Sedimentary Environment and Model for Organic Matter Enrichment: Chang 7 Shale of Late Triassic Yanchang Formation, Southern Margin of Ordos Basin, China

: Shale oil is an unconventional oil resource that needs to be developed and utilized urgently. However, the Chang 7 shale in the Ordos Basin, as the most typical continental source rock in China, is limited by the study of organic matter (OM) enrichment factors in continental lacustrine facies, and there are still controversies about the controlling factors, which limit the progress of oil and gas exploration. This paper aims to reconstruct the paleoenvironment of Chang 7 shale in the southern margin of Ordos Basin and reveal the controlling factors of organic rich shale by organic and elemental analysis, X-ray diffraction (XRD) analysis, thin section observation, and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) analysis. The results show that during the deposition period of Chang 7 shale, the climate was warm and humid, the lake water has strong reducing, low salinity and rapid depth changes. Total organic carbon (TOC) is positively correlated with salinity and hydrothermal action and inversely proportional to terrigenous input. The high productivity, low consumption and low dilution result in high enrichment of shale OM in the southern margin of Ordos Basin.


Introduction
Shale is widely known in nature, accounting for about 55% of the distribution area of sedimentary rocks [1]. Organic-rich black shale is the main rock type to form shale oil and gas. Shale can be formed in three types of sedimentary environments (marine facies, transitional facies of marine and continent and lacustrine facies) [2][3][4][5]. The formation of organic-rich black shale requires two important conditions: the development of plankton in surface waters and high productivity [6,7], and it has the conditions conducive to the preservation, accumulation and transformation of sedimentary rock organic matter (OM) [8,9]. Various sedimentary models are used to summarize the formation mechanism of black shale: silled-basin model or modified silled-basin model [10], upwelling model [11], oxygen-minimum zone model [11], model of an oxygen minimum zone (OMZ) in a shelf sea [12,13], silled-basin transgressive chemocline model [14], photic zone euxinia model [15] and estuarine circulation nutrient model [16]. The terrestrial lacustrine basin has limited sedimentary water, and the water circulation capacity is less than that of the ocean, so that the rich-organic black shale mainly develops in two depositional models (water The internal structure of Ordos Basin is very asymmetric. The western edge of the basin is steep and the eastern edge is gentle, and the northwest is high and the southeast is low [56][57][58]. It can be divided into six primary structural units: the Western Thrust Belt, the western Tianhuan Depression, the central Yishan Slope, the eastern Jinxi Fault-fold Belt, the southern Weibei Uplift and the northern Yimeng Uplift [35,55,59].
At the period of Middle and Late Triassic, the Ordos Basin entered the development stage of inland lake basin under the influence of the Indosinian movement. There were multiple sets of source-reservoir-cap rocks combination condition in the longitudinal direction. The Yanchang formation completely recorded the whole process of Lake Basin from formation and development to shrinkage [59,60]. The lake facies with thickness of 400~800 m was formed, which interbedded with fluvial facies and delta facies. In fact, during the Chang 7 sedimentary period, the lake transgression scale in the basin reached its largest, leading to the emergence of deep lacustrine facies deposition and the development of thick high-quality shale [31,61].
According to the lithologic characteristics, Chang 7 Member can be divided into three sections vertically. From top to bottom: Chang 71, Chang 72 and Chang 73 [44,62]. At the period of Chang 7 sedimentary, the water depth reached its deepest point. The shales deposited during this period is the best quality in the Triassic strata, which can provide a good hydrocarbon generation basis for the Yanchang Formation [58]. Following the deposition of Chang 71 and Chang 72, the water depth reduced gradually, and the sediment was mainly deep lacustrine facies deposition, which was interbedded with siltstone and The internal structure of Ordos Basin is very asymmetric. The western edge of the basin is steep and the eastern edge is gentle, and the northwest is high and the southeast is low [56][57][58]. It can be divided into six primary structural units: the Western Thrust Belt, the western Tianhuan Depression, the central Yishan Slope, the eastern Jinxi Fault-fold Belt, the southern Weibei Uplift and the northern Yimeng Uplift [35,55,59].
At the period of Middle and Late Triassic, the Ordos Basin entered the development stage of inland lake basin under the influence of the Indosinian movement. There were multiple sets of source-reservoir-cap rocks combination condition in the longitudinal direction. The Yanchang formation completely recorded the whole process of Lake Basin from formation and development to shrinkage [59,60]. The lake facies with thickness of 400~800 m was formed, which interbedded with fluvial facies and delta facies. In fact, during the Chang 7 sedimentary period, the lake transgression scale in the basin reached its largest, leading to the emergence of deep lacustrine facies deposition and the development of thick high-quality shale [31,61].
According to the lithologic characteristics, Chang 7 Member can be divided into three sections vertically. From top to bottom: Chang 7 1 , Chang 7 2 and Chang 7 3 [44,62]. At the period of Chang 7 sedimentary, the water depth reached its deepest point. The shales deposited during this period is the best quality in the Triassic strata, which can provide a good hydrocarbon generation basis for the Yanchang Formation [58]. Following the deposition of Chang 7 1 and Chang 7 2 , the water depth reduced gradually, and the sediment was mainly deep lacustrine facies deposition, which was interbedded with siltstone and mudstone, and thin-layer source rocks were developed in some areas (Figure 1c).

