High-quality lacustrine black shale was developed in the 7th member of the Triassic Yanchang Formation in the Ordos Basin (hereinafter referred to as the Chang 7 Member), with an extremely high abundance of organic matter. The total organic carbon (TOC) content is generally more than 5%, with the highest value over 30%. The maturity of lacustrine black shale is moderate, generating a large amount of oil. This set of black shale is not only the main source of the conventional oil in the Ordos Basin, which supports the rapid increase in reserves and production of the Ordos Basin, but also contains rich shale oil resources. This has drawn great attention of the scholars all over the world^[1,2,3,4,5]. To make clear about the development environment of the black shale and the favorable layers for shale oil exploration and development, a fully cored well (Well Feng 75) from the Chang 7 to Chang 9 Members of the Yanchang Formation was drilled in the northwestern margin of the Middle and the Late Triassic lacustrine basin in 2018, and systematical analysis of the petrological and organic geochemical characteristics of this black shale was carried out.

The analysis results of the organic carbon isotopic compositions revealed that there is a clear negative drift phenomenon of isotopic compositions in the Chang 7 Member at the depth of 2753-2777 m. Previous studies have been done on the negative drift of organic carbon isotopic compositions, and there are two main explanation on its genesis^{[6,7,8,9,10,11]}. One is related to the source of organic matter, and the other is related to the sedimentary environment of organic matter. Hu et al.^[6] believed that a clear drift of organic carbon isotopic compositions is related to the extreme hot events, and the extreme hot events were caused by volcanic activities. The negative drift indicated that the volcanic activity occurred on land, while the positive drift indicated that volcanic activity occurred on the ocean. If volcanic activities occurred on land, the eruption was accompanied by the release of a large amount of CO₂, CH₄, H₂S, and other gases, forming an anoxic environment. When a large amount of light organic carbon isotope gas entered into the atmosphere-ocean system, a negative drift of organic carbon isotopic compositions formed. When volcanic activities occurred in a deep-sea environment, the primary productivity of the ocean was enhanced due to the released heat and nutrients directly entering the ocean, forming a submarine hypoxic environment. Large-scale burial of the organic matter prevented carbon from returning to the atmosphere-ocean system. Ther organic carbon isotopic compositions show a positive drift. The two different mechanisms lead to different ecological effects, which further affected the prosperity of organisms and the preservation of organic matter, which in turn affected the development of source rocks and the potential evaluation of hydrocarbon generation^[6,7,8,9]. It can be seen that the organic carbon isotopic composition drift can be used as an important indicator of volcanic activities, and volcanic activities plays an important role on promoting the enrichment of organic matter in source rocks^{[10,11,12,13,14,15]}.

In previous studies on the Triassic lacustrine shale in the Ordos Basin, the important influence of the volcanic activity on the enrichment of organic matter by providing nutrients has been discussed^[10,11]. However, the studies are few about the effects of volcanic activity on the climate, the oxidation-reduction environment of water, the storage of organic matter, and the enrichment of shale oil.

In this study, with Well Feng 75 as an example, through high-density analysis of inorganic elements, biomarkers, and carbon isotopic compositions of the Chang 7 Member, the climatic conditions and the water redox environment during the formation of the black shale are analyzed. This study determines the source of the parent material for hydrocarbon generation and the characteristics of generated hydrocarbon, providing a scientific basis for the selection of shale oil-enriched intervals.

The Well Feng 75 is located at the northwestern margin of the middle-late Triassic lacustrine basin in the Ordos Basin (Fig. 1), with about 350 m core length. According to the stratigraphic division scheme of the Ordos Basin, the continuous black shale interval from 2663 m to 2778 m in the well has been identified as the Chang 7 Member, with black shale accounting for more than 95%, interbedded with a small amount of gray mudstone, thin argillaceous siltstone and tuff. Using the data from adjacent wells, the Chang 7 Member is further subdivided into three segments: the Chang 7₁, Chang 7₂, and Chang 7₃ sub-members (Fig. 1c).

