Organic-rich fine-grained sedimentary rocks not only act as important hydrocarbon source rocks, but also provide storage space for shale oil and gas resources^[1,2]. In recent years, as fine-grained sedimentary rocks become a major domain for increasing reserves and production, study on this kind of rock has become a hot spot^[3,4,5]. However, there is still huge controversy over the sedimentary environment and mode of organic-rich fine-grained sedimentary rocks, which hinders the prediction of this kind of rock and exploration and exploitation of unconventional oil and gas resources in them.

Fine-grained sedimentary rocks generally include shale, mudstone, and claystone^[6], which are traditionally described as homogeneous, unstructured, in massive and/or stratified rock units, and finer than 62 μm in grain size generally. Their mineralogical components are mainly clay, silty sand, carbonate, and organic matter^[4]. Specifically, fine-grained sedimentary rocks with bedding are generally referred to as shale, while those without bedding, argillaceous rocks^[7]. These fine- grained sedimentary rocks are conventionally speculated to be deposited through suspended sedimentation in deep-water basin environment with quiet water body and permanent hypoxia^{[8, 9]}. But, some recent research indicates some of the rock may be of shallow-water origin^[10,11,12]. In general, most clay particles finer than 10 μm tend to flocculate, and flocculation is conducive to long-distance transport of large quantity of muddy sediments in the marine environment^[13,14]. In contrast, clay particles larger than 10 μm tend to settle down in the form of single particles^[15,16]. Morphologically monotonous laminar or massive mudstones are deposited in turbulent environment, in the form of aggregate particles^{[3, 5, 11]}. A series of sedimentary features including scour surfaces, reworked lag deposits, both normal and reverse grading of particles, hummocky cross-stratification consisting of currents, waves and ripples, and gray-green shale intercalations are observed in the shallow-water black shale^{[10, 17]}. A recent study reports direct overlapping of the organic-rich Devonian Chattanooga shale over a major unconformity in the southern Appalachian Basin^[18], which again supports the possibility that organic-rich fine-grained sedimentary rocks could be formed in relatively shallow-water environment with flowing currents and oxygen-bearing bottom water that supports benthic life.

As the largest petroliferous basin in China, the Tarim Basin has been in a heated debate on its main source rocks^[19,20]. Specifically, the Lower Cambrian Yuertus Formation with wide distribution in the Aksu area is considered as an important set of source rock^[21]. However, no consensus has been reached when it comes to the depositional environment of the Yuertus Formation in the Aksu-Wushi-Keping area. Most researchers proposed that the Yuertus Formation was deposited in deep-water shelf to deep basin environments based on analysis of petrology, geochemistry and organic matter type^[22,23,24]. Some researchers believed it was deposited in middle-deep ramp environments based on limited drilling and seismic data^[25]. A few researchers argued it was deposited in shallow-water environment through investigation into the sedimentary sequences and petrographic characteristics of siliceous rocks^[26,27].

The latest study shows source rock as the fine-grained sedimentary rock richest in organic matter often occurs very next above an unconformity^[18], which indicates possible overlapping of organic-rich fine-grained sedimentary rocks on the unconformities in a shallow-water environment during the early transgression. Recently, we observed typical overlapping of the Yuertus Formation on the regional unconformity at the top of the Sinian Qigebulak Formation in the northwestern Tarim Basin, which also suggests it may be deposited in shallow-water environment. It should be noted that deep-water environments in abovementioned research generally refer to undercompensated deep basin environment, middle-deep ramp environment, and deep-water continental shelf environment. In comparison, shallow-water environments refer to shallow-water (shallower than tens of meters) continental shelf environments with compensated to overcompensated sedimentation. In this study, taking the Yuertus Formation in the northwestern Tarim Basin as an example, the lithofacies, micropaleontological fossils, trace and constant elements, and seismic data of fine-grained sedimentary rocks on some profiles were investigated carefully to figure out its sedimentary environment and depositional model. Results of this study can not only provide strong sedimentological evidence for identifying the sedimentary environment of the Yuertus Formation black rock series in the northwestern Tarim Basin, but also enrich the sedimentological theory of fine-grained sedimentary rocks, so they are of great scientific and practical significance.