Samples
TOC, major and trace elements were determined in 12 shale samples from Chang 7 outcrop in the east of Weibei uplift, Ordos Basin. Moreover, four samples were collected for biomarker identification, which were from four well core of Chang 7 shale in the southern margin of Ordos Basin. Twelve shale samples of Chang 7 outcrop are used for X-ray diffraction (XRD) analysis. There are six samples (No. H3, H9, H12, Z71, Z59, Z40) used for thin section observation and Scanning Electron Microscopy-Energy Dispersive Spectrometer (SEM-EDS) analysis.

Experimental Methods
TOC measurement was performed using the Leco CS-200 carbon and sulfur analyzer. Grind the sample to a particle size of less than 0.2 mm. After removing inorganic carbon (>10 g) from the sample with dilute hydrochloric acid, the sample was burned under high-temperature oxygen flows to convert TOC to CO 2 . Then, the TOC in the sample was determined by an flame ionization detector (FID).
The major elements, such as SiO 2 , TiO 2 , Al 2 O 3 , CaO, Fe 2 O 3 , K 2 O and MgO, were measured by a wavelength dispersive X-ray fluorescence (XRF) spectrometer (ZSX PrimusII). The XRF analysis follows "Verification regulation for wavelength dispersive X-Ray fluorescence spectrometer" (the national measurement standard of the People's Republic of China JJG 016-2004). There are 60 g powder of each sample used for the analysis. The sample was dissolved with anhydrous lithium tetraborate, ammonium nitrate as oxidant, lithium fluoride and a small amount of lithium bromide as cosolvent and release agent, respectively. The ratio of sample to reagent is 1:8. Glass samples were prepared at 1150-1250 • C using a sample melting machine. Ten major elements were obtained for each sample. The contents of six metal ions were determined by conversion. The analytical error of the major elements is less than 1%.
The trace elements, such as Ba, Mn, Sr, Zn, V, Zr, Cr, Rb, Ni, in the samples were directly determined by the external standard method of inductively coupled plasma mass spectrometry (Perkin Elmer NexION 350X). There was 60 g powder of each sample used for the analysis. First, the sample was dissolved in hydrofluoric acid and nitric acid in a closed vessel. The hydrofluoric acid was then removed by evaporation using a hot plate. Then, the sample was dissolved in nitric acid under sealed conditions for determination. A total of 40 trace elements were detected from the samples. The trace element data were compared with the data from the international standard substance of United States Geological Survey. The analytical error of the trace elements was less than 5%.
The more samples there are, the more obvious the statistical significance is and the more geological significance can be reflected. To establish the relationship between TOC and the different major/trace elements, statistical analysis was performed in Chang 7 Member of the study area. Due to the low number of samples of Chang 7 2 Member, the correlation coefficient (R 2 ) data refers only to Chang 7 3 Member.
After the pretreatment of source rock samples, the source rock extract was obtained by a Soxhlet extractor method. There are 100 g powder of each sample used for the analysis. Gas chromatography-mass spectrometer (GC-MS) analysis was obtained by Agilent 7890A/7000 GC/MS system. Firstly, the source rock extract and crude oil were dilutedthrough n-hexane and centrifuged for 5 minutes. After that, supernatant was taken and separated by silica a gel/alumina (3:2) chromatography column. Then, 25 mL of n-hexane was slowly dripped into the column to obtain saturated hydrocarbons. Next, a GC-MS system was used to analyze the separated saturated hydrocarbon components. To analyze the saturated fractions, set the initial GC oven temperature to 50 • C for 1 min, whereupon the temperature will rise 20 • C every minute to 120 • C, and then the temperature will rise 3 • C every minute to 310 • C and stay at this temperature for 30 min. Calculate the peak area, the results of geochemical parameters of the samples are obtained.
The XRD analysis is mainly based on "X-ray diffraction analysis method of clay minerals and common non clay minerals in sedimentary rocks" (the oil and gas industry standard of the People's Republic of China SY/T 5163-2010), and the whole rock XRD analysis of shale samples is carried out by using X-ray diffractometer (Panalytical X'Pert PRO). There was 50 g powder of each sample used for the analysis. The 2-theta range of the XRD analysis is 5-45 • . The test was completed by the Nuclear Industry Geological Analysis and Test Research Center of China.
The thin section observation was completed mainly according to the national standard of the People's Republic of China GB/T 35206-2017, and the thin sections of shale were observed by a polarizing microscope (Lecia DM4500P/DFC450C). This work was carried out by the modern analysis and testing center of Xi'an Shiyou University. The SEM-EDS analysis follows the national standard of the People's Republic of China GB/T 17,359-2012 and uses a field emission scanning electron microscope (JEOL-7600F) and an energy spectrometer (X-Max50) to analyze shale samples. There was an 80-g block of each sample used for the analysis. The coating type used was gold, the range of working distance was 8.0-16.0 mm, the range of offset was 2.5-3.0 mm. The test was completed by the Experimental Test Center of Xi'an Geological Survey Center of China Geological Survey.

