The microscopy identification of kerogen component uses a biological microscope with the function of projecting white light and epi-fluorescence to identify the microscopy kerogen components to determine the types of kerogen. The seven samples from the section are microscopically examined. The microscopy kerogen components are mainly amorphous sapropelinite with a little inertinite. The seven samples are observed under the microscope, and a large amount of brown dispersed and flocculent saprolite, a minor amount of black inertinite, rare colorless transparent minerals are found, determining that the kerogen of all samples are Type II₁ (Figure 4).

The microscopic observation (left) shows that the sample P1701-3-3 includes amorphous brown dispersed and flocculent sapropelinite (A), black inertinite (B) and colorless transparent mineral (D). The microscopic observation (right) shows that all organic components do not fluoresce under blue light excitation.

It is classified according to the classification method of organic matter type index (T_i) (Shuai Qin et al., 2012) [10] , T_i = (content of sapropelinite × 100 + content of exinite × 50 − content of vitrinite × 75 − content of inertinite × 100)/100. The type index of the seven samples varies between 56 and 78. The samples can be judged to be Type II₁ sapropelic kerogen, as shown in Table 2.

For the hydrocarbon-forming parent material of kerogen, its stable carbon isotope composition has not changed much after a long and complicated evolutionary stage [11]. According to previous studies, the organic matter of the Mesoproterozoic source rocks in northern North China is mainly enriched in light carbon isotope [12]. Golyshev et al. [13] believe that the sapropelic kerogen is also enriched in light carbon isotopes and the range of δ¹³C is between −27.5‰ and −32.2‰. According to the values of protein data bank (PDB) of the seven samples (Table 3), it can be concluded that the type is sapropelic kerogen, and the source of the parent material may be algae dominated lower aquatic organisms. According to the correlation diagram between kerogen types and δ¹³C (Tissot et al., 1974) [14] , most of the samples fall in the overlapped intervals of Type I and Type II kerogen, and one sample falling outside the interval may be due to over maturity (Figure 5). In summary, it can be identified that the samples may be Type I kerogen (Sapropelic) and Type II₂ kerogen (humic-sapropelic).

Biomarker compounds contain many geochemical characteristics of source rocks, which have unique advantages in judging organic matter types. Shanmugam [15] uses the plot of Pr/n-C₁₇ vs. Ph/n-C₁₈ to determine the types of organic matter. Through the sampling point (Figure 6), all seven samples fall within the intervals of type I and type II₁. The ratio of Pr/nC₁₇ is low, indicating that it is formed in a partial reduction depositional environment.

The relative contents of saturated hydrocarbon, aromatic hydrocarbon, non-hydrocarbon, and asphaltene are obtained by extracting the section samples. These values can be used not only to study the parent material characteristics of the samples but also to judge the organic matter types by the relationship between the contents. Seven samples are analyzed for the composition of chloroform bitumen A group (Table 4). The saturated hydrocarbon content ranges from 13.16% to 41.07%, most of which are between 20% and 40%; the saturated/aromatic hydrocarbon ratio ranges from 0.83 to 3.7, mainly between 1 and 3. The non-hydrocarbon plus asphaltene content ranges from 21.62% to 61.91%, demonstrating that it’s Type II (mixed type) kerogen, and Type II₁ is dominated.

For layers containing little vitrinite, the vitrinite reflectance of high-grade plant debris cannot be used as the criterion for maturity [17]. The sapropelic reflectance (R_D) and the reflectance after conversing the equivalent vitrinite reflectance were used for analysis (Zhong Ningning et al. 1995) [18]. The formula is: R_o = 1.302 × R_D + 0.185. The organic matter reflectance R_o of the seven samples on the section ranges from 1.68% to 1.94%, with an average of 1.83%, which shows a high maturity stage with the potential to generate oil and gas.

The odd/even carbon number predominance (OEP) value is the predominant odd/even carbon value obtained by taking the two values near the main peak carbon. As the thermal evolution level of organic matter increases, the odd carbon number predominance will gradually decrease and tend to become 1. The OEP value of the seven samples varies from 0.87 to 1.11, with an average of 1.05, showing a higher thermal evolution level.

The phenanthrene series compounds are widely used in judging the maturity of crude oil and source rocks, especially at the high maturity stage [19] [20].

Regarding the organic matter reflectance value of the samples, the conversion formula R_c = −0.50MPI + 2.27 (Radke & Welte, 1981) [21] is used to obtain the parameters by calculation (Table 5). It can be seen that R_c ranges from 1.63% to 1.81%, and the average value is 1.71%, demonstrating high maturity and obvious potential of hydrocarbon generation.