As a large craton basin in the northwestern Xinjiang, China, the Tarim Basin, with an area of about 56×10⁴ km², is developed on a continental crustal basement composed of Archean and Early-Middle Proterozoic metamorphic rocks^[28]. The study area geographically spans Aksu-Wushi-Keping counties, while it is tectonically in the Keping block uplift on the northwestern edge of the Tarim platform (Fig. 1). During Nanhua-Sinian periods, the Tarim plate was mainly located in the northwestern margin of the Eastern Gondwana supercontinent, in direct contact with the Australian plate. At that time, it was in the mid-low latitudes of the northern hemisphere^{[29, 30]}, but it started to drift southward, reaching the equator during the early Cambrian, and starting to detach from the Australian plate. By then, the entire plate periphery was in a strongly tensional rifting environment, accompanied by formation of extensive open sea at the continental margin of the northern Tarim and the intracontinental rift at the southern Tarim^[30]. Within the Tarim plate, the water depth was deep in the east and shallow in the west. Correspondingly, there was a deep- water basin in the Korla-Ruoqiang-Qiemo area of the eastern Tarim and a shallow-water platform in the Kuqa-Minfeng- Aksu area of the western Tarim^[31].

The Lower Cambrian Yuertus Formation is widely and stably distributed in the Tarim Basin. It lithologically consists of gray and black phosphorous siliceous rock, thin-layered dolomite, black carbonaceous shale, gray-green shale containing sandy dolomitic limestone, thin-layered gray-white microcrystalline dolomite, and nodular dolomite containing thin-layered yellow-green and purple-red shale from bottom to top (Fig. 2). Small shelly fossils, algae and Bradoriids are observed under the microscope^[32]. The overlying Lower Cambrian Xiaoerbulake Formation^[33] is mainly composed of trilobite-containing dolomite-limestone deposits, and is believed to be in conformable contact with the Yuertus Formation by some researchers. The underlying Upper Sinian Qigebulak Formation is in parallel unconformable contact with the Yuertus Formation and mainly consists of gray mudstone and algal dolomite^[34]. A penecontemporaneous exposure surface is present, and it can be traced laterally for a long distance (Fig. 2). Organic-rich shales in the study area are thinly interbedded with either siliceous rocks or carbonate rocks (Fig. 2). Specifically, organic-rich shales interbedded with siliceous rocks mainly occur at the bottom of the Yuertus Formation, which can be traced laterally. In comparison, organic-rich shales. interbedded with carbonate rocks are mainly observed in the middle and upper parts of the Yuertus Formation, with lithofacies changes in some sections (Fig. 2).

As shown in Fig. 3A, organic-rich shales are interbedded with thin-layered dark black siliceous rocks.

2.1.1. Lithofacies

The siliceous rocks come in thin layers and dark black. According to structural characteristics, two types of siliceous rocks are identified, siliceous rock with granular structure and that with residual stromatolithic structure. Siliceous particles are mainly phosphorous oncolite, organic algal spherulite, algal filaments, (micro) biological debris (such as small shelly fossils and filamentous cyanobacteria), and organic matter- clay composites^[26] (Fig. 3b and 3c). These particles are moderately to well sorted and rounded, 0.1-0.5 mm in grain size. Coarse crystalline siliceous cements are observed filling between dissolved dolostone particles with original outlines (Fig. 3c). The siliceous rock with residual stromatolithic structure has alternate bright and dark horizontal laminae, along-bedding dissolution pores and vugs (Fig. 3d and 3e) filled with coarse-grained siliceous cements of generational features (Fig. 3e), and algae particles and silicified particles in scattered distribution.

Shales are mainly dark-gray to gray-black, occasionally gray-green (Fig. 3f). They are mainly composed of quartz, dolomite, apatite, and clay minerals, with some pyrite and iron nodules. Organic matter is commonly observed under the microscope, including massive amounts of benthic algae fragments, siliceous calcareous sponge spicules, and biological fragments^{[35,36,37,38]}. According to a previous study on organic matter contents in shale samples from ten outcrop sections^[39], the shale samples thinly interbedded with siliceous rock have higher contents of organic matter, with TOC of more than 2% in general, and up to 11.5% locally.

