterogeneity, so the karst paleotopography needs to be described in detail to find favorable reservoirs. The “L”-type marginal zone has strong tectonic activities, making it difficult to preserve natural gas reservoirs, so the relatively structure stable zone needs to be sort out for exploration. In view of this, the authors made the following suggestions: (1) To strengthen basic geological research. This mainly includes the study on the fine characterization of pre-Carboniferous weathering crust karst paleotopography in the Qingyang Paleo-uplift, the identification of subsalt gas source and the confirmation of lithologic trap targets of the Ordovician in the central and eastern parts of the basin, interpretation of geological structure and trap effectiveness of the “L-shaped” platform margin zone, and the shale gas exploration potential and evaluation research. Meanwhile, paleo-tectonics, paleo-hydrocarbons, paleo-sedimentation and paleo-accumulation of the Mesoproterozoic in the Gansu-Shaanxi rift trough, Shanxi-Shaanxi rift trough, and the Cambrian estuary should be examined comprehensively, to lock in the main exploration targets as soon as possible. (2) To strengthen the research on key seismic technology. It is recommended to take the middle and northern sections in the Tianhuan Depression-Majiatan area, the southern section of the Tianhuan Depression-Zhengning area, and the Qingyang Paleo-uplift as the key areas, to deploy high-precision two-dimensional seismic lines, to improve the deep seismic signal- to-noise ratio, to tackle key technologies on the reservoir prediction and fine structure interpretation of karst vug zone, platform margin reef shoal, and to select targets to carry out risk drilling.

The Ordos Basin and its periphery has three sets of source rocks, Mesoproterozoic, Lower Cambrian bottom, and the Middle and Upper Ordovician. Among them, the Mesoproterozoic source rocks are uniformly distributed in the deep continental shelf in the periphery of the basin and the Gansu-Shaanxi and Shanxi-Shaanxi rift troughs. The source rock at the bottom of Lower Cambrian is newly discovered, which mainly developed in the southern margin of the basin-the northern Qinling area (rift) and the deepwater continental shelf environment. The Middle-Upper Ordovician marine source rock mainly developed in the “L”-type trough continental shelf-slope belt and the intra-platform salt sag.

Controlled by the coastal environment, paleo-uplift and platform marginal environment, the Lower Paleozoic mainly has four types of reservoirs, namely the Lower Cambrian (Xinji- Zhushadong Formations, Mantou-Maozhuang Formations) and the Mesoproterozoic littoral shallow sea quartz sandstone, Middle-Upper Cambrian-Ordovician weathering crust reservoirs, and the Ordovician “L”-type platform margin reef shoal.

The accumulation combinations in the study area vary greatly, the intra-platform accumulation combinations are dominated by “source-above-reservoir” type (Carboniferous- Permian source rock, the Cambrian-Ordovician reservoir and caprock), followed by “source-reservoir-in-one”. The platform margin zone has many types of accumulation combinations, including marine shale gas reservoirs and conventional gas reservoirs, “source-above-reservoir” type and “source-reservoir-in-one” type (Cambrian-Ordovician source and reservoir). In addition, regardless of the intra-platform or the platform margin, “source-below-reservoir” accumulation combinations would occur (source of the Changcheng System, reservoir in the Cambrian) in the case that deep and large faults connect the ancient rift trough in the Changcheng System.

In the Mesoproterozoic of the Ordos Basin, there are four natural gas exploration fields, pre- Carboniferous weathering crust of the Qingyang paleo-uplift, the “L”-type marginal belt, Ordovician subsalt combination and the Cambrian-Mesoproterozoic rift trough (estuary). Among them, the pre-Carboniferous weathering crust and the “L”-type marginal facies belt are more realistic replacement areas. The Ordovician subsalt combination and the deep Mesoproterozoic-Cambrian in the basin have certain exploration potentials, and are worth further exploring.

The authors express their sincere gratitude to Sun Liuyi and Ren Junfeng from the Research Institute of Petroleum Exploration and Development of PetroChina Changqing Oilfield Company and Guo Yanru, Zhou Jingao, Wu Xingning, Zhao Zhenyu, Liu Junbang, Zhang Yueqiao, Gao Jianrong, Zhang Yanling, Li Ningxi and others from PetroChina Research Institute of Petroleum Exploration and Development for their work in the study.