The total organic carbon content (TOC) of Maoming oil shales ranges from 5.1 to 28.9% (Table 1), and the TOC contents in most samples are close to or greater than 10%, except these carbonaceous shale samples from the bottom of the Youganwo Formation. The TOC content also shows significant variation in different section of the profile, as the TOC content of the lower section oil shales on the profile from 840 to 852 m is in the range of 10.3 to 28.9% (with an average of 18.5%), while the TOC content of the upper section oil shales on the profile from 815 to 840 m vary from 9.2 to 19.1% (with an average of 13.3%). Obviously, the organic matter abundance in the lower section oil shales is higher than that of the upper section oil shales on the profile.

The thermal pyrolysis data suggested that these Maoming oil shales are thermally immature or low mature (Peters and Cassa, 1994), as their T_max values range from 425 to 437°C and most of them were under 435°C (Table 1 and Figure 2). The free hydrocarbon content (S₁) ranges from 0.33 to 1.89 mg/g (with average of 0.96 mg/g), indicating a high variability of liquid hydrocarbon contents in these shales, and the hydrocarbon content released from the thermal crack of kerogen (S₂) vary from 19.99 to 234.79 mg/g (with an average of 111.99 mg/g). The hydrogen index (HI) and oxygen index (OI) values of Maoming oil shales are in the range of 391–901 mg/gTOC and 4–64 mg/gTOC, respectively (Table 1). The S₁ + S₂ values varied from 20.35 to 236.59 mg/g (with an average of 112.94 mg/g), and most of them were around 100 mg/g, which illustrate high hydrocarbon generation potential of Maoming oil shales.

The composition and distribution of aliphatic hydrocarbon compounds in Maoming oil shales include a series of compounds such as n-alkanes, branched and acyclic isoprenoids, hopanes and steranes, with n-alkanes are the dominant components. The chromatograms of aliphatic fraction in representative oil shale samples are displayed in Figure 3. The carbon number of the n-alkanes is principally distributed in the range of n-C₁₃ to n-C₃₃, while these normal alkanes display a clear bimodal distribution with clear rear peak group dominance as the n-C₂₇ being the predominance n-alkane peak on the chromatograms of all samples on the profile. The front peak group of low carbon number alkanes is dominated by n-C₁₇ or n-C₁₅, while the rear peak group of high carbon number n-alkanes is solely dominated by n-C₂₇. The high carbon number n-alkanes display obviously a carbon number predominance of odd-over-even, as the carbon-preference index (CPI) of n-C₂₄ to n-C₃₄ is in the range of 2.33 to 4.54 on the whole profile (Figure 2), indicating low maturity or immaturity. Although the overall dominance of rear peak group over the front peak group of n-alkanes, the relative abundance of front peak group vs. the rear peak group show some variation along with burying depth on the profile, and these variation are likely due to the slightly change in the source inputs of organic matter. The relative abundance of low and medium carbon number alkanes of n-C₁₃ to n-C₂₁ and n-C₂₂ to n-C₂₆ varies along with bearing depth and the whole profile could be divided into three sections according to the distribution of the n-alkanes. The section I ranging from 815 to 828 m and the section III ranging from 840 to 854 m are characterized by relative lower abundance of low and medium carbon number n-alkanes, while the section II ranging from 828 to 840 m are characterized by relative higher abundance of low and medium carbon number n-alkanes.

Generally, the low carbon number n-alkanes (carbon number ≤ n-C₁₉) are considered as originated from algae and photosynthetic bacteria inputs (Gelpi et al., 1970; Meyers, 2003; Kristen et al., 2010), the medium-chain n-alkanes (n-C₂₀ ∼ n-C₂₆) often indicate aquatic plant sources (Ficken et al., 2000; Mügler et al., 2008; Aichner et al., 2010; Damsté et al., 2011), while the high carbon number n-alkanes (carbon number ≥ n-C₂₇) with an odd-over-even predominance are widely considered as related with terrestrial higher plant inputs (Eglinton and Hamilton, 1967). Therefore, some samples in section II (for example, MYY-24 at 829.6 m depth) contain high abundance of the medium carbon number n-alkanes, which suggested an incremental input of algae and macrophyte (Gelpi et al., 1970).