To study about the abundance and types of organic matter in the black shale of the Chang 7 Member, high-density sampling has been carried out in this well interval. A total of 304 samples were collected in the 115 m well interval to carry out Rock-Eval and total organic carbon scanning, which can obtain the basic geochemical information of black shale. A total of 73 samples were selected for the fine organic geochemical analysis based on the data given above. The analysis items included organic carbon isotopic compositions, biomarkers, and many others. The analysis and testing were carried out in the Key Laboratory of Petroleum Geochemistry of China National Petroleum Corporation in accordance with relevant national and industry standards. In addition, the analysis and testing of the major elements and trace elements of the 304 samples were completed in the Analysis and Testing Research Center of the Beijing Geological Research Institute of Nuclear Industry.

The key parameters of TOC and S₂, TOC and (S₁+S₂) indicating the abundance of organic matter of the source rock in the Chang 7 Member of Well Feng 75 both showed a good positive correlation, that is, the higher the organic carbon content, the greater hydrocarbon generation potential (Fig. 2). The TOC values of 304 samples were 0.76%-9.42%, with an average value of 4.63%, showing as high-quality source rocks. However, it is lower than that of the oil shale of the Chang 7 Member in the center part of the lacustrine basin, and the maximum TOC value of the latter can be higher than 30%^[10,11,12]. The (S₁+S₂) value is 2.84-39.75 mg/g, with an average value of 17.39 mg/g. In comparison, the organic matter abundance in the Chang 7₃ Sub-member was the largest one with an average TOC value of 5.40%; it is followed by the Chang 7₂ Sub-member with an average TOC value of 4.55%; the Chang 7₁ Sub-member was slightly lower with an average TOC value of 3.55%.

The hydrogen index (HI) value can reflect organic matter types under conditions of similar maturity. The HI value of the Chang 7 Member of the Well Feng 75 is generally 250-400 mg/g, and the HI value of the source rock of the Chang 7₃ Sub-member is the highest, followed by the Chang 7₂ Sub-member, and the HI value of the source rock of the Chang 7₁ Sub-member is the smallest (Fig. 3), indicating that the organic matter type of the lacustrine source rock of the Chang 7 Member of the Ordos Basin is dominated by type II₁, which is different from the source rock of the Cretaceous Qingshankou Formation in the Songliao Basin, which is also formed in the freshwater lacustrine basin. The latter are mostly type I organic matter^[16]. The value of the T_max in the Chang 7 Member of the Well Feng 75 is about 440 °C, with HI value of about 300 mg/g. The major samples fall within the range of type II₁ organic matter, but a few samples of the Chang 7₁ Sub-member fall within the range of type II₂ organic matter. Some samples of the Chang 7₃ Sub-member fall within the range of type I organic matter (Fig. 3a).

The Oxygen Index (OI) values of the source rocks of the Chang 7₃ and Chang 7₂ Sub-members are extremely low, and most of the samples are less than 10 mg/g, which is much lower than that of the source rocks in other basins, and even lower than left boundary line of the intersection diagrams of the general source rocks. The OI values of source rocks of the Chang 7₁ Sub-member mostly fall within the range of type II₁ organic matter (Fig. 3b). The too low OI values are related to the extremely oxygen-deficient depositional environment at that time.

Vertically, the organic matter abundance of the Chang 7 Member gradually decreases from lower to upper, and the types gradually deteriorate, which is related to the preservation environment of organic matter. The OI value, which represents the degree of oxygen content in the water, gradually increases in ascending order (Fig. 4). The Chang 7₃ Sub-member has the highest TOC and HI values, with average values of 5.70% and 345 mg/g, respectively, but it has the lowest OI values with an average of only 6 mg/g. The TOC and HI values of the Chang 7₂ Sub-member are reduced, but the OI value increase with an average reaching 11 mg/g. However, the OI value of the Chang 7₁ Sub-member increase significantly with the highest value of up to 93 mg/g (average value 29 mg/g), and the corresponding TOC and HI value further reduce to 3.55% and 269 mg/g respectively, reflecting the substantial increase in oxygen content in the sedimentary water, and the gradual deterioration of organic matter storage conditions. A section with significantly lower TOC and HI values and higher OI values appeared on the 2755-2758 m well interval in the middle of the Chang 7₃ Sub-member. The main reason is that the lithological change occurred in this section, changing from black shale to argillaceous siltstone interlayer.

