Numerous significant tectonic and environmental events occurred from the Ediacaran to Cambrian, including the assembly of Gondwana, the extinction of Ediacaran fauna, the emergence of small shell fauna, the sea level rise, and the anoxic event in shallow shelf areas^{[1,2,3,4,5,6,7,8,9,10,11]}. Under this background, a set of black organic-rich shale was deposited globally (e.g. in Amman, Morocco, Siberia, Canada, and Southern China) at the bottom of the Lower Cambrian^[8,9,10,11]. This black shale is referred as the Lower Cambrian Yurtus Formation source rock in the northern Tarim. In recent years, the hydro- carbon generation potential of the Yurtus source rock has attracted considerable interest^{[12,13,14,15,16,17,18,19,20,21]} due to the expansion of oil and gas exploration to deep basin. The black organic-rich shale in Yurtus Formation at the Akesu-Keping region has the highest organic carbon content (18%) among marine shales in China^[18]. The updated oil source correlation parameters also confirm that the Lower Paleozoic oil and gas in the Tabei uplift and the western Tazhong uplift are mainly sourced from the Yurtus Formation^[14].

Currently, only one well has drilled Yurtus Formation, with a thickness of 33 m, in the interior of the Tarim Basin, while other six wells in the Tadong area drilled Xishanbrak Formation, a possibly contemporaneous deposit with a thickness below 100 m. Because of small thickness and deep burial, it is very difficult to identify and trace the Yurtus Formation in seismic section. Therefore, its distribution was usually inferred from the Middle Cambrian paleogeographic pattern of the “East Basin and West Platform” that is bounded by a nearly NS-direction platform margin. The Yurtus Formation in the east is of basin facies with a thickness of 10-20 m, and that in the west is of carbonate platform facies with the maximum thickness of 50 m^{[17,18,19,20,21,22]}. The lithologic assemblage of the Lower-Middle Cambrian in the Sichuan Basin is similar to that in the Tarim Basin^{[21,22,23,24]}, which consists of clastic rock, carbonate, and evaporite from the bottom up, indicating similar basin evolution and filling processes. However, the Early Cambrian paleogeographic pattern of the Sichuan Basin is different from that in the Middle Cambrian^[23,24], as marked by the extinction of the Early Cambrian NS-trending rift trough and the formation of the Middle Cambrian central Sichuan paleo-uplift. In other word, the Middle Cambrian paleogeographic pattern had changed greatly with the emergence of evaporite. Therefore, it is unreliable to infer the distribution of the Lower Cambrian Yurtus Formation from the Middle Cambrian paleogeographic pattern.

Multiple reservoir-cap assemblages occur onto the Yurtus source rock in the northern Tarim^[23]. More than 10 wells have been drilled over last 20 years since the discovery of the high-quality reservoir-cap assemblage of the Lower Cambrian Xiaoerbulake dolomite and the Middle Cambrian Shayilike evaporite from the well He 4. However, industrial oil and gas were only found in the Middle Cambrian intra-salt (well Zs 1: 6 426-6 487 m) and subsalt (well Zs 1: 6 597- 6785 m, Zs 1C well: 6 861-6 944 m) layers in the Tazhong uplift. Since the distribution of the Middle Cambrian gypsum-salt caprock has been confirmed by seismic and drilling data^{[13, 20-22]}, the distribution and scale of the Yurtus source rock as well as its spatial coupling with reservoir are the key to studying the subsalt and deep-basin hydrocarbon geology. Based on the Precambrian tectonic evolution, this paper discusses the Early Cambrian syndepositional structure and the paleogeographic pattern it controlled using field observation, drilling, and seismic data in attempt to establish the Cambrian subsalt source-reservoir configuration and hydrocarbon accumulation models that would provide new insight into the deep oil and gas investigation and exploration in the Tarim Basin.