The diamictic grainstone comes in greyish-white to greyish-yellow and medium-thick layer, and has flat base and convex top macroscopically and uneven thickness laterally (Fig. 4a and 4b). Microscopically, within the diamictic grainstone interval, the lower part is mainly dolomitic fine sandstone, with a grain size range of 0.1-0.25 mm, a particle content of 60%-80%, and a dolomite content of about 20%-40%. The terrigenous detrital particles are mainly quartz, with a small amount of lithic fragments and feldspars (Fig. 4c and 4d). In the middle of the diamictic grainstone interval, with the decrease of terrigenous detrital debris content, sandy dolomite gradually takes the dominance (Fig. 4e and 4f). In the upper part of the diamictic grainstone interval, terrigenous detrital debris disappears, and crystal dolomite with residual granular structures becomes dominant (Fig. 4g). Observation of thin sections on a piece of 70 g A4 paper under microscope shows the original rock fabric features typical granular structure, clearly visible residual intergranular dissolved pores (Fig. 4h), and a general grain size range of 0.25-0.5 mm (Fig. 4h).

2.1.2. Sedimentary sequences and sedimentary structures

The sedimentary sequence is composed of interbedded thin organic-rich shale and siliceous rock layers and diamictic grainstone. Vertically, interbedded thin organic-rich shale and siliceous rock layers gradually transit into diamictic grainstone (Fig. 4a), and they are often in continuous and gradual contact. In general, the diamictic grainstone layer features reverse grading of particles and wave-built cross-bedding in its upper part (Fig. 4i).

Another occurrence of shale is thin interbeds with limestone, as shown in Fig. 5a. This kind of shale is limited in distribution in the study area, and it changes in lithofacies in some sections (Fig. 2).

2.2.1. Lithofacies

The limestone comes in thin-very thin layers, greenish-gray, granular structure, with occasional dolomite, and gradual contact with black shale with even contact surface. The thick shale intervals often contain siderite nodules and limestone lenses (Fig. 5b), as well as pyrite and other authigenic minerals occasionally. In comparison, the shales are black and sometimes gray-green, composed of calcite, quartz and dolomite minerals, and have massive amounts of biological fragments and algae clearly seen under microscope^[38]. The shale samples interbedded with limestone generally have a TOC range of 1.5%-4.5%^[39].

2.2.2. Sedimentary sequence and sedimentary structure

This sequence is composed of thin shale and limestone interbeds. The shale layers vary greatly in thickness from 1 to 60 cm. Vertically, the shale layers become thinner gradually upward (Fig. 5a), while carbonate layers become thicker upward. Some thin limestone layers have reverse grading of particles clearly (Fig. 5c). Observed under the microscope, the limestone layers in the lower part have calcite grains finer than 0.1 mm (Fig. 5f), while calcite grains become coarser in the middle and upper parts (Fig. 5d and 5e), reach a maximum size of 0.25-1.5 mm at the layer top. These coarse-grained calcite monocrystals at the layer top were argued to be fragments of echinodermata^[27].

The upward-shallowing sequence of the Yuertus Formation in the study area is characterized by gradual lithologic change at the base and sudden lithologic change at the top. The lower part of the sequence is the relatively deep-water deposit of organic rich shale or thin interbeds of organic rich shale and siliceous rock, while the upper part is mainly diamictic grainstone. Due to the effect of vertical accretion, the diamictic grainstone grew rapidly upward, and finally stopped growing and exposed above the seal level, as the water body turned shallow gradually.