The inorganic element analysis results can indicate the water environment when organic matter is deposited. P, Ba, and other nutrients related to biological growth showed a trend of gradually increasing in ascending order, indicating that nutrients gradually increased in ascending order, and there was a partial enrichment layer of nutrient elements in the Chang 7₃ Sub-member. The elements such as U and Mo, which indicate the oxidation-reduction environment of water, showed an downward trend in ascending order, indicating that the oxygen content in the water increases upward gradually. Relatively speaking, the bottom oxygen content is lower and the organic matter storage conditions are better (Fig. 5). Combined with the comprehensive histogram of Rock-Eval parameters in Fig. 4, it can be seen that the preservation conditions of organic matter are more important for the Formation of source rocks with high organic matter abundance.

The element intersection diagram indicates the hypoxia degree in the water ^[17,18,19]. The hypoxia degree of the Chang 7₃ Sub-member is the highest, and it is generally in an anaerobic - oxygen-deficient environment. The following is the Chang 7₂ Sub-member, most of which are in an oxygen-deficient environment, with a few falling in an anaerobic environment. The Chang 7₁ Sub-member is in an oxygen-deficient environment on the whole (Fig. 6). It reflects that the reduction degree of the water in the deposition process of black shale gradually weakens in ascending order, which is consistent with the result reflected by the OI values of source rocks in Fig. 4.

The biomarkers are important means to indicate the organic matter source and its sedimentary environment. On the whole, the black shale from the Chang 7 Member is characterized by freshwater lacustrine deposits. In terms of terpanes, the content of tricyclic terpane (TT) is much less than that of the pentacyclic triterpane, and the contents of C₁₉TT, C₂₀TT, C₂₁TT and C₂₃TT increase in sequence, with more Ts than Tm, medium contents of C₂₉Ts and C₃₀ rearranged hopanes, extremely low content of gammacerane, C₃₄ and C₃₅ hopanes. In terms of steranes, the content of rearranged sterane is high, with the highest content of C₂₇ steranes, followed by C₂₉ steranes, and C₂₈ steranes are the lowest. These characteristics mentioned above indicate that the organic matter mainly came from lacustrine aquatic organisms and contains a small number of terrestrial organisms. In addition, the content of ββ steranes is much higher than that of the αα steranes, and the content of 20S steranes is equivalent to the content of 20R steranes (Fig. 7), indicating that it has a medium maturity for the organic matter and is in oil window stage^[20].

There is a certain difference in biomarkers for different layers. In the vertical direction, it showed a regular trend of change in the biomarkers. The major sources of Pr (pristane) and Ph (phytane) are from the chlorophyll a in photosynthesis and the lateral arm plate chain of the bacterial chlorophyll-a and -b from the purple sulfur bacteria^[21]. The reduced or hypoxic environment is favorable for the rupture of the lateral arm plate chain to form phytol and finally phytane. While the oxidation conditions promote the preferential formation of pristane from phytol, and therefore, the pristine-to-phytol ratio is an important parameter that can reflect the formation environment of organic matter. When the pristine-to- phytol is more than 3, it mainly reflects that the organic matter is formed in an oxidized or weakly oxidized terrestrial environment; when the pristine-to-phytol ratio is less than 1, it reflects that the organic matter is formed in a typical reducing environment^[20]. The pristine-to-phytol ratio of the black shale of the Chang 7 Member is low with an average of less than 1, indicating that the organic matter in the black shale of the Chang 7 Member was formed in a strongly reducing environment. However, in comparison, the pristine-to-phytol ratio of the black shale from Chang 7₃ Sub-member is significantly higher than that of Chang 7₂ and Chang 7₁ Sub-members, and it has a trend of increasing first and then decreasing in ascending order for the Chang 7₃ Sub-member (Fig. 8). The formation environment of organic matter indicated by the pristine-to-phytol ratio is not consistent with the severely hypoxic sedimentary environment of the water mentioned above, indicating that the organic matter is not completely formed by aquatic organisms, and a small amount of organic matter is from terrestrial higher plants which gathered and formed in a strong reducing water environment after a certain transportation process.