Total Organic Carbon Contents
The TOC of 12 samples in Chang 7 2 and Chang 7 3 shale is distributed, ranging from 4.84-6.03%, with an average value of 5.68%. The TOC of all samples are more than 4%, and the TOC of most samples is between 5-6% (Table 1), which could imply high oil-generation potential [63].

Major and Trace Element Geochemistry
Concentrations and ratios of trace elements could be useful parameters for the reconstruction of sedimentary paleoenvironment [64][65][66]. The major elements of the studied samples have SiO 2 and TiO 2 , and the ratio of the major element oxides CIA is also included ( Table 1). The trace elements of the studied samples are mostly presented in the form of ratios, which are mainly related to these trace elements (Ba, Al, P, Fe, Mn, Ti, Sr, V, Cr, Ni, U, Th) ( Table 1).
The Ba/Al ratio in Chang 7 shale is between 53.14-118.26, with an average value of 90.52. The P/Ti ratio is between 0.31-0.91, and the average value is 0.63. The V/Cr ratio is between 2.18-5.27, and the average value is 3.72. The V/(V + Ni) ratio is between 0.82-0.92, and the average value is 0.88. The Sr/Ba ratio is between 0.27-0.57, and the average value is 0.39. The Fe/Mn ratio is between 28. 81-190.13, and the average value is 83.95. The ratio of Si/(Si + Al + Fe) is between 0.18-0.64, and the average value is 0.57. The Fe/Ti ratio is

n-alkanes and Isoprenoids
The carbon number of n-alkanes in Chang 7 3 shale is between C 13 -C 33 , and the main peak is between C 16/17 -C 22 , which has the advantage of low carbon number and presents the characteristics of front, and Pr/Ph are less than 1 (Figure 2a,b and Table 2).
is 0.39. The Fe/Mn ratio is between 28. 81-190.13, and the average value is 83.95. The ra of Si/(Si + Al + Fe) is between 0.18-0.64, and the average value is 0.57. The Fe/Ti ratio between 14.11-37.60, with an average of 26.57. The U/Th ratio is between 0.91-6.39, w an average of 3.52 (Table 1).

n-alkanes and Isoprenoids
The carbon number of n-alkanes in Chang 73 shale is between C13-C33, and the ma peak is between C16/17-C22, which has the advantage of low carbon number and presen the characteristics of front, and Pr/Ph are less than 1 (Figure 2a,b and Table 2).

Steranes and Terpanes
The main peak of Chang 73 shale is C30H(17α(H)-C30hopane), and the abundance C30diaH(17α(H)-C30diahopane) is the least relatively; however, the relative abundance norhopane (Ts and Tm) is different in different samples. In fact, the Ts value of sample the north of the area (such as well Z59 and well Z71) is significantly greater than the T value, and Ts/Tm > 1, C30H/C30diaH < 10 ( Figure 2a). Meanwhile, the one in the south the area (such as well Z25 and well Z40) is exactly the opposite. The distribution chara teristics of regular steranes of ααα-20R are relatively uniform, characterized by C29 > C2 C28, and C27/C29 values are less than 1 (Figure 2a,b).

Mineral Constituent of Shale
In order to make clear the mineral constituent of Chang 72 and Chang 73 shale, t whole rock XRD analysis is mainly performed on outcrop shale. The minerals were d tected by XRD, including quart, feldspar, dolomite, calcite, pyrite, illite/smectite fo mation, illite, kaolinite, and chlorite (Table 3) illite/smectite formation, illite, kaolinite, a chlorite belongs to clay minerals. The average contents of dolomite, calcite, kaolinite, a chlorite are all less than 5% ( Table 3). The content of illite/smectite formation is betwe

Steranes and Terpanes
The main peak of Chang 7 3 shale is C 30 H(17α(H)-C 30 hopane), and the abundance of C 30 diaH(17α(H)-C 30 diahopane) is the least relatively; however, the relative abundance of norhopane (Ts and Tm) is different in different samples. In fact, the Ts value of sample in the north of the area (such as well Z59 and well Z71) is significantly greater than the Tm value, and Ts/Tm > 1, C 30 H/C 30 diaH < 10 ( Figure 2a). Meanwhile, the one in the south of the area (such as well Z25 and well Z40) is exactly the opposite. The distribution characteristics of regular steranes of ααα-20R are relatively uniform, characterized by C 29 > C 27 > C 28 , and C 27 /C 29 values are less than 1 (Figure 2a,b).