The Tarim Craton is composed of the Neoarchean to Neoproterozoic metamorphic basement and the overlying Nanhua to Cenozoic marine-continental deposits^[25,26] (Fig. 1). In the Early Neoproterozoic, the collision and collage of the southern and northern Tarim blocks and adjacent blocks formed the unified cratonic basement to constitute a part of the Rodinia supercontinent^{[1, 25]}. Along with the breakup of Rodinia during the Nanhua period, intensive rifting occurred in the interior and margin of Tarim. Recent studies indicate that the northern and southern rift systems might result from different dynamic mechanism, and thus showing distinct distribution pattern and evolution process^{[27,28,29,30,31]}. Specifically, the southern rift might initiate in the early Nanhua (about 780 Ma) and was closely associated with the global super-mantle plume activity. The northern rift, extending in nearly EW direction^{[27,28,29,30,31]}, probably initiated at 740 Ma and was controlled by the back-arc extension in response to the subduction of the Pan-Rodinia oceanic plate at the supercontinent margin^{[32,33,34,35]}. In the middle and Late Sinian, the South Tianshan Ocean to the north and the West Kunlun-Altyn-Qilian Ocean to the south were opened to produce passive continental margins around Tarim, and continued to develop in the Early Cambrian^{[28, 35-38]}. Therefore, the northern Tarim was an Early Cambrian passive continental margin basin.

The Lower Cambrian in the northern Tarim is composed of the Yurtus, Xiaoerbulake, and Wusongger formations at NW Tarim and the Xishanbrak and Xidashan formations at NE Tarim. According to the recent paleontological, stratigraphic, and carbon isotopic data^{[39,40,41,42]}, the Yurtus and Xishanbrak formations are isochronous deposits that correspond to the Fortunian and stage 2 of Terreneuvian, and the Xiaoerbulake, Wusongger, and Xidashan formations are isochronous deposits that correspond to the stages 3 and 4 of series 2 (Fig. 2).

Syndepositional structure is defined as the structure type that develops in the depositional period and controls the depositional pattern of the basin^[43]. The Early Cambrian syndepositional structures in northern Tarim mainly include the syndepositional faults and related siliceous hydrothermal activities during the Yurtus period, which governed the distribution of both the bedded chert and black shales in the Yurtus Formation and the platform margin reef in the overlying Xiaoerbulake Formation.

Bedded chert and black shale assemblage is widely developed at the bottom of the Lower Cambrian in northern Tarim, with a significant upward decrease of siliceous content and appearance of two sets of black shale^[44,45,46] (Fig. 3). The major-, trace- , and rare earth-element geochemical results show that the bedded chert is mainly of hydrothermal origin^{[44,45,46,47,48]}. A lot of fractures and caves filled with siliceous veins (Fig. 3a) were found at the top of the Qigebulake Formation at the Kule section near Aksu (Fig. 1). The bedded chert has the maximum thickness (9.8 m) at Kule where hydrothermal spout- related flow structure occurred as well (Fig. 3b). Its thickness decreases at the sections to the southwest, with 6.7 m at Shiairike, 2 m at Linkuanggou, and 0.8 m at Kungaikuotan^[45]. These evidences indicate that the Kule section was closer to the hydrothermal vent, whereas the Shiairike, Linkuanggou, and Kungaikuotan sections were far away from the hydrothermal vent. In the Kuruktag region where deep water occurred during the Early Cambrian, the bedded chert at the Wuligezitag and Qiakemaketieshi sections is in a thickness above 50 m, whereas it is only about 10 m, with significant muddy component, at the Yaerdangshan section. Moreover, no

bedded chert was found in the well Yl 1 that drilled pyrite-bearing mudstone in a similar deep water environment^[45], and the bedded chert was also not drilled at the Tadong uplift. The distribution of bedded chert is, therefore, not related to the water depth and even the sedimentary facies, but may be associated with the distance to hydrothermal vent. Generally, the closer the hydrothermal vent is, the greater the thickness of bedded chert.

The hydrothermal origin of the bedded chert at the bottom Lower Cambrian indicates intense extensional tectonic activity in northern Tarim during the Early Cambrian^{[44-45, 48]}, and the reactivated Nanhua faults under this extensional environment are probably the hydrothermal eruption channel. It can thus be inferred that the Early Cambrian deposition center was distributed along the Nanhua faults in northern Tarim, in other words, the Nanhua rift had syndepositional structure in the Early Cambrian.

The thickness of the Yurtus and Xishanbrak formations in northern Tarim is below 100 m, and the Manjiaer depression has subsided for thousands to tens of thousands of meters, making the Early Cambrian syndepositional structure in amplitude of tens to hundreds of meters easy to be ignored in seismic interpretation (Fig. 4). However, it can be recognized by magnifying local seismic profiles and flattening the related horizon (Fig. 5a, b).