Other biomarkers also indicate that the Chang 7₃ Sub-member has relatively more terrestrial organic matter input. Tricyclic terpenes are usually used as important indicating compounds for the organic matter source^[22,23]. The previous study of terrestrial source rocks in the Kuqa Depression showed that the organic matter is dominated by aquatic organisms, with a relatively low content of tricyclic terpanes. While the organic matter derived from terrestrial higher plants has higher content of tricyclic terpenes. The lower the carbon number, the higher the content^[24,25]. The overall performance of the biomarkers in the Chang7 Member of the Well Feng 75 is that the contents of C₁₉TT, C₂₀TT, C₂₁TT, and C₂₃TT increase sequentially, reflecting that the organic matters are mainly from lacutrine aquatic organisms, but there is a trend of C₁₉TT to C₂₃TT ratio decreasing gradually in ascending order, which indicates that the Chang 7₃ Sub-member has relatively more terrestrial organic matter input (Fig. 8).

The C₂₄ tetracyclic terpane (C₂₄TeT) mainly indicating the sedimentary environment of carbonate rocks and evaporites, and abundant C₂₄TeT is also found in the continental crude oil of Australia^{[20, 26]}. There is also abundant C₂₄TeT in the terrestrial organic matter-based source rock of the Chang 8 Member of the Ordos Basin, and the C₂₄TeT content is much higher than that of the C₂₆TT. The ratio of C₂₄TeT to C₂₆TT is above 2, and some samples are over 10. Therefore, the ratio of C₂₄TeT to C₂₆TT can be used as an indicator of terrestrial organic matter input. The distribution of this ratio in the Chang 7 black shale ranges from 0.7 to 1.9, indicating small amount of terrestrial organic matter input. It shows a trend of increasing first and then decreasing for the Chang 7₃ Sub-member, and the significantly increased layers are basically in accordance with these layers with increased pristine-to-phytol ratio and C₁₉TT-to-C₂₃TT ratio (Fig. 8).

Affected by the paleoclimate conditions and other factors, there is certain difference in the organic carbon isotopic compositions in different ages and sedimentary environments. Zhang et al.^[27] analyzed the organic carbon isotopic composition data of a large number of source rock samples on land in China, and found that the overall organic carbon isotopic compositions showed a trend of gradual heaviness with the stratigraphic age from old to new, and organic carbon isotopic compositions of the Lower Cambrian to Devonian marine strata is generally lighter than -29‰, with the Sinian, the Cambrian, and the Ordovician having isotopic compositions of -31.67‰, -29.30‰, and -28.90‰, respectively. The average organic carbon isotopic compositions of the Carboniferous-Permian marine-terrestrial transition facies and the Mesozoic-Cenozoic terrestrial strata are about -24‰. However, the organic carbon isotopic compositions have a wide distribution range in different ages, especially in Mesozoic and Cenozoic terrestrial strata, ranging from -30‰ to -20‰. It presents a bimodal or even multimodal distribution pattern, which is closely related to its depositional environment.

The organic carbon isotopic composition is usually used to indicate the organic material source and sedimentary water environment^{[27,28,29,30,31,32,33,34]}. In general, the carbon isotopic composition of organic matter from aquatic organisms is lighter, while the carbon isotopic composition of organic matter from terrestrial higher plant sources is heavier. For example, the carbon isotopic composition of the measured coal source rocks is heavier than that of lacustrine source rocks. From a large number of test data^{[28,29,30,31,32,33,34]}, the organic carbon isotopic composition of freshwater lacustrine source rocks (such as Cretaceous in Songliao Basin, Paleogene in Bohai Bay Basin, Triassic in Kuqa Depression, etc.) generally ranges from -31‰ to -29‰; Marine source rocks in the Tarim and Sichuan basins are similar to lacustrine source rocks, and the carbon isotopic composition of the Cambrian source rocks is lighter, with general values of -32‰. The saltwater lacustrine source rocks (such as the Paleogene in the Qaidam and Jianghan Basins) organic carbon isotopic composition is heavier, ranging from -26‰ to -24‰. The organic carbon isotopic composition of coal-measure source rocks is generally heavier than -27‰ (such as the Carboniferous-Permian in the Ordos Basin, the Triassic Xujiahe Formation in the Sichuan Basin, Jurassic in Northwest China, etc.).