Mineral Constituent of Shale
In order to make clear the mineral constituent of Chang 7 2 and Chang 7 3 shale, the whole rock XRD analysis is mainly performed on outcrop shale. The minerals were detected by XRD, including quart, feldspar, dolomite, calcite, pyrite, illite/smectite formation, illite, kaolinite, and chlorite (Table 3) illite/smectite formation, illite, kaolinite, and chlorite belongs to clay minerals. The average contents of dolomite, calcite, kaolinite, and chlorite are all less than 5% ( Table 3). The content of illite/smectite formation is between 18.4-27.3%, and the average value is 23.2%. The content of illite is between 10.6-18.2%, with an average value of 13.9%. The content of pyrite is between 14.2-19.3%, and the average value is 16.7%. The content of quartz is between 23.0-34.3%, with an average value of 29.0%. The content of feldspar is between 9.6-20.4%, and the average value is 15.1% (Table 3).

Volcanic Tuff and Hydrothermal Mineral in Shale
Under a polarizing microscope, the outcrop shale thin section of Chang 7 was observed [67,68]. Ash tuff was found in sample H3, and pyrite was sporadically distributed (Figure 3a). Crystal debris tuff appeared in sample H9, in which the crystal debris was mainly albite and potash feldspar. The surface of albite was relatively clean, and the surface alteration of potash feldspar was obvious (Figure 3b). In sample H12, vitric-crystal debris tuff was observed, vitric debris was mostly striped or irregular, crystal debris was mainly albite, and there was pyrite distribution (Figure 3c). The appearance of these tuffs indicates that the Chang 7 deposit was affected by volcanic activity in the study area.
Through SEM-EDS analysis, subglobular marcasite and ortho-octahedral pyrite were found in the Chang 7 3 shale SEM image of well Z71 (Figure 3d). Framboidal pyrite appeared in the Chang 7 3 shale SEM image of Well Z59 (Figure 3e,f). Petaloid anhydrite could be seen in the Chang 7 3 shale SEM image of Well Z40 (Figure 3g,h). These characteristic minerals of SEM-EDS identification are considered as typical minerals reflecting low-temperature hydrothermal activities of the depositional period.    Biological productivity plays an important role in the enrichment of OM [7,43,69,70]. It is generally believed that there is a positive correlation between biological productivity and shale OM content [71,72]. As a necessary nutrient element for biology, P is usually used to judge biological productivity after Ti correction, P/Ti > 0.79 could be indicators of high productivity, 0.34 < P/Ti < 0.79 represents medium productivity and P/Ti < 0.34 represents low productivity [66]. The Ba contained in organism will form one of the elemental components of barite after biological death, resulting in relative aggregation of Ba, so that the paleoproductivity intensity can also be roughly judged by Ba/Al [65,73,74].
The P/Ti distribution range of Chang-7 shale is between 0.31-0.91, with an average value of 0.63, and the Ba/Al distribution range is between 53.14-118.26, with an average value of 90.52 (Table 2). Moreover, those indices indicate that Chang 7 shale has mediumhigh biological productivity.
It can be found that there is hardly any correlation between TOC and Ba/Al ( Figure 4a) and a positive correlation between TOC and P/Ti (Figure 4b) with the cross-plot of TOC and biological productivity index in Chang 7 shale.

Biological Productivity and Its Impact on OM Enrichment
Biological productivity plays an important role in the enrichment of OM [7,43,69,70]. It is generally believed that there is a positive correlation between biological productivity and shale OM content [71,72]. As a necessary nutrient element for biology, P is usually used to judge biological productivity after Ti correction, P/Ti > 0.79 could be indicators of high productivity, 0.34 < P/Ti < 0.79 represents medium productivity and P/Ti < 0.34 represents low productivity [66]. The Ba contained in organism will form one of the elemental components of barite after biological death, resulting in relative aggregation of Ba, so that the paleoproductivity intensity can also be roughly judged by Ba/Al [65,73,74].
The P/Ti distribution range of Chang-7 shale is between 0.31-0.91, with an average value of 0.63, and the Ba/Al distribution range is between 53.14-118.26, with an average value of 90.52 (Table 2). Moreover, those indices indicate that Chang 7 shale has mediumhigh biological productivity.
It can be found that there is hardly any correlation between TOC and Ba/Al ( Figure  4a) and a positive correlation between TOC and P/Ti (Figure 4b) with the cross-plot of TOC and biological productivity index in Chang 7 shale. It can be found that there is a strong positive correlation between TOC and biological productivity index of Chang 73 shale, indicating that high biological productivity is the premise of OM enrichment (Figure 4).

Biomarker Assemblages and Depositional Environments
The distribution of n-alkanes and isoprenoids can provide information on the origin and sedimentary environment of OM, which is of great significance to oil source and oil and gas exploration [60,75,76]. Generally, the carbon number distribution range of n-alkanes in shale formed with aquatic algae is less than C20, and the n-alkane distribution curve is the frontal peak type, reflecting a deep water sedimentary environment. The carbon number distribution of n-alkanes in the hydrocarbon source rocks formed with higher plants on land and the main peak is relatively backward, showing shallow water sedimentary environment [44,77]. The shale sample of Chang 7 is characterized by the frontal peak type curve of n-alkane distribution, indicating that its parent material is aquatic algae. It has also been reported that low molecular weight n-alkanes may also indicate bacterial modification under some conditions [78].
Steranes and terpanes are the most common biomarkers and have good indicative significance for maturity, sedimentary environment and oil source correlation [79,80]. The relative contents of Pr and Ph are related to the redox environment during OM deposition. It can be found that there is a strong positive correlation between TOC and biological productivity index of Chang 7 3 shale, indicating that high biological productivity is the premise of OM enrichment (Figure 4).