The Xiaoerbulake platform margin reef has been clearly identified from the 3D seismic profile by flattening the bottom of the Lower Cambrian Xiaoerbulake Formation in the Lungu area of Tabei uplift (Fig. 5a). Its interior is characterized by a progradational reflection structure (green arrow in Fig. 5c). Under the Xiaoerbulake Formation, a series of strong-amplitude and continuous seismic reflection axes may be related to the Yurtus Formation, Sinian System, and Nanhua System. According to the calibration and tracing from well Xh 1 (100 km to the west), the bottom of the Yurtus Formation is located at the trough below the time zero line. This reflection axis was significantly disrupted below the platform margin slope of the overlying Xiaoerbulake Formation (red arrow in Fig. 5a), probably due to the fault cutting. The significant increase of seismic reflection axis to the east of the fault indicates the fault is a SE-dipping normal fault (F1) that cuts the Nanhua, Sinian, and Yurtus sequences. The bottom of the Yurtus Formation in the hanging wall may be located at the strong-amplitude and highly-continuous trough 100 ms below the time zero line, and the overlapping towards the footwall is present in the interior of the Yurtus Formation (blue arrow in Fig. 5c).

The Nanhua rift structure has been clearly observed on the 2D seismic profile with the flattening bottom of the Xiaoerbulake Formation at the northern slope of the Tazhong uplift (Fig. 5b). The boundary fault occurs at the north of the rift (F2, red arrow in Fig. 5b).The Xiaoerbulake platform margin reef and progradation reflection structure develop at the footwall (green arrow in Fig. 5d), and a set of overlapping sequence onto the rift edge develops at the hanging wall (blue arrow in Fig. 5d), with a maximum time-thickness of ca. 100 ms. This is similar to the Lower Cambrian sedimentary structure in the hanging wall and footwall of the Nanhua fault (F1) at the southern slope of the Tabei uplift. It can thus be inferred that the overlapping sequence at the hanging wall of the boundary fault F2 is the Yurtus Formation deposited during the Early Cambrian subsidence of the Nanhua rift.

These evidences indicate that Early Cambrian syntectonic sedimentation occurred in the Nanhua rift. The hanging wall of the boundary fault subsided synchronously with the sedimentation of the Yurtus Formation, and subsequently, the Yurtus Formation thickened and overlapped onto the edge of the Nanhua rift, whereas the footwall controlled the distribution of the Xiaoerbulake platform margin reef. The maximum thickness of the Yurtus Formation in these two areas is 250 m using a seismic wave velocity of 5000 m/s. In addition, regional seismic profiles show that a basement uplift zone between the Tabei and Tazhong uplifts separates the northern and southern Nanhua rifts (Fig. 4), controlling the two deposition centers of the Yurtus Formation with a maximum thickness of 250 m. These two deposition centers are possibly similar to the Anyue rift trough in the Early Cambrian Qiongzhusi Formation in the Sichuan Basin (Fig. 2)^{[21,22,23,24]}.

Based on the above syndepositional structure model, 76 processed 2D seismic lines of 37 006 km were used to systematically interpret and trace the Nanhua rifts and the Early Cambrian syndepositional faults in northern Tarim (Fig. 6). Four reliable syndepositional faults (F1, F1-1, F2, and F2-1) are preliminarily identified, which are located at the southern slope of the Tabei uplift and the northern slope of the Tazhong uplift, respectively. They extend in a NEE-EW direction, and control two deposition centers of the Yurtus Formation. The nearly EW-extending southern deposition center has a thickness of 120 m and an area of 15 000 km² (length of 250 km and width of 50-80 km); and the northern one extends in a NEE direction with a thickness of 120 m and an area of ca. 17000 km² (length of 300 km and width of 50-100 km) (Fig. 6).

