According to the GB/T 18602–2012 standard for “Rock Pyrolysis Analysis,” rock pyrolysis experiments were conducted using the ROCK EVAL 6 pyrolyzer to calculate total organic carbon content (TOC), hydrogen index (HI), oxygen index (OI), and hydrocarbon generation potential (He et al., 2016; Li et al., 2019a; Qin et al., 2018; Cui et al., 2023). The source rock samples were dried and crushed to an appropriate particle size to ensure uniformity. The pretreated samples were placed in the pyrolyzer, where the temperature was gradually increased to release organic matter. During the pyrolysis process, the TOC, HI, and OI of the samples were determined. Additionally, the hydrocarbon generation potential (S₁ + S₂) was calculated, where S1 represents the hydrocarbons that have been generated but not yet expelled from the sample, and S2 represents the potential hydrocarbons that can still be generated.

Following the GB/T 19145–2022 standard for “Determination of Total Organic Carbon in Sedimentary Rocks,” total organic carbon content in the source rock samples was measured using a carbon-sulfur analyzer (CS744). The pretreated samples were placed in the analyzer, where high-temperature combustion decomposed the organic carbon in the samples, simultaneously measuring the amount of CO₂ released (Xu et al., 2023).

Based on the SY/T 5125–2014 standard for “Identification and Classification Methods of Microscopic Components in Transmitted Light-Fluorescence Kerogen,” the microscopic component characteristics and vitrinite reflectance (Huo et al., 2019; Li et al., 2019b; Yang et al., 2020) of the source rock thin sections were observed using a polarized light microscope. The samples were examined to identify the microscopic components of kerogen (such as amorphous matter, alginite, and lignin) and to record their relative abundances. Vitrinite reflectance (R_o) was measured using a reflected light microscope to assess the thermal maturity level of