The organic carbon isotopic composition of the Chang 7 Member of the Well Feng 75 ranges from -30.6‰ to -28.4‰, with an average value of -29.3‰, showing characteristic of typical freshwater lacustrine source rocks. It has an obvious negative drift trend of organic carbon isotopic composition for Chang 7₃ Sub-member, and a deviation of about 2% from -28.4‰ at the well depth of 2777 m to -30.6‰ at the well depth 2753 m, and then gradually becomes positive drift with a value of -29.2‰ at the depth of 2730 m, after which it remains stable overall (Fig. 9). According to the previous research results of the biomarkers, there is a relatively large amount of terrestrial organic matter input for the shale of the Chang 7₃ Sub-member, but it is characterized by lighter carbon isotopic composition. There is a contradiction between these two aspects, and the main reason is due to the major geological events.

The important geological events are not only recorded in the marine strata, but also in the continental lacustrine basins, and the shift of organic carbon isotopic composition is a direct response to the important geological events. Xu et al.^[35] found that there is a significant negative drift of organic carbon isotopic composition in the Da'anzhai Member of the Jurassic Ziliujing Formation in the Sichuan Basin, which is considered to be a response to the Toarcian ocean hypoxia event in the Early Jurassic, and is closely related to global volcanic activity and carbon cycle. Jones et al.^[36] found that the organic carbon isotopic composition of the source rocks of the Cretaceous Qingshankou Formation in the Songliao basin has a positive shift, which is a response to third stage OAE3 (Oceanic Anoxic Event 3) of the Late Cretaceous ocean anoxic event. The drift of organic carbon isotopic composition that existed in the black shale from Well Feng 75 in the Ordos Basin has a similar formation mechanism as that of the Jurassic in the Sichuan Basin and the Cretaceous in the Songliao Basin. The volcanic activity in the ancient Qinling region caused by the extremely hot and anoxic geological events is the main reason^[37,38,39]. Due to the volcanic activity that occurred on land, volcanic ash floated in the air and entered into the Ordos Basin. This continental volcanic activity caused a negative drift of organic carbon isotopic composition. The negative isotope drift amplitude is also consistent with the thickness of volcanic ash. Vertically, there are multiple sets of volcanic ash in the black shale of the Chang 7 Member, mainly concentrated in the Chang 7₃ Sub-member, and a small amount of thin volcanic ash in the Chang 7₂ Sub-member, but almost no volcanic ash in the Chang 7₁ Sub-member, indicating that the volcanic strength is gradually weakened in ascending order.

The extreme hot events caused by volcanic activities have an important influence on the deposition and preservation of organic matter. The Carnian Pluvial Episode (CPE) event that occurred in the Carnian Period of the Late Triassic was due to extreme hot events induced by volcanic activities, resulting in a long-term large-scale rainfall and the surface torrents much higher than normal^{[40,41,42,43]}. The top and bottom sedimentary ages of the Chang 7 Member black shale are 241.06±0.12 Ma and 241.558±0.093 Ma, respectively^[44], belonging to the Mesozoic Ladinian (Latinian) period, earlier than the CPE. They have similar influence on stratigraphic deposits. It contains a small amount of siltstone and carbonaceous mudstone in the fine-grained sediments of the Chang 7₃ Sub-member in the Well Feng 75, which are all related to surface torrents. In addition, volcanic activity has increased the content of CO₂, H₂S, CH_4, and other gases in the air, resulting in a turbid atmosphere and decreasing in O₂ content, forming a severely hypoxic environment. This mechanism can reasonably explain the contradictions previously mentioned. On one hand, the extreme weather caused a relatively large amount of terrestrial organic matter input during the deposition of the Chang 7₃ Sub-member. On the other hand, because of the more anoxic preservation environment, it is rich in hydrogen and poor of oxygen, resulting in low values of the OI and high values of the HI of the organic matter in the Chang 7₃ Sub-member.

Besides the Chang 7 Member from the Ordos Basin, multiple layers of volcanic ash have been discovered in the Cretaceous Qingshankou Formation in the Songliao Basin. The sedimentary tuffs are abundant in the Permian Lucaogou Formation in the Junggar and Santanghu Basins, indicating that the formation of high-quality source rocks is closely related to volcanic activity to a certain extent. The enrichment of organic matter by volcanic activity is mainly reflected in the following two aspects: (1) A large amount of nutrients were brought due to volcanic eruption, which has a "fertilization" effect on the aquatic organisms in the lake, and it is favorable for the proliferation of organisms. (2) The hypoxia environment formed by the flowering of life and rapid burial, coupled with the rapid burial of volcanic ash, is favorable for the preservation of organic matter.