The early Cambrian syndepositional faults might primarily occur in the area where the Nanhua faults were well preserved, e.g. the NW-SE trending seismic profile across the Tabei and Tadong uplifts shows distinct structural and sedimentary features in these two regions (Fig. 7). No thickened deposits and obvious platform margin reef occurred in the Xishanbrak and Xidashan formations at the Tadong region. In addition, no wells drilling through the Lower Cambrian in the Tadong uplift encountered bedded chert, suggesting that the syndepositional faults as the pathway for deep siliceous hydrothermal fluids did not occur at the Tadong region. Wu summarized that the Cambrian sequence has a parallel unconformable contact with the Nanhua-Sinian System in the Tabei and western Tazhong uplifts, while there are truncation or angle unconformities in the Tadong, Bachu, and eastern Tazhong uplifts^[31]. In the parallel unconformity area, the tectonic activity was weak at the end of the Sinian period that preserved the Nanhua rifts, and thus the syndepositional faults presumably occurred during the Yurtus period. In contrast, the Nanhua rifts in the truncation and angle unconformity area were uplifted and eroded by the intense tectonic activity at the end of the Sinian period, resulting in a lack of syndepositional faults and the formation of gentle slope inclined to the subsidence center during the Yurtus period (Fig. 8). Therefore, the Late Sinian to Early Cambrian structure patterns varied greatly among different regions at northern Tarim. This difference not only controlled the distribution of syndepositional faults in the Yurtus period, but also controlled the paleogeographic patterns that further generated distinct platform types in the Xiaoerbulake period.

From the Late Sinian to Early Cambrian variation of tectonic activity and sedimentary pattern in different areas of northern Tarim (Fig. 7) and the distribution of two NEE-EW extending deposition centers in the Yurtus period (Fig. 8), we determine that the syndepositional faulting activities occurred in the center of basin during the Early Cambrian, with a great thickness of the Yurtus Formation between the southern slope of the Tabei uplift and the northern slope of the Tazhong uplift, while the Tadong uplift and the Manxi area were characterized by a gentle ramp without syndepositional faulting activity that had a small thickness of the Yurtus or Xishanbrak formations. Seismic data also reveal that the Lower Cambrian sequence shows overlapping and thinning trends from the Manjiaer depression to the Tadong uplift^{[27, 29]}. This indicates that the Tadong region was in a higher position close to the provenance area during the Early Cambrian (Fig. 8). Margin reefs grew to form a NEE-EW extending rimmed platform at the footwall of syndepositional fault at the northern slope of the Tabei uplift and the northern slope of the Tazhong uplift during the Xiaoerbulake/Xidashan period (Fig. 9). The Lower Cambrian Xishanbrak and Xidashan formations at the Tadong region are dominated by siliceous mudstones at the bottom and mudstone-calcareous mudstone-dolomitic mudstone assemblages at the upper (Fig. 2) that confirms the presence of a hybrid platform (Figs. 9 and 10). The Xiaoerbulake Formation at the Aksu and Manxi regions consists mainly of grained, algal, and argillaceous dolomites that are indicative of a rimmed platform system including margin reef, intra-platform bank, and interbank depression^{[21-22, 49]}. Recently, some researchers hold that the Xiaoerbulake Formation in these two regions is reef-bank deposits under a gentle slope environment, and thus classified as a ramp carbonate platform^[50]. However, the Wensu salient to the north of Aksu and the Tabei uplift are both the Early Paleozoic uplift^[51], which may have a similar control on the Early Cambrian paleogeographic pattern. Therefore, this paper employs the deposition model of the Tabei uplift margins to classify the margins of the Wensu salient and the Mnaxi region into rimmed and ramp platforms, respectively (Figs. 9 and 10).

The activity intensity of the Early Cambrian syndepositional fault (i.e. fault displacement) not only controlled the thickness of the Yurtus Formation at the hanging wall, but also controlled the scale of the Xiaoerbulake platform margin reefs at the footwall (Fig. 10). The syndepositional fault F2 at the middle of the northern slope of the Tazhong uplift has the largest activity intensity that resulted in a large scarp altitude difference and an overlying large-scale platform margin reef. With the fault displacement decreasing westward, the scarp altitude difference and the scale of margin reef decrease correspondingly (Fig. 11a-c). When the fault displacement decreases to zero, the margin reef disappears (Fig. 11d), the steep scarp becomes a gentle ramp, and the platform changes from rimmed to ramp one (Fig. 9). The southern slope of the Tabei uplift shares similar model, i.e., the scale of the platform margin reef at the footwall decreases gradually until it disappears (Fig. 11e-f) along with the westward decrease of displacement of the syndepositional fault F1. In addition, the fault horst bounded by the syndepositional faults F1-1 and F2-1 divides the Yurtus basin into the northern and southern subsidence centers (Fig. 9), where high-energy reef and bank deposits may develop in an external ramp but relatively high position after the transgression in the Xiaoerbulake period (Fig. 11f).