Biomarker Assemblages and Depositional Environments
The distribution of n-alkanes and isoprenoids can provide information on the origin and sedimentary environment of OM, which is of great significance to oil source and oil and gas exploration [60,75,76]. Generally, the carbon number distribution range of n-alkanes in shale formed with aquatic algae is less than C 20 , and the n-alkane distribution curve is the frontal peak type, reflecting a deep water sedimentary environment. The carbon number distribution of n-alkanes in the hydrocarbon source rocks formed with higher plants on land and the main peak is relatively backward, showing shallow water sedimentary environment [44,77]. The shale sample of Chang 7 is characterized by the frontal peak type curve of n-alkane distribution, indicating that its parent material is aquatic algae. It has also been reported that low molecular weight n-alkanes may also indicate bacterial modification under some conditions [78].
Steranes and terpanes are the most common biomarkers and have good indicative significance for maturity, sedimentary environment and oil source correlation [79,80]. The relative contents of Pr and Ph are related to the redox environment during OM deposition. Pr/Ph < 1 reflects the weak reducing sedimentary environment, Pr/Ph between 1.0-2.0 reflects the weak reduction weak oxidation mixed sedimentary environment and Pr/Ph > 2 reflects the partial oxidizing sedimentary environment [81][82][83]. The Pr/Ph of Chang 7 shale samples is less than 1, indicating that the environment of Chang 7 in the area was reductive during the depositional period.
The values of Ts/Tm and C 30 H/C 30 diaH are related to the maturity of OM. With the increase of burial depth, the values of Ts/Tm and C 30 H/C 30 diaH will usually decrease [80,84]. When there are the same maturity conditions, the ratio also reflects the organic facies of source rocks, which is closely related to the type of source rocks and clay catalysis. The sedimentary environment with high terrigenous clay content (Table 3) and sub-oxidation are more conducive to the formation of Ts and rearranged sterane (such as C 30 diaH) [85,86]. The northern part of the area is close to the middle of the lake basin, with deep water environment and relatively high reducing and a low content of terrigenous clay. However, the Ts and C 30 diaH of sample are higher than those at the southern part of the area at the edge of the lake basin. This indicates that the reason for the difference between the north part and the south one has nothing to do with clay catalysis and oxygenation reducing but has a greater relationship with maturity.
The biomarker assemblages are different in shale from different parent material. In humic shale, the relative content of C 29 sterane is higher and that of C 27 sterane is lower. On the contrary, better quality shale with sapropelic parent material shows the characteristics that the relative content of C 27 sterane is greater than that of C 29 sterane [87,88]. The high C 29 sterane of sample reflects the input of some proportion of terrigenous higher plants in the sedimentary period of Chang 7 shale, which is consistent with the sedimentary background that the area is located at the edge of the basin and is subject to terrigenous input from the Qinling Mountains.

Climate Conditions
Through the restoration of ancient latitude and the analysis of a large number of sporopollen fossils, previous researchers confirmed that the southern Ordos basin should be under the tropical-subtropical warm and humid paleoclimate during the Chang 7 depositional period [89][90][91].
The composition, distribution and relative content of elements in lake sediments can reflect the characteristics of paleoclimatic conditions accurately. In warm and humid climate, elements such as Fe, Mn, Cr, V, Ni and Co are more vulnerable to weathering and denudation, thus transporting to the lake [92,93]. On the other hand, in arid and hot climate, a large number of elements will precipitate, such as Ca, Mg, K, Na, Sr, Ba, etc., due to the continuous evaporation of water and the enhancement of water alkalinity [94,95]. Paleoclimatic conditions can be reconstructed by using the ratio (C value) of these two kinds of elements [96,97]. According to the C value of Table 1, climate can be divided into arid, semi-arid, semi-humid and semi-arid, semi-humid and humid, and they correspond to C < 0.2, 0.2 < C < 0.4, 0.4 < C < 0.6, 0.6 < C < 0.8 and C > 0.8, respectively. In addition, paleoclimate can affect the weathering intensity and rock composition of parent rock. Chemical Index of Alteration (CIA) is an effective indicator parameter of paleoclimate, and its calculation equation is shown in Table 1 [95]. According to the CIA values, climate can be divided into cold, dry, warm and humid, and the correspond to 40-50, 50-65, 65-85 and 85-100, respectively.
The C value of Chang 7 shale is between 0.28-0.71, with an average value of 0.44, while the CIA value is between 68.81-76.40, with an average value of 71.85 (Table 1). This indicates that the sedimentary period of Chang 7 is a warm and humid climate. On the cross-plot of paleoclimate indicators (C value and CIA) and TOC ( Figure 5), it is difficult to find a correlation between them, indicating that paleoclimate conditions are not the main controlling factor affecting the enrichment of OM in Chang 7 shale.