As mentioned above, thin shales and siliceous rock interbeds gradually transit into diamictic grainstone upward (Fig. 4a and 4b) which is characterized by reverse grading of particles and decrease of terrigenous debris content that reflect upward-shallowing environment (Fig. 4c-4h). Iron crusts and reddish-brown near-surface breccias are observed at the top of the diamictic grainstone (Fig. 6a and 6b). The breccias are mainly composed of well-rounded, dolomitic fine sandstone (Fig. 6b and 6c). Above the breccia layer is the dark shale of the next cycle with typical overlapping phenomenon (Fig. 6d). Reticular or vertical karrens are widely seen below the breccias, and they are usually filled with yellow-brown dolomitic mud or carbonate sand (Fig. 6e). In addition, sac-like karst caves are found to be filled with breccia and carbonate sand (Fig. 6f and 6g). Dissolved fractures and karrens filled with insoluble materials are seen in the fine-crystal dolomite (Fig. 6h). Under the influence of karstification, grains near dominant channels become much finer, and intercrystal dissolved pores are mostly filled with insoluble materials (Fig. 6i).

In order to more clearly characterize the exposure features of the top of diamictic grainstone shore, zoomed-in photos of the Shiairike section are presented (Fig. 7). Above the exposure surface is the grey-green shale developed during the early stage of the next transgression cycle (Fig. 7a to 7e). Below the exposure surface are near-surface breccias, which, together with weathered hematite, form iron crust (Fig. 7b to 7e). Further below the near-surface breccias are nearly horizontal fractures filled with grey-green argillaceous materials and sac-like karst caves filled with calcite, which gradually transit into in-situ breccias cut by dominant karst channels (Fig. 7f). Further downwards, sac-like karst caves filled with macrocrystalline calcite are observed (Fig. 7a and 7c), associated with hematite filling resulting from infiltration (Fig. 7c).

Yuertus Formation in the study area has two types of overlapping. One type is typically seen at the bottom of the Yuertus Formation, where thin shale and siliceous rock interbeds overlap on the karst landform of the Sinian Qigebulake Formation (Fig. 8a and 8b), indicating the thin shale and siliceous rock interbeds could be deposited in very shallow and limited water environment. The other type is observed at the exposure surface of the lower part of the Yuertus Formation. Such exposure surface is closely related to the upward-shallowing sedimentary sequence and is thus categorized as syngenetic or penecontemporaneous exposure during intermittent sedimentary hiatus. Below this kind of exposure surface is diamictic grainstone and underlying thin shale and siliceous rock interbeds. Influenced by imbricated syngenetic normal faults, stepped micro-geomorphologic undulations often occur on the top surface of diamictic grainstone. Subsequently deposited thin greenish-gray (weathered) shale and limestone interbeds gradually overlap on the micro-geomorphologic high, and often terminate at the slope break of the exposure surface (Fig. 8c and 8d).

It can be seen from the regional seismic stratigraphic distribution (Fig. 9) that the Yuertus Formation has an overall stable thickness, though it slightly thickens from southwest to northeast. The overlapping pinch-out point is located near the platform margin of the Xiaoerbulake Formation and to some extent inherits features of the depression in depositional stage of Sinian Qigebulake Formation. During deposition of the Yuertus Formation, the depocenter was in the Manjar sag (Fig. 1a), and the sedimentation was compensated to over-compensated according to features of the inherited sag and thickness of stratigraphic filling.

The Lower Cambrian Yuertus Formation in the Tarim Basin with a set of black organic-rich source rock has long drawn attention from researchers^[39]. Previous mainstream viewpoints held that it deposited in deep-water shelf to deep basin environment^[22,23,24], or middle-deep ramp environment^[25], and that the Sinian to Middle Cambrian formations in the Tarim Basin are typical aragonite sea sediments under dry and arid paleoclimate^[40]. The Tarim Basin was connected with the Tarim platform during the Early Cambrian, with a ramp between them^[31]. In the following sub-sections, we will demonstrate that the organic-rich fine-grained sedimentary rocks in the Tarim Basin were formed in anoxic-suboxic shallow-water environment from multiple perspectives.