In the Middle Cambrian, the southwestern margin of Tarim transformed from the passive margin to the active margin^[52]. The compression led to the uplift of the southwestern Tarim, the Middle Cambrian platform margin began to migrate eastward (Fig. 11a-b), and the ramp to the west of the margin evolved into a restricted-evaporation platform^[21,22] with the development of widespread salt-lake, gypsum-dolomite flat, and mud-dolomitite flat deposits. The platform margin at the southern slope of the Tabei uplift had migrated southeastward in a distance of 77 km until the middle Ordovician (Fig. 7), which gradually formed the paleogeographic pattern of “East basin and West platform”. This is consistent with the fact that the turbidite and black mudstone of deep-water undercompensated facies in eastern Tarim occurred after the Early Cambrian and were strictly restricted by the Middle-Late Cambrian to Ordovician platform margin and slope^[21,22].

Two NNE-EW extending depocenters of the Yurtus Formation occurred in the northern Tarim, the interior of the Tarim Block (Fig. 6), which were located at deep-water zone separated from the open sea within the Early Cambrian shallow shelf. Although no data directly prove whether high-quality source rock is present or not, the hydrocarbon generation potential of these two depocenters can be determined using the following three aspects of evidence.

(1) The opening of the South Tianshan Ocean promoted the development of oceanic upwelling in the Early Cambrian. The Yurtus Formation at the Aksu region is characterized by a C-Si-P-G assemblage of organic shale, siliceous rock, phosphorite, and glauconite (Fig. 2) that is a typical rock assemblage in the modern upwelling zone such as the continental shelfs in Namibia, western Peru, and eastern Brazil^[53,54]. In addition, the trace element and isotopic compositions of the Yurtus Formation are similar to those of the upwelling deposits during the geological period, e.g. the Triassic Shubilk Formation at Alaska and the Permian Phosphoria Formation at the Rocky Mountain^[45,53-54]. The South Tianshan Ocean to the north of Tarim was opened during the Late Sinian, and evolved into a mature oceanic basin in the Early Cambrian^{[35,36,37,38]}. This process changed the oceanic current system, enhanced the extension of upwelling to the interior of the Tarim shelf, brought the deep eutrophic water to shallow shelf, and promoted the blooming of algae to form the organic-rich deposits.

(2) Restricted water contributed to the preservation of organic matter and the formation of high-quality source rock. The average Mo/TOC (molybdenum content to total organic carbon content ratio) value of black shale can be used to express the limitation extent of water, i.e., the less the average Mo/TOC value, the more restricted the water^[53]. The two Yurtus black organic-rich shale sets at the Aksu region have average Mo/TOC values of 8.11 and 4.97^[45], respectively, which are close to the values of the modern Black Sea (4.5±1) and the Norway Framvaren Fjord (9±2) (Table 1). This indicates that the water in northwestern Tarim may not exchange well with the open sea during the Early Cambrian. The Black Sea and Framvaren Fjord are typical examples of high organic matter concentration in sediments isolated from the open sea. The two depocenters of the Yurtus Formation in the northern Tarim Basin are located in the shelf with insufficiently exchanged water that enables to form a hypoxic environment beneficial for the preservation of organic matter and the development of high-quality source rock.

(3) The siliceous hydrothermal process provided nutrients to promote the growth of hydrocarbon-forming algae. The two depocenters of the Yurtus Formation in northern Tarim were controlled by the syndepositional fault activities accompanied by siliceous hydrothermal process. As mentioned above, the layered siliceous rock and black shale assemblage was widely present at the bottom Lower Cambrian, and the TOC value of the lower black shale layer is significantly higher than that of the upper layer^[18,46], reflecting the positive contribution of siliceous hydrothermal fluid to organic enrichment^[45,46]. Nutrients (e.g. silicon, phosphorus, nitrogen, and potassium) provided by the hydrothermal process promoted the flourishing of microorganisms, especially the hydrocarbon-forming algae, to significantly increase the paleo-productivity^[45,46]. Therefore, the Yurtus shale that is closer to the hydrothermal vent or syndepositional fault has higher organic abundance.