The oil-bearing analysis for shale shows that the hydrocarbon content of the interval affected by volcanic activity is lower than that of other intervals, whether it is the Rock-Eval parameter or free hydrocarbon S₁, chloroform bitumen “A”, or the rock hydrocarbon index (pre-extraction value of S₂ subtract that of post-extraction and then plus S₁) proposed by Jarvie^[45] and Li et al. ^[46], it is not the true result of hydrocarbon content. The light component content in this section is high, and the hydrocarbon content has been underestimated due to its low volatility (Fig. 10).

This can be clearly shown from the hydrocarbon composition in the rock. In this study, 3 group parameters of the mass ratio $\text{n}{{\text{C}}_{21-22}}/\text{n}{{\text{C}}_{28-29}}$, $\sum\limits_{i=0}^{2}{\text{n}{{\text{C}}_{\text{15+2}i}}}/\sum\limits_{i=0}^{2}{\text{n}{{\text{C}}_{\text{27+2}i}}}$, $\sum\limits_{i=0}^{2}{\text{n}{{\text{C}}_{23+2i}}}/\sum\limits_{i=0}^{2}{\text{n}{{\text{C}}_{27+2i}}}$ of saturated hydrocarbon com-pounds in the whole oil gas chromatogram were selected to represent medium relative molecular mass/high relative molecular mass, low relative molecular mass odd- numbered carbon/high relative molecular mass odd-numbered carbon, and medium relative molecular mass odd- numbered carbon/high molecular weight odd-numbered carbon n-alkanes, reflecting the ratio of the content of light components or medium components to heavy components. It can be seen from Fig. 11 that the trend variation of these three sets of parameters is almost the same, it has a good corresponding relationship with the drifting trend of organic carbon isotopic composition, especially close to the minimum value of organic carbon isotopic composition (well interval of 2750-2754 m), and the content of light and medium components is much higher than that of heavy components. This is related to the more input of terrestrial organic matter because terrestrial organic matter is more favorable for the formation of light hydrocarbon components. It can be seen that as a result of hypoxic events, the source rocks contain more light and intermediate components, which can serve as important "sweet spots" layers for the shale oil.

Moreover, the volcanic ash produced by volcanic eruptions is mainly composed of quartz and feldspar, both of which are highly brittle and can be easily fractured. A large number of related research results have already been reported^[32,33], so it is unnecessary to repeat them in this study.

The lithology of the Chang 7 Member in the Well Feng 75 in the northwestern margin of the Triassic lacustrine basin in the Ordos Basin is dominated by black shale, with a small amount of argillaceous siltstone and tuff. It is rich in organic matter, and it is mainly the type Ⅱ₁ organic matter at the oil window maturity. The OI value of the black shale is extremely low, much lower than that of other source rocks in lacustrine basin source rocks, which is related to the extremely oxygen-deficient sedimentary environment. Vertically, the OI value increases gradually in ascending order, reflecting the gradual increase in oxygen content of sedimentary water.

The biomarkers show regular change, and a clear "inflection point" appeared in the well interval of the Chang 7₃ Sub-member at a depth of 2753-2777 m, indicating an increase of the input of terrestrial organic matter. However, there was a negative shift in the composition of organic carbon isotopes near the "inflection point", which is in contradiction with the results of biomarkers. The extreme hot and hypoxia induced by continental volcanic activities in the ancient Qinling area is the main reason for this phenomenon. Affected by extreme hot and hypoxic events, the source rocks have large amount of light and intermediate components, which are favorable for the oil flow, with the possibility of becoming an important "sweet spot" interval for shale oil enrichment.

HI—hydrogen index, mg/g;

OI—oxygen index, mg/g;

S₁—free hydrocarbon content in the rock, mg/g;

S₂—pyrolysis hydrocarbon content in the rock, mg/g;

T_max—the highest peak temperature of rock pyrolysis, °C;

TOC—total organic carbon, %.