3.1.1. Thinly interbedded shales and siliceous/carbonate rocks

The thin-layered siliceous rock at the bottom of the Lower Cambrian Yuertus Formation in the Tarim Basin is speculated to be deposited during a particular period, and thus is a good indicator of the paleo-environment^{[26, 37]}. It is concluded from geochemical analysis by some researchers that it is hydrothermal-related siliceous rock, and thus inferred that the sedimentary environment of Yuertus Formation is deep-sea reducing environment^[22]. But by looking at structural characteristics of siliceous rocks, lithological changes of adjacent strata and the regional tectonic background, some other researchers proposed that the siliceous rock was deposited in shallow sea-bathyal environment rather than deep sea^{[23, 41-42]}. By examining the macroscopic and microscopic petrological characteristics of the siliceous rock, Yang Zongyu et al. suggested that the siliceous rock-phosphatic rock-black shale assemblage was deposited on the deep ramp below the base of storm waves where the environment was mainly sub-anoxic and anoxic^[37]. Through inorganic and organic petrological analyses of siliceous rock samples from different outcrops, Yang Chengyu et al.^[26] concluded that the siliceous rock was formed in high-salinity and alkaline closed water body under the rifting background, where hydrodynamic changes resulted in layered and clastic structures. In this study, we observed a series of typical structures in the layered siliceous rock samples, including residual granular structure, laminated structure and cemented pores and vugs (Fig. 3b and 3c), which indicate their parent rocks may be silicified granular limestone and stromatolite with relatively good roundness. Obviously, these structural characteristics can’t be explained by deep water or supersaturated hydrothermal deposition and indicate that the siliceous rock was formed in a relatively high-energy environment with turbulent water body. Basically, thin shale indicates low-energy environment, while the interbedded siliceous rock indicates high-energy environment, therefore, they are inferred to be deposited in an environment near the wave base (Fig. 4). The diamictic grainstone sequence with reverse grading of particles and wave-built cross-bedding at the top above the thin fine-grained sedimentary rock and siliceous rock interbeds constitute an upward-shallowing sequence, and the diamictic grainstone was formed above the wave base, supporting that the sequence was formed near the wave base (Fig. 4i). Meanwhile, such argument is supported by the overlapping of the thin shale and siliceous rock interbeds over the geomorphological karst highland of the Sinian Qigebulake Formation (Fig. 8a and 8b).

In addition, in the thin shale and carbonate interbeds, the thin carbonate layers have reverse grading of particles and overlap on the micro-geomorphological karst highland, which also suggest this kind of sediment was formed in shallow water and limited environment. In the meantime, the correlation between the stepped karst landform and the imbricated normal faults indicates the regional regression and exposure may be related to episodic rifting events in this period.

3.1.2. Eogenetic exposure

Tan Xiucheng and Xiao D et al. concluded that karstification in eogenetic stage had the following typical identifiable features^{[43,44,45,46]}: (1) Its distribution is greatly controlled by sedimentary facies and layers. (2) Pores and caves are mostly filled with dissolved and dissociated bedrock particles and bioclastic particles as well as carbonate sand and mud, and the interfaces between pores and caves and surrounding rocks are fuzzy and often show gradual change. (3) Multi-cyclic high- frequency superimposition is common (Fig. 2). In the Yuertus Formation, iron crust and near-surface breccias occur at the top of diamictic grainstone; below the breccias are horizontal and sac-like karst caves filled by overlying argillaceous matter, hematite and marco-crystalline calcite. The karstification shows typical facies and layer control features and multiple cycles of high frequency. Microscopically, karrens, dissolution fractures and residual intergranular dissolution pores are found to be filled with insoluble matter (Figs. 4g, 4h, 6h and 6i). These imply that the top of the diamictic grainstones exposed in epigenetic stage, which, together with the thin shale and limestone interbeds, suggest that the diamictic grainstone was deposited in shallow-water environment near and slightly below the wave base.

3.1.3. Bioassemblages of hydrocarbon-forming organisms

Bioassemblages are a major part of source rock study. Hydrocarbon-forming organisms in the study area are mainly benthic algae, represented by red algae^[38]. Large amounts of planktonic algae (including dinoflagellates, green algae, leiosphaeridia, hystrichosphaera, and volvox) are only seen in the Sugaite section^{[24, 38]}. The above petrological evidences presented show that the Yuertus Formation organic-rich shale in the study area is a set of shallow-water sediment depositing during the early transgression. Within the transgression sequence, bioassemblages of hydrocarbon-forming organisms show some regular variations, the bottom part has mainly benthic algae and a small amount of planktonic algae; the middle part has decrease of benthic algae and increase of planktonic algae; and the upper part has the highest ratio of planktonic algae. Overall, the planktonic algae tend to increase from bottom to top^[24]. In conclusion, as the transgression continuously proceeds and the sea area expands, the bioassemblage of hydrocarbon-forming organisms tends to evolve from benthic-dominated to planktonic-dominated.