The Yurtus source rock at the hanging wall of the Early Cambrian syndepositional fault and the Xiaoerbulake platform margin reefs at the footwall constitute a predictable source- reservoir assemblage. A series of significant hydrocarbon discoveries in recent years^[55,56,57] have confirmed that this near source assemblage contributes greatly to the large-scale hydrocarbon accumulation. However, the Lower Cambrian source-reservoir assemblage related to the syndepositional fault is generally deeper than 8 000 m (Fig. 9). Except for the well Ts 1 region at the southern slope of the Tabei uplift, nearly all the source-reservoir assemblages at the middle and eastern segments of the faults have a depth more than 10 000 m, and those at the western segment range between 8 000 and 11000 m. By calculating the kinetic parameters of the thermal simulation experiment of the marine normal crude oil and the cracking of the crude oil into gas, Zhu believes that the depth of initial crude oil cracking in northern Tarim is 7500- 8 000 m and the depth of large-scale cracking is 8 800-9 500 m^[58]. Currently in this area, the deepest-buried Paleozoic marine oil accumulation in the world has been discovered from Ordovician at 7 750 m depth by the Sinopec^[20]. In addition, the development capacity of fractures associated with deep fault and the collapse depth of carbonate caves calculated by numerical models show that the depth threshold of high- quality reservoirs can be 8 500 m, and the closer to the fault, the better developed the reservoirs are, and large caves will disappear at a depth of 11 000 m^[20].

In summary, there is great oil and gas exploration potential in the Cambrian subsalt and deep strata at northern Tarim. However, the low-quality seismic data from deep basin cannot meet the needs of precise interpretation and mapping of syndepositional structures. In recent years, high-precision seismic exploration has achieved significant progress in the Kuqa and Tabei regions^[59]. A near NS-trending survey line across the well Xh1 (the only well drilling the high-quality Yurtus source rock) and the well Zs 1 structure (the only structure unit discovering large-scale Cambrian subsalt oil and gas) needs to be arranged using this technology. A more realistic geological model will be established through the integrated seismic processing and interpretation, which can provide reliable geological and geophysical parameters for the subsequent high-precision seismic arrangement. It is beneficial to better understand the Early Cambrian syndepositional structure and the source-reservoir relationship it controlled. Based on this, we can search the platform margin reef and ramp reef-bank of Xiaoerbulake Formation along the faults westward, and then drill risk exploration wells at sites where the target layer is shallow in depth.

The Late Sinian tectonic activities in the northern Tarim varied greatly among different areas and controlled the sedimentary pattern of the Early Cambrian basin. Early Cambrian syndepositional fault activities occurred in the Nanhua rifts at the southern slope of the Tabei uplift and the northern slope of the Tazhong uplift, forming two NEE- EW extending subsidence and depositional centers with a predicted maximum thickness of the Yurtus Formation up to 250 m. During the Xiaoerbulake period, rimmed platforms formed at the footwall of syndepositional faults. The two depocenters of the Yurtus Formation were located at the deep-water zone within the Early Cambrian shallow shelf. Multiple factors including the opening of the South Tianshan Ocean during the late Sinian, the presence of the Early Cambrian upwelling and siliceous hydrothermal process and accompanied nutrients, and the anoxic environment due to the separation from the open sea all contributed to the development of hydrocarbon source centers.

The Yurtus black shale at the hanging wall and the Xiaoerbulake platform margin reef at the footwall of the Early Cambrian syndepositional fault constitute a predictable source-reservoir assemblage. The activity intensity of the fault not only controlled the thickness of the Yurtus black shale, but also controlled the scale of the Xiaoerbulake platform margin reef. The southern slope of the Tabei uplift, the western part of the northern slope of the Tazhong uplift, and the Manxi area are adjacent to the two hydrocarbon source centers; the Xiaoerbulake platform margin reef and ramp reef-bank and the Middle Cambrian gypsum-salt caprock were also developed, which are favorable regions for the oil and gas exploration in the Cambrian subsalt and deep field. High-precision seismic exploration should be arranged in the future to precisely locate the Early Cambrian syndepositional faults and search reef-beach bodies and mound-shoal bodies through risk exploration drilling in favorable traps along the fault westward at sites where the target layer is shallow in depth.