The main acyclic isoprenoids include the pristane (Pr) and the phytane (Ph) which occur in high abundance in all samples of whole profile. The Pr/Ph ratio (Figure 2) vary in the range of 0.80 to 2.42 on the profile and displays profile variation as it is in the range of 0.80 to 1.67 for section I and section II, and 1.15 to 2.42 for section III, respectively. Since the Pr/Ph ratio between 0.80 and 3.5 could not be solely used as depositional redox indicator, the Pr/Ph ratios of these oil shales should be combined with other proxies to reconstruct the paleoenvironment variation.

A great number of biomarkers including steroid and hopane compounds are detected in all the oil shale samples (Figures 4, 5).

The steranes identified in Maoming oil shales are mainly included the regular steranes, rearranged steranes (diasteranes), 4-methyl steranes and 4-methyl rearranged sterenes (4-methyl diasterenes). The sedimentary steranes are often transformed from steroids and sterenes that originated from organisms. The composition and distribution of regular steranes (including C₂₇, C₂₈, C₂₉ steranes) are consistent in most samples of the whole profile, with the high abundance of C₂₇-cholestanes and C₂₉-cholestanes and the low abundance of C₂₈-cholestanes. The 4-methyl steranes are consisted of a high abundance of the C₃₀-4-methyl steranes, low abundance of C₂₉-4-methyl steranes and C₂₈-4-methyl steranes. The rearranged sterenes are mainly consisted of trace amounts of C₂₈-rearranged sterenes, high abundance of the C₂₇-rearranged sterenes and C₂₉-rearranged sterenes. The 4-methyl rearranged sterenes are dominated by a high abundance of the C₃₀-4-methyl rearranged sterenes and a relative high amount of C₂₈-4-methyl rearranged sterenes, while the C₂₉-4-methyl rearranged sterenes are only detected in trace amount of few samples (Figure 4).

The hopanes are widely present in high abundance in all samples of the whole profile. The main compounds of hopanes are trisnorhopanes, norhopanes, hopanes, homohopanes, Δ¹⁷⁽²¹⁾-hopenes, and Δ¹³⁽¹⁸⁾-neohopenes, but the gammacerane is not detected (Figure 5). The hopenes are dominant compounds of pentacyclic-terpane as there are abundant of the Δ¹⁷⁽²¹⁾-hopenes are detected, which are considered as medium products which the ββ-hopanes transformed to βα-hopanes and αβ-hopanes (Zhou et al., 1998). The occurrence of high abundance of ββ-hopanes, Δ¹⁷⁽²¹⁾-hopenes and Δ¹³⁽¹⁸⁾-neohopenes in the samples also indicate that these oil shales of Youganwo Formation are of low maturity.

The composition and distribution of the hopanes and the hopenes in these oil shales are generally similar on the whole profile. However, the abundance of the Δ¹⁷⁽²¹⁾-hopene compounds are very low as the 30-Δ¹⁷⁽²¹⁾-norhopenes and Δ¹⁷⁽²¹⁾-trisnorhopenes are absent in section III. On the other hand, the abundance of the Δ¹³⁽¹⁸⁾-neohopene and Δ¹³⁽¹⁸⁾- norneohopene compounds appear to be significantly increased on the section III. Some previous studies suggested that the Δ¹³⁽¹⁸⁾-neohopene is generated from the parent material through molecular bond broke during the late diagenesis; and the others considered that the Δ¹³⁽¹⁸⁾-neohopene was transformed from thermodynamically unstable Δ¹⁷⁽²¹⁾-hopene as suggested by the δ¹³C compositon of the hopenes in the peat samples (Zhou et al., 1998).