3.1.4. Redox conditions

The Mo/TOC ratio can be used to evaluate how restricted the water body in an oceanic basin is^[47], while the MO/U ratio can reflect redox conditions of water body^[48]. As this study is mainly based on comprehensive petrological work, we only analyzed geochemical data that has been previously published^{[39, 49]}, and the sampling positions of the data basically cover the entire range of Yuertus Formation shale.

Through analysis of the Mo-TOC values, it is found the Yuertus Formation shale is richer in Mo element and low in TOC content (Fig. 10). The Mo content is in positive correlation with TOC, indicating the shale is formed in anoxic environment^[49]. Moreover, the Mo/TOC ratios of samples mostly fall in a range between the Sanich Bay and Black Sea, only those of a few samples indicate extremely restricted environment. Therefore, as product of a hypoxic, slightly-moderately restricted shallow-water environment, the black shale in the study area might be formed in a semi-restricted shallow sea shelf environment with a small amount of seawater exchange. According to previous research^[50], strong aerobic weathering on the continent and narrowly fluctuating marine sulfide water could result in high Mo concentration in seawater. Therefore, the black shale in the study area could be formed above the sulfide surface. In addition, recent research on modern environments also shows the enrichment of redox-sensitive trace elements doesn’t necessarily need water column hypoxia^[51]. Conversely, if the redox boundary is close to the sediment- water surface, oxygen may exist in the bottom water^[51]. Moreover, as long as the productivity is high enough, source rock also could be formed in the oxygen-bearing bottom water^[52]. The source rock of the Yuertus Formation in the study area is characterized by high TOC content and high thermal maturity^[53], so it can’t be ruled out that part of the Yuertus Formation organic-rich shale was formed in oxygen-bearing water body.

Mo and U elements in seawater mainly come from terrigenous weathering^[49], while enrichment of Mo and U elements in marine sediments would only be triggered by oxygen depletion and sulfide dissolution in seawater^[54,55]. There are a variety of autogenous Mo-U co-variation modes in the study area, but all of them show positive Mo_auth-U_auth covariant characteristics (Fig. 11). Specifically, samples from the Xiaoerbulak section present a maximum U_auth EFS (enrichment factor) value of 50, a maximum Mo_authEFS value of 50; assuming that Mo/U ratio equals to e, and a Mo/U ratio of the sample from East Xiaoerbulak section is 0.1e-0.3e, all indicating oxidized-suboxidized environment. In contrast, samples from the Sugaite section present a maximum U_auth EFS value of 60, a maximum Mo_auth EFS value of 550, and a Mo/U ratio range between 0.3e and 1.0e, all indicating an anoxic environment. Moreover, samples from the East Xiaoerbulak section present a maximum U_auth EFS value of 850, a maximum Mo_auth EFS value of 1020, and a Mo/U ratio range between 0.3e and 1.0e, two singular points have a Mo/U ratio range of 0.2e and 1.5e. Samples from the Youermeinak section present a maximum U_auth EFS value of 110, a maximum Mo_auth EFS value of 60, and a Mo/U ratio range between 0.1e and 0.3e, and only a few samples have Mo/U ratios of larger than 1.0e, indicating an oxidized-suboxidized environment, with occasionally anoxic conditions. In a word, there is not a modern oceanic counterpart for the Mo-U covariant model of the Yuertus Formation in the study area, the covariant trend of Mo-U is generally parallel to that of normal seawater, with large fluctuations ranging from 0.1e to 3.0e. Therefore, it could be inferred that the redox conditions of the depositional shallow water body should have been largely influenced by the sea level fluctuations, but the overall environment should be hypoxic to sub-oxidizing.