Paleowater Conditions
Siderophile elements such as V, Ni, U, Mn and Cr are redox-sensitive elements. They have different chemical behaviors in different redox environments and can be used as the discrimination index of a redox environment. V/(V + Ni) > 0.77 indicates a reductive environment, and V/(V + Ni) < 0.6 indicates an oxygen-enriched environment [98,99]. In addition, V/Cr can also be used as an indicator of redox environment. V/Cr < 2 indicates oxygen enrichment conditions, 2 < V/Cr < 4.25 indicates transition conditions and V/Cr > 4.25 indicates anoxic environment [100,101]. The occurrence forms of Sr and Ba are bicarbonate in the lake. When the salinity of a lake increases, Ba precipitates before Sr, which is why the value of Sr/Ba can reflect the change of salinity. Generally, Sr/Ba < 1 represents a fresh water environment and Sr/Ba > 1 represents a salt water environment [102,103]. The Fe/Mn ratio can be used to indicate the water depth. The greater the ratio, the shallower the water depth [104,105].
The  Table 1). The results show that the depositional environment of Chang 7 shale is reductive freshwater.
On the cross-plot of redox conditions, salinity, water depth and TOC, it can be found that there is a weak positive correlation between redox conditions (V/Cr and V/(V + Ni)) and TOC (Figure 6a,b). To some extent, this indicates that a reducing environment is beneficial to the preservation of OM. It can also be seen from the relationship between water depth (Fe/Mn) and TOC (Figure 6c). In addition, there is a weak positive correlation between salinity (Sr/Ba) and TOC (Figure 6d), indicating that a fresh water environment may be conducive to the enrichment of OM in the background of overall fresh water.

Paleowater Conditions
Siderophile elements such as V, Ni, U, Mn and Cr are redox-sensitive elements. They have different chemical behaviors in different redox environments and can be used as the discrimination index of a redox environment. V/(V + Ni) > 0.77 indicates a reductive environment, and V/(V + Ni) < 0.6 indicates an oxygen-enriched environment [98,99]. In addition, V/Cr can also be used as an indicator of redox environment. V/Cr < 2 indicates oxygen enrichment conditions, 2 < V/Cr < 4.25 indicates transition conditions and V/Cr > 4.25 indicates anoxic environment [100,101]. The occurrence forms of Sr and Ba are bicarbonate in the lake. When the salinity of a lake increases, Ba precipitates before Sr, which is why the value of Sr/Ba can reflect the change of salinity. Generally, Sr/Ba < 1 represents a fresh water environment and Sr/Ba > 1 represents a salt water environment [102,103]. The Fe/Mn ratio can be used to indicate the water depth. The greater the ratio, the shallower the water depth [104,105].
The  Table 1). The results show that the depositional environment of Chang 7 shale is reductive freshwater.
On the cross-plot of redox conditions, salinity, water depth and TOC, it can be found that there is a weak positive correlation between redox conditions (V/Cr and V/(V + Ni)) and TOC (Figure 6a,b). To some extent, this indicates that a reducing environment is beneficial to the preservation of OM. It can also be seen from the relationship between water depth (Fe/Mn) and TOC (Figure 6c). In addition, there is a weak positive correlation between salinity (Sr/Ba) and TOC (Figure 6d), indicating that a fresh water environment may be conducive to the enrichment of OM in the background of overall fresh water.

Terrestrial Inputs and Hydrodynamic Conditions
As stable terrestrial major elements, SiO 2 , TiO 2 , Al 2 O 3 , K 2 O and Na 2 O are used to indicate terrestrial inputs distinctly [106,107]. In addition, Fe + K, which is stable as a kind of terrestrial trace elements index, is also a common index for characterizing the input of terrigenous debris. The higher its value, the stronger the terrestrial inputs [108,109]. Si/(Si + Al + Fe) can provide information related to the distance from the provenance area. This value decreases with the increase of the distance from the provenance area. A higher ratio means that it is closer to the provenance area and vice versa [110].
The content of TiO 2 + Al 2 O 3 + SiO 2 is between 19.56-66.72 ppm in Chang 7 shale, with an average of 54.70 ppm. The content of Fe + K is between 7.07-9.47 ppm, with an average of 8.16 ppm. The content of Si/(Si + Al + Fe) is between 0.18-0.64, with an average value of 0.57 (Table 1). The results indicate that there are some terrestrial inputs of Chang 7 shale, and the area is not far from the provenance area.