Since the maturity of oil shale samples show little change on the profile (Figure 2), the lower abundance Δ¹⁷⁽²¹⁾-hopene and the higher abundance for both Δ¹³⁽¹⁸⁾-norneohopene and Δ¹³⁽¹⁸⁾-neohopene in section III than section II might not be related with the diagenesis process, but may be related to the variation in the sedimentary condition. As we have discussed above, the Pr/Ph ratio (Figure 2) shows an increasing trend along with the buried depth in section III, this may suggest that the deposition condition of section III could be more oxidized than that of upper sections. Therefore, the Δ¹³⁽¹⁸⁾-neohopene would be better preserved or formed than Δ¹⁷⁽²¹⁾-hopene under an oxidizing sedimentary condition.

It is worthwhile to point out that C₃₃-botryococcanes and C₃₁-botryococcanes are source-specific biomarkers and has been intermittently detected in high abundance in most samples of the top section at a depth from 815 to 821 m (Figures 3, 6), as the peak height of one C₃₃-botryococcane compound is even higher than that of the dominant n-alkane compound of n-C₂₇. The botryococcanes are a kind of irregular acyclic isoprenoids and are considered as transformed from botryococcenes, which originated from Botryococcus braunii which is a kind of algae living in fresh to brackish water or even estuarine area (Moldowan and Seifert, 1980; McKirdy et al., 1986; Smittenberg et al., 2005; Volkman, 2014). Since the Botryococcus braunii is sensitive to the variation of the water condition, the occurrence of botryococcanes in the sediments suggests obvious change in the environmental condition and a decline in the abundance of botryococcanes are often considered as indication of earlier anoxic condition (Smittenberg et al., 2005). The intermittently high abundance presence of the botryococcanes on the top section of the profile indicates that the lake environment was frequently favorable for the thriving of Botryococcus braunii B. and therefore there is a significant fluctuation in lake environment during the later stage of sedimentation in the Maoming basin.

The δ¹³C composition of n-alkanes in representative samples of oil shales varies from −31.7 to −16.2‰ on the profile. The δ¹³C composition of n-C₁₇ and n-C₁₉, which are considered as derived from bacteria and algae, range from −26.9 to −19.6‰ and −29.6 to −20.0‰, respectively. The δ¹³C of n-C₂₃ and n-C₂₅, which are often considered as originated mainly from aquatic macrophytes (Ficken et al., 2000), range from −30.2 to −18.4‰ and −29.5 to −18.5‰, respectively. While the δ¹³C of these n-C₂₇, n-C₂₉, n-C₃₁, and n-C₃₃, that are considered as a predominantly contribution from terrigenous plants, range from −28.8 to −16.2‰, −28.9 to −20.5‰, −30.9 to −21.1‰, and −31.7 to −20.1‰, respectively. The δ¹³C composition of the same n-alkane compounds varies among these samples on the profile (Figure 7).

The profile variation trends of the δ¹³C composition of the medium odd carbon number n-alkanes of n-C₂₃ and n-C₂₅ are similar to that of the high odd carbon number n-alkanes of n-C₂₇, n-C₂₉, and n-C₃₁. These shows that the δ¹³C composition variation of individual hydrocarbon compounds are covariant on the profile and therefore suggests that these changes were caused by similar factors, such as changes in organic matter inputs (the direct carbon source) or of partial pressure (pCO₂) of atmospheric CO₂ (the indirect and the elementary carbon source).