A shallow-water overlapping model of organic-rich fine- grained sedimentary rocks of Yuertus Formation in northwestern Tarim Basin has been established based on the above analysis (Fig. 12). The organic-rich fine-grained sedimentary rocks mainly occur in the Mangar sag and some limited shallow-water areas in the platform of the study area. The eastern Mangar sag receives the thickest filling mainly composed of thin-layered radiolarian siliceous rock, siliceous mudstone, and shale in the basin. The paleo-karst landforms during the Qigebulak period was inherited during deposition of the Yuertus Formation, and the restricted low lands in the landform were anoxic frequently due to poor water body exchange with seawater, providing anoxic reducing environment suitable for the development of source rock. Spouting of undersea hydrothermal fluids led to certain seawater exchange and thus blend of oxygen-enriched seawater in the semi-restricted environment, providing sufficient material basis for the formation of organic-rich fine-grained sedimentary rocks^[60]. Consequently, this set of widely distributed shallow-water organic-rich fine-grained sedimentary rocks was deposited in the suboxidized-hypoxic environment slightly below the wave base, which has been validated by the presence of a physical boundary that distinguishes overlying and underlying sediments from each other in the sedimentary composition and structures. From the Mangar sag to the study area, the organic-rich fine-grained sedimentary rocks tend to become thinner and overlap toward the shallow water area (Fig.12a to 12c). The whole overlapping process can be summed up as three cycles: In the first cycle, restricted by paleotopographic conditions, organic-rich fine-grained sediments formed in lowlands; and they gradually overlapped on the Qigebulake Formation karst landform as the relative sea level rose. In the second cycle, the relative sea level drop due to rifting tectonic activities made the shoal sediments on the highs of the ancient landform subjected to karstification during the eogenetic stage, then the karstified landforms was overlapped by transgressive sediments of the next stage (Fig. 12c and 12d). During the third cycle, a large set of carbonate sediments developed on the exposed sediments formed by regression at the end of the second cycle, but no shale deposited anymore (Fig. 12e and 12f). The study shows that there are high-quality source rocks in both the Mangar sag and the shallow water area of study area. But the source rock in the study area has higher thermal maturity and TOC content than that in the Mangar sag^[60]. Therefore, it should be safe to conclude that the shallow-water exposure during the eogenetic stage didn’t impact the formation and preservation of the source rock. Conversely, the study area might have higher paleoproductivity and better preservation conditions than the Mangar sag. Meanwhile, locally present secondary sags in the platform might have thicker source rock. This understanding may help predict distribution of sweet spots in source rocks with similar sedimentary backgrounds. Moreover, sweet spots of shale gas reservoirs can be predicted through reconstruction of the microgeomorphy during the sedimentation.

The organic-rich Yuertus Formation shale in the study area of the northwestern Tarim Basin, China mainly occurs in two kinds of assemblages, one, thin shale and siliceous rock interbeds which transit into diamictic grainstone with wave-built cross-bedding and eogenetic karrens at the top vertically, forming a upward-shallowing sedimentary sequence, the other, thin shale and carbonate interbeds with and shale layers thinning-upward and carbonate layers thickening upward. Some thin limestone layers show reverse grading of particles.

The comprehensive petrological analysis shows that the organic-rich Yuertus Formation shale was mainly deposited in a shallow-water environment near and below the wave base. Redox-sensitive elements indicate the shale was formed in hypoxic-suboxidizing shallow sea shelf environment. Further analysis demonstrates the high paleoproductivity and poor oxygen exchange controlled the formation of the shale jointly.

It is found that the organic-rich fine-grained sedimentary rocks overlap on tectonic highlands (unconformities) on both seismic and outcrop sections, based on which a shallow-water overlapping model of the organic-rich fine-grained sediments has been established. As shown in the stratigraphic filling pattern, the topography was relative flat during deposition of the Yuertus Formation, and organic-rich fine-grained sediments are often distributed in shallow water and lowlands, and overlap layer-by-layer from the Mangar sag to the shallow-water area.