Organic matter abundance affected the hydrocarbon generation capacity of source rocks and the scale of a reservoir. Common evaluation indicators of organic matter abundance include TOC content, hydrocarbon generation potential (PG), chloroform bitumen “A,” total hydrocarbon content (HC), and hydrogen index (HI) (Lu and Zhang, 2010). Experimental results show that the average TOC content of the J₁da² shale from well Y1 in Yuanba area are mainly in the two ranges of 1.0–2.0% and 0.5–1.0%. Samples with TOC content between 1.0% and 2.0% account for about 57%, which is significantly higher than that of the J₁da¹ and J₁da³ shale, and is regarded as the main section of high-quality source rock in J₁da in Yuanba area. However, TOC content of the J₁da shale from well T1 in Puguang area ranges from 0.5% to 2.71%, with an average value of 1.24%, which is slightly higher than that of Yuanba area, and the TOC content increases as the burial depth becomes shallower (Figure 2).

Rock pyrolysis analysis shows that hydrocarbon generation potential (PG = S ₁ + S ₂) of the J₁da shale from well Y1 in Yuanba area is between 0.78 and 1.98 mg/g, with an average value of only 0.72 mg/g, which is much lower than that of the evaluation standard of a source rock (>2 mg/g). However, PG values of the J₁da shale from well T1 in Puguang area range from 0.15 to 10.77 mg/g, with an average value of 3.83 mg/g, which can be evaluated as a good source rock (Figure 2). Combined with the other parameters, it is thought that the above difference is mainly due to the widely varying burial depth and organic matter maturity of shale samples between the two areas. Previous studies indicated that, with the increase of thermal evolution degree, hydrocarbon generation potential of source rocks would decrease significantly (Lu et al., 2003; Pang et al., 2014). Therefore, the hydrocarbon generation potential index PG is more suitable for the immature-low mature shale samples in Puguang area. According to Peters and Cassa (1994), the relative content of TOC and S ₂ could be used for source rock evaluation, and this method weakened the influence of maturity on a single parameter (TOC or S ₂) to a certain extent. Figure 3 shows a good correlation between TOC and S ₂ content of the J₁da shale samples in this study; that is, with the increase of TOC, S ₂ has a corresponding increasing trend. From the overall evaluation, it is thought that the J₁da shales in the northeastern Sichuan Basin belong to good-general source rocks.

Chloroform bitumen “A” and total HC reflected the contents of soluble organic matter and hydrocarbons in sedimentary rocks, so they were also important abundance indicators for source rock evaluation (Lu and Zhang, 2010). However, they could be affected by organic matter type and maturity, so they were generally not used alone when evaluating the organic matter abundance of a source rock (Chen et al., 2009). In this study, the content of chloroform bitumen “A” of the shale samples from J₁da in Yuanba ranges from 0.008% to 0.066%, with an average value of 0.025%, which gradually increases from bottom to top. However, the chloroform bitumen “A” content of the J₁da shale samples in Puguang is between 0.007% and 0.302%, with an average of 0.113%, and the values in the upper part is significantly higher than the lower part, which is consistent with the distribution characteristics of PG. Compared with the Dongyuemiao Member in Fuling area, HC values of the J₁da shale are significantly lower, while the asphaltene content is significantly higher, reflecting the differences in kerogen type, thermal evolution degree, and sedimentary environments between different members of Jurassic shale strata in the northeast Sichuan Basin (Shu et al., 2021).

To sum up, the organic matter abundance of J₁da shale in Puguang is slightly better than that in Yuanba area. The upper part of the second section of J₁da is better than other sections according to well Y1 and well T1, which can be evaluated as a good-general source rock on the whole, which is a favorable sweet spot for the J₁da shale in the northeastern Sichuan Basin.