The δ¹³C variation trends of individual hydrocarbon compounds are also similar to that of bulk organic matter (δ¹³C_org) in the profile (Figure 7). The coefficiency of the δ¹³C composition among individual n-alkane compounds are analyzed by Pearson correlation methods (level of significance alpha = 0.050). The results show that the δ¹³C of n-C₁₇ has low correlation coefficients with that of n-C₂₉ (0.388), n-C₃₁ (0.440), and n-C₃₃ (0.514), while the δ¹³C of n-C₁₉ also shows low correlation coefficients with that of n-C₂₉ (0.496), n-C₃₁ (0.551), and n-C₃₃ (0.531), respectively. On the other hand, the δ¹³C of n-C₁₉ are closely correlated with the δ¹³C of bulk organic matter, while the δ¹³C of n-C₂₃, n-C₂₅, and n-C₂₇ are closely correlated with that of the δ¹³C of bulk organic matter and the δ¹³C of all other n-alkane homologs (Table 2). So, the primary sources of organic matter in the sediments of Youganwo Formation were mainly composed of aquatic algae and macrophytes, including some of the aquatic (or terrestrial) higher plants input. Since the δ¹³C_org are highly correlated with the δ¹³C of n-alkanes (Table 2), the δ¹³C variation of medium and high carbon number n-alkanes on the profile is likely not caused by the changes in the source inputs of organic matter, but possibly correlated with the variation of atmospheric pCO₂ during the late Paleocene. In addition, the δ¹³C of all individual n-alkane compounds on the profile display a positive excursion trend from the bottom to the top, and the δ¹³C of the n-alkanes display dramatic changes in the section II of the profile (Figure 7). The general upward positive excursion and the dramatic changes of the δ¹³C of n-alkanes on the profile may correlate with the general decline trend and with the frequent fluctuations in the atmospheric pCO₂ during the late Paleocene (McKay et al., 2016).

Among the steranes, only C₃₀-4-methyl steranes are present in high abundance in all samples of the whole profile. The C₃₀-4-methyl steranes are considered as derived from Dinoflagellates or methylotrophic bacteria. The methylotrophic bacteria prefer to consume isotopically lighter methane generated from the methanogens and would consequently result in isotopically lighter lipids (biomarkers) (Games et al., 1978), while the δ¹³C composition of C₃₀-4-methyl steranes are in the range of −11.93 to −6.28‰ (Table 3) which are very heavier. Therefore, the high abundance of C₃₀-4-methyl steranes was most likely derived from Dinoflagellates due to its much heavier δ¹³C composition.

Meanwhile, the δ¹³C composition of the botryococcanes is very heavier and ranges from −4.5 to −8.4‰ (Table 3). The exceptional heavier δ¹³C of C₃₃-botryococcenes could not be explained by sedimentary conditions alone. Under an aquatic condition of CO₂ (aq) exhaustion, B. braunii had to take alternative carbon source by triggering its bicarbonate (HCO₃^–) consumption mechanism (Cepak and Lukavsky, 1994). Since the partial pressure of atmospheric carbon dioxide (pCO₂) during the late Paleocene was significantly lower (Galeotti et al., 2016; Steinthorsdottir et al., 2016) and this could result in low concentration of dissolved CO₂ (aq) in the lake water, and therefore triggered bicarbonate consumption mechanism of the B. braunii (Huang et al., 1999). HCO3- assimilation by B. braunii would cause a significant heavier δ¹³C composition of organic matter generated by the B. braunii and other algae. Hence, the extreme heavy δ¹³C composition of the C₃₃-botryococcanes and the C₃₁-botryococcanes in the samples on the top section of the Youganwo Formation suggests that the atmospheric pCO₂ during the late Paleocene decreased significantly in the low altitude continental region and frequently resulted in lower CO₂(aq) concentration in lake water and therefore would have triggered bicarbonate consumption mechanism of the B. braunii.

According to the previous studies, the atmospheric pCO₂ in the late Paleogene displayed a general decreased trend but associated with sharp fluctuations (Pagani et al., 2005; Pearson et al., 2009; Galeotti et al., 2016). So, a general positive excursion associated with significant fluctuation in the δ¹³C composition of the n-alkanes on the profile of Maoming oil shales, especially the frequently and sharply variation in the section II, could be considered as correlated to the atmospheric pCO₂ variation during the late Paleogene, while the heavier δ¹³C composition of some source specific biomarkers would be indication of the lower concentration aquatic CO₂ in the lake water due to the lower atmospheric pCO₂ which caused the bicarbonate consumption mechanism of algae such as B. braunii. Therefore, the variation in the δ¹³C composition of the n-alkanes on the profile and a general positive excursion from the bottom to the top could be considered as responses to the atmosphere pCO₂ decline and variation toward the late Paleogene.