Terrestrial Inputs and Hydrodynamic Conditions
As stable terrestrial major elements, SiO2, TiO2, Al2O3, K2O and Na2O are used to indicate terrestrial inputs distinctly [106,107]. In addition, Fe + K, which is stable as a kind of terrestrial trace elements index, is also a common index for characterizing the input of terrigenous debris. The higher its value, the stronger the terrestrial inputs [108,109]. Si/(Si + Al + Fe) can provide information related to the distance from the provenance area. This value decreases with the increase of the distance from the provenance area. A higher ratio means that it is closer to the provenance area and vice versa [110].
The content of TiO2 + Al2O3 + SiO2 is between 19.56-66.72 ppm in Chang 7 shale, with an average of 54.70 ppm. The content of Fe + K is between 7.07-9.47 ppm, with an average of 8.16 ppm. The content of Si/(Si + Al + Fe) is between 0.18-0.64, with an average value of 0.57 ( Table 1). The results indicate that there are some terrestrial inputs of Chang 7 shale, and the area is not far from the provenance area.
There is an obvious negative correlation between terrestrial inputs indicators (TiO2 and SiO2) and TOC (Figure 7), indicating that the parent material of OM is mainly aquatic organisms. Moreover, terrestrial inputs will dilute the OM and is not conducive to the preservation of OM. There is an obvious negative correlation between terrestrial inputs indicators (TiO 2 and SiO 2 ) and TOC (Figure 7), indicating that the parent material of OM is mainly aquatic organisms. Moreover, terrestrial inputs will dilute the OM and is not conducive to the preservation of OM.

Volcanism and Hydrothermal Activity
Volcanism and hydrothermal activities at the bottom of lake would bring a lot of heat and nutrient elements, which is conducive to the development of organisms, and then affect the enrichment of OM. The study of volcanism and hydrothermal activities in the sedimentary period of Chang 7 shale is a research hotspot in recent years [111]. Generally, sediments formed by volcanism and hydrothermal activities have abnormal element geochemical characteristics, including obvious Eu anomaly, enrichment of heavy rare earth elements and high Fe/Ti [112][113][114].
The Eu value of Chang 7 shale is high, between 1.04-1.84 ppm, with an average value of 1.33 ppm; Fe/Ti is between 14.11-37.60, with an average value of 26.57; U/Th is between 0.91-6.39, with an average value of 3.52 (Table 1). In addition, hydrothermal sediments and tuff formed by volcanic ash developed in Chang 7 shale and dense microfossils of

Volcanism and Hydrothermal Activity
Volcanism and hydrothermal activities at the bottom of lake would bring a lot of heat and nutrient elements, which is conducive to the development of organisms, and then affect the enrichment of OM. The study of volcanism and hydrothermal activities in the sedimentary period of Chang 7 shale is a research hotspot in recent years [111]. Generally, sediments formed by volcanism and hydrothermal activities have abnormal element geochemical characteristics, including obvious Eu anomaly, enrichment of heavy rare earth elements and high Fe/Ti [112][113][114]. The Eu value of Chang 7 shale is high, between 1.04-1.84 ppm, with an average value of 1.33 ppm; Fe/Ti is between 14.11-37.60, with an average value of 26.57; U/Th is between 0.91-6.39, with an average value of 3.52 (Table 1). In addition, hydrothermal sediments and tuff formed by volcanic ash developed in Chang 7 shale and dense microfossils of thermophilic algae are seen in thin sections under the microscope, indicating that the Chang 7 shale experienced volcanism and hydrothermal activities during the sedimentary period. However, scholars have not found tectonic evidence of volcanic activity around Ordos Basin until the present moment.
At present, there is controversy about the impact of volcanism and hydrothermal activities at the bottom of the lake on the enrichment of OM in Chang 7 shale. Some scholars believe that volcanism and hydrothermal activities have brought rich nutrient elements, promoted the growth of organisms and increased the enrichment of OM in sediments [115]. However, other scholars believe that hydrothermal activities will destroy the inherent OM in sedimentary rocks and adversely affect the development and preservation of OM [116]. During the Indosinian movement, Ordos Basin experienced basement fault activity. Frequent volcanic eruptions and hydrothermal upwelling in deep water in Qinling orogenic belt coincided with the deposition of Yanchang formation. Volcanism and hydrothermal activities at the bottom of the lake had an important impact on Chang 7 shale [111,[117][118][119]. There is a positive correlation between the hydrothermal activity indexes and TOC (Figure 8), indicating that the hydrothermal activity at the bottom of the lake has played a positive role in the enrichment of OM in Chang 7 shale and improved the abundance of lake sedimentary OM. The discovery of tuff, XRD data, SEM images and EDS data confirmed that the Chang 7 depositional period in this area was once affected by volcanic and hydrothermal activities at the bottom of the lake from the aspects of petrology and mineralogy, which increased the temperature of Chang 7 deposition. In this study, a series of low-temperature hydrothermal minerals were found in Chang 7 shale: dolomite, calcite, marcasite, pyrite, anhydrite, etc. (Table 3, Figure 3).
Previous researchers have suggested that relatively low temperature hydrothermal fluids can promote the enrichment of organic matter in source rocks, while relatively high temperature hydrothermal fluids may destroy the preservation of organic matter in source rocks [120]. Therefore, the petrological and mineralogical evidence inferred from the above results can also show that volcanism and hydrothermal activities at the bottom of the lake have played a positive role in the enrichment of OM in Chang 7 shale and improved the abundance of lake sedimentary OM in the southern margin of Ordos Basin.