The distributions of organic macerals in source rocks were related to the organic matter provenance and depositional environment (Marynowski, et al., 2007). Identifying the organic macerals and compositional characteristics under the microscope was an effective method to determine the organic matter type of source rocks (Hu and Huang, 1991). In this study, 30 and 20 J₁da shale samples were selected for organic petrological analysis from well Y1 and well T1, respectively. Microscopic observation and identification show that the organic macerals of J₁da shale samples in northeastern Sichuan Basin consist of vitrinite, fusinite, solid asphalt, and microsomes. Vitrinite are generally developed in all samples. The vitrinite content of samples from well Y1 is 40–92% with an average of 79%, and it ranges from 25% to 90% with an average of 55% in samples from well T1, indicating that the organic matter type of J₁da shale in Puguang area is generally better than that in Yuanba area. The vitrinite types in the studied samples are mainly vitrodetrinite and occasionally phyllovitrinite, which are mostly lump-shaped or strip-shaped, with a uniform surface and gray-gray-white color under microscopic observation (Figures 4B,D). It is speculated that they are mainly formed by humification and gelatination from lignocellulosic tissues of higher plants (Raj, 1979). The content of fusinite is second only to vitrinite, and it is mostly bright white with bulges under the microscope (Figures 4A,E), which transform from the lignocellulosic tissue of higher plants by fusainization. Microsomes are dispersed in minerals and clay, so the contents are not easy to estimate. The solid asphalt is distributed only in local intervals and is mainly found along the rock cracks or mineral intergranular filling. It is generally considered that solid asphalt is the product of crude oil cracking, that is, the post-oil asphalt (Figure 4C). Some solid asphalts have orange-yellow fluorescence, which is speculated to be the former-oil asphalt (Figure 4F) (Nishikawa et al., 2009). Mineral matrix asphalt that is uniformly mixed with inorganic minerals is common in J₁da samples from well T1, accounting for about 22% of the total organic macerals content. The higher development degree of solid asphalt reflects the better organic matter type of J₁da source rock and stronger shale oil and gas generation capacity, to a certain extent (Liu et al., 2021).

The carbon isotope of kerogen was related to the depositional environment and the origin of parent materials, so the carbon isotope characteristics inherited from the parent material can be used for the classification of kerogen types (Omokawa, 1982; Beukes et al., 1990). It was generally believed that the higher the content of organic matter in terrestrial higher plants, the heavier the stable carbon isotope in it, while the lower hydrobiont showed just the opposite (Cheng et al., 2008). According to the values of kerogen carbon isotope δ¹³C, organic matter types of J₁da shale are classified, shown in Figure 5. It indicates that organic matter types of J₁da shale in the study area are mainly type II₁ and II₂, reflecting the parent materials are derived from the mixed sources of terrestrial higher plants and lower hydrobionts. The organic matter type of J₁da shale in Yuanba is mainly type II₂, while it is mainly type II₁ in Puguang, both of which show a trend of better types as the burial depth becomes shallower, indicating that the organic matter type in the upper part of the J₁da shale strata is better (Figure 5).

The value of organic matter maturity determines the stage of thermal evolution degree of a source rock, so as to determine whether it is given priority to generate oil or gas, which is very important for evaluating the exploration potential of shale oil and gas (Garrigues et al., 1988; Lineva et al., 2018). Vitrinite reflectance (R _o) was currently the most effective indicator for determining the organic matter maturity (Qiu et al., 2004). Results show that R _o values of J₁da shale in Yuanba area vary in the range of 1.44–1.83%, with an average of 1.67%, based on five samples of well Y1, which indicates the shale strata have reached a mature-high mature evolution stage and is currently in the peak of gas generation period. However, experimental data of 14 J₁da shale samples from Puguang area show that R _o values are 0.76–1.18%, with an average value of 1.01%, which is within the oil-generating window.

The maximum pyrolysis peak temperature (T _max) tends to increase with the increase of burial depth and the age of the formation, so it can also be used as an effective indicator for evaluating the maturity of source rocks (Zhou et al., 2014). Rock-Eval of 40 shale samples of well Y1 shows that the T _max values range from 472°C to 496°C, with an average of 487°C, indicating the J₁da shale in Yuanba is in a high-mature evolution stage, even over-mature stage on the whole. However, the T _max values of 18 shale samples in Puguang range from 444°C to 453°C, with an average of 449°C, indicating that they are in the mature stage (Figure 6).

Based on the above analysis of geochemical characteristics, it can be concluded that the organic matter content and type of J₁da shale in northeastern Sichuan Basin are moderate on the whole, and parameters from well T1 in Puguang are generally better than that from well Y1 in Yuanba. However, the maturity of J₁da shale in Yuanba is significantly higher than that in Puguang, resulting in the difference of oil generation or gas generation. Vertically, J₁da² develops the most favorable source rock interval in the J₁da shale strata, and the hydrocarbon generation potential of the upper part is better than that of the lower part, so it is the favorable target for the Da’anzhai Member in the northeastern Sichuan Basin.