Enrichment Model of Lacustrine OM
The geochemical data show that the TOC of Chang 7 shale has a strong positive correlation with biological productivity, an obvious negative correlation with terrestrial input, a weak positive correlation with reducing, salinity and hydrothermal activity intensity and has no inevitable connection with climate conditions and other environmental indicators. In fact, it indicates that high productivity, low consumption and low dilution have created high OM of Chang 7 shale. The discovery of tuff, XRD data, SEM images and EDS data confirmed that the Chang 7 depositional period in this area was once affected by volcanic and hydrothermal activities at the bottom of the lake from the aspects of petrology and mineralogy, which increased the temperature of Chang 7 deposition. In this study, a series of low-temperature hydrothermal minerals were found in Chang 7 shale: dolomite, calcite, marcasite, pyrite, anhydrite, etc. (Table 3, Figure 3).
Previous researchers have suggested that relatively low temperature hydrothermal fluids can promote the enrichment of organic matter in source rocks, while relatively high temperature hydrothermal fluids may destroy the preservation of organic matter in source rocks [120]. Therefore, the petrological and mineralogical evidence inferred from the above results can also show that volcanism and hydrothermal activities at the bottom of the lake have played a positive role in the enrichment of OM in Chang 7 shale and improved the abundance of lake sedimentary OM in the southern margin of Ordos Basin.

Enrichment Model of Lacustrine OM
The geochemical data show that the TOC of Chang 7 shale has a strong positive correlation with biological productivity, an obvious negative correlation with terrestrial input, a weak positive correlation with reducing, salinity and hydrothermal activity intensity and has no inevitable connection with climate conditions and other environmental indicators. In fact, it indicates that high productivity, low consumption and low dilution have created high OM of Chang 7 shale.
Based on the discussion of this study, the enrichment model of Chang 7 shale OM in the southern margin of Ordos Basin is proposed (Figure 9): during the deposition period of Chang 7 shale, the sedimentary environment of Ordos Basin is stable, supporting the large-scale development of deep lacustrine facies. Hydrothermal activities at the bottom of the lake brought a lot of nutrients and promoted the flourish of organisms; in warm and humid climate, evaporation was weak, water salinity was low, salinity stratification interface was deep (according to the discussion of "5.1.4 Paleowater conditions") and the reduction and brackish environment formed in the middle and lower part of the deep water area of the lake basin was conducive to the preservation of OM. In shallow water, OM underwent strong oxidation and decomposition. Terrigenous inputs not only brought terrestrial OM, but also stirred the lake water, inhibited the development of aquatic organisms and increased the oxidation and decomposition extent of OM (according to the discussion of "5.1.5 Terrestrial inputs and hydrodynamic conditions"), which was not conducive to the enrichment and preservation of OM.

Conclusions
The parent material of Chang 7 shale is mainly algae in the south of Ordos Basin. The sedimentary environment is deep lake, which is reductive and affected by the inputs of terrestrial higher plants.
The climate was warm and humid, the water body was reductive and there was low salinity in the sedimentary period of Chang 7 shale. The area was not far from the provenance area, the water deep changed rapidly, the main origin of OM was algae, but it was also affected by terrestrial OM to a certain extent, and there are very few parts of OM that are terrestrial, and there is also hydrothermal activity at the bottom of the lake. The paleoclimate of Chang 7 shale in the south of Ordos Basin was temperate and subtropical warm and humid or had a humid climate, the topographic slope in the south of the lake basin was steep and the supply source of Qinling Mountain in the South was sufficient.
The hydrothermal activity at the bottom of the lake basin promotes the proliferation of algal organisms and the highly reductive environment is conducive to the preservation of OM. However, terrigenous inputs inhibit the development of aquatic organisms and increase the oxidation and decomposition extent of OM, which is not conducive to the enrichment and preservation of OM.
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Conclusions
The parent material of Chang 7 shale is mainly algae in the south of Ordos Basin. The sedimentary environment is deep lake, which is reductive and affected by the inputs of terrestrial higher plants.
The climate was warm and humid, the water body was reductive and there was low salinity in the sedimentary period of Chang 7 shale. The area was not far from the provenance area, the water deep changed rapidly, the main origin of OM was algae, but it was also affected by terrestrial OM to a certain extent, and there are very few parts of OM that are terrestrial, and there is also hydrothermal activity at the bottom of the lake. The paleoclimate of Chang 7 shale in the south of Ordos Basin was temperate and subtropical warm and humid or had a humid climate, the topographic slope in the south of the lake basin was steep and the supply source of Qinling Mountain in the South was sufficient.
The hydrothermal activity at the bottom of the lake basin promotes the proliferation of algal organisms and the highly reductive environment is conducive to the preservation of OM. However, terrigenous inputs inhibit the development of aquatic organisms and