N-Alkanes were widely distributed in organisms such as fungi, algae, and higher plants and were the main components of the saturated hydrocarbon fraction of source rock extracts, so they could be used for indicating the provenance of organic matter (Gelpi et al., 1970; Moldowan et al., 1985; Peters et al., 2005). In this study, GC analysis of saturated hydrocarbon were carried out on 10 shale samples from well Y1 in Yuanba area, and biomarker compound analysis was carried out on shale samples from both well Y1 and well T1. Figure 7 and Table 1 show the statistical results of key parameters based on the laboratory tests. In general, there are similar chromatographic characteristics of n-alkanes among different samples of J₁da shale strata in Yuanba area; that is, the carbon number of n-alkanes has a wide distribution range of nC₁₅–nC₃₇, with the main peak carbon number being mainly C₁₉, which agrees with the general compositional characteristics of continental crude oil (Zhu et al., 2013). The chromatograms of J₁da² shale samples from well Y1 show that some samples are pre-single peak with the main peak carbon being nC₁₉ or nC₁₇, reflecting a single provenance of organic matter, and most are derived from plankton such as algae (Jin et al., 2016). However, some samples show double peaks with the main carbon peaks being mostly nC₁₉ and nC₂₅, indicating that the organic matter may be a mixed source of both algae and higher plants (Figure 7). Xu (2015) concluded that the main peak carbon number of n-alkane was mainly nC₂₃ and nC₂₅, which showed the characteristics of post-single peak, by analyzing the saturated hydrocarbon chromatography of shale samples taken from Ziliujing Formation in Puguang area. Based on the above analysis, it is believed that the organic matter of the Jurassic J₁da shale in the study area is a typical terrestrial–aquatic mixed input.

∑nC₂₁₋/∑nC₂₂₊ and (nC₂₁ + nC₂₂)/(nC₂₈ + nC₂₉) represented the relative content of light hydrocarbons and heavy hydrocarbons in n-alkanes, which is not only controlled by thermodynamics but also affected by depositional environment and original organic matter (Gelpi et al., 1970; Moldowan et al., 1985; Peters et al., 2005). From Table 1, the value of ∑nC₂₁₋/∑nC₂₂₊ of the J₁da² shale samples in Yuanba is 0.075–2.09, with an average of 1.028, showing a general equilibrium of C₂₁₋ and C₂₂₊, reflecting that the source of organic matter is a mix of lower aquatic organisms with terrestrial higher plants. Previous study showed that the ∑nC₂₁₋/∑nC₂₂₊ values of the Ziliujing Formation in Puguang were 0.26–0.37, reflecting a main hydrobiont input (Xu (2015)). Philippi (1974) believed that at the same organic matter maturity, lower hydrobionts had higher (nC₂₁ + nC₂₂) content, while terrestrial higher plants had higher (nC₂₈ + nC₂₉) content. In this study, the values of (nC₂₁ + nC₂₂)/(nC₂₈ + nC₂₉) of J₁da² shale sample from well Y1 ranges from 0.344 to 6.448, with an average of 3.285 (Table 1), indicating that the organic matter provenance is an obvious mixed bio-inputs, and the proportion of different sources vary greatly among layers.

The relative proportion of regular steranes could be used for the analysis of organic matter provenance (Kamp and Leake, 1995). Generally, the predominance of C₂₉-sterane indicated a strong terrigenous input, while lower hydrobionts were rich in C₂₇-sterane (Huang and Meinschein, 1979; Li et al., 2021). Abundant steroids were detected in the J₁da shale samples from both wells Y1 and T1 in the study area. As a whole, C₂₇–C₂₉ regular steranes play a dominant role, followed by the rearranged steranes, and the abundance of pregnane is extremely low. The average contents of C₂₇, C₂₈, and C₂₉ regular steranes in samples from Yuanba area are 4.74%, 4.03%, and 6.69%, respectively, and the values in samples from Puguang are 1.84%, 0.99%, and 2.32%, respectively. The typical chromatogram shows that the C₂₇-C₂₈-C₂₉ regular steranes in the ααα-20R configuration show a feature of “V-shaped” or “inverse L-shaped” distribution, that is, C₂₈ < C₂₇ < C₂₉ (Figure 8), further reflecting the J₁da shale in the study area has dual biogenic origin of terrestrial higher plants and lower algae and dominated by the former (Seifert and Moldowan, 1986). The previous studies considered the high content of 4α-methyl sterane as a sign of dinoflagellates in the freshwater lake; however, no 4α-methyl sterane was found in the studied samples (Goodwin et al., 1988). Therefore, the existing evidence was not enough to determine the source and cause of algae in the study area. Combined with other scholars' studies, it was speculated that it may originate from planktonic algae or bacteria (Brassell et al., 1986; Chen et al., 1994; Lin, 2017).

In the composition of terpenoids, tricyclic terpenes were generally considered to be derived from microorganisms and algae, whose content and carbon number distribution were quite different among different organic matter types of source rocks (Dutta et al., 2006; Wang et al., 2008), while tetracyclic terpenes may be derived from the degradation of hopane precursors, which indicated terrestrial higher plants (Zhou et al., 2016). According to the experimental tests, the carbon number of tricyclic terpenes in J₁da shale samples from well Y1 in Yuanba area is mainly distributed between nC₁₉ and nC₂₉, whose total content is 15.39–24.71% with the mean value of 19.16%, and the content of C₂₄ tetracyclic terpenes is 0.95–1.82% with an average of 1.04%. For J₁da shale samples from well T1 in Puguang area, the carbon number of tricyclic terpenes is also distributed in the range of nC₁₉–nC₂₉, whose total content is 8.44–20.88% with a mean value of 11.26% (Figure 9). It shows that in addition to the contribution of algae, there are also certain inputs of terrestrial higher plants during the depositional period of the Jurassic J₁da strata in the northeastern Sichuan Basin.

The ratio of pristane and phytane (Pr/Ph) was usually used to indicate the redox environment where the source rocks and organic matter deposited (Haven et al., 1987). Generally, Pr/Ph value greater than 3.0 reflected an oxidation environment, while less than 1.0 reflected a reducing environment (Wang, 1990; Peters et al., 2005; Tang et al., 2020b). Abundant pristane (Pr) and phytane (Ph) are detected in the J₁da² shale samples from well Y1 in this study. The values of (Pr/Ph) are in the range of 0.214–1.405, with an average of 0.652, shown in Table 1, which shows a characteristic of phytane dominance, indicating that J₁da² shale samples in this study are formed in a reducing or transitional environment. The ratios of isoprenoids to their adjacent n-alkanes, Pr/nC₁₇ and Ph/nC₁₈, can also indicate the redox conditions of water body during paleo-sedimentation (Alias et al., 2012; Yu et al., 2021). The crossplot of the values of Pr/nC₁₇ and Ph/nC₁₈ shows that the depositional environment of J₁da² shale in Yuanba area is mainly a reducing–weak oxidation environment (Figure 10), which is the typical characteristics of semi-deep lake–shallow lake deposition.

Hopanes were common pentacyclic triterpenoids in source rocks, whose carbon number distribution was related to the redox properties of the sedimentary environment (Peters et al., 2005; Lu and Zhang, 2010; Zhu et al., 2021). Hopanes in the studied samples are mainly conventional 17α(H), 21β(H)-hopanes and a small amount of rearranged hopanes, which means that the terrestrial plants in the petrologen are less affected by the bacterial action under the oxidation environment, and the overall hydrocarbon generation condition is in a reducing environment (Zhu et al., 2013). Figure 11 shows two typical hopanes chromatograms of shale samples in the study area. One is shown in Figure 11A; the C₃₁–C₃₅ hopanes show a decreasing distribution pattern with the increase of carbon number, which is common in freshwater lacustrine source rock. This pattern is mainly distributed in the upper section of the J₁da shale strata of well Y1 in Yuanba area and also generally seen in the J₁da shale strata of well T1 in Puguang area. The other is shown in Figure 11B; the C₃₁–C₃₅ hopanes show a “tail-warped” distribution pattern, which is commonly found in saltwater lacustrine source rock (Zhu et al., 2007). This pattern distributes locally in the middle and lower sections of the J₁da shale strata of well Y1 in Yuanba area. From the above analysis, it can be concluded that the J₁da shales in the northeastern Sichuan Basin are not deposited in a single water environment, and the sedimentary facies change fast. It is formed in a freshwater lacustrine sedimentary environment in Puguang area; it is dominated by freshwater deposition as a whole in Yuanba area, with salty or brackish sedimentary environment in local intervals.

High content of γ-cerane was known as a mark of strong reduction environment in salty or brackish sediments, and the γ-cerane content of source rocks in saline sedimentary environment was usually greater than 1.0 (Zhang et al., 1998; Marynowski et al., 2000). The content of γ-cerane in J₁da shale samples in the study area is generally high, which is 1.78–2.50 with an average of 2.11 in well Y1 and 1.06–3.95 with an average of 2.02 in well T1, indicating that the salinity of the water body is high during the deposition period of the Da’anzhai Member. Some scholars believed that high content of γ-cerane did not always exist in the strong reducing and hypersaline environments, and it was essentially a maker of water stratification (Zhang et al., 1999). From this point of view, high content of γ-cerane in the shale samples in this study indicates that the lake water body is stratified during the deposition of the J₁da, which leads to the circulation flow stagnation of the bottom water, and the lakebed is in an anoxic and reducing environment, conducive to the preservation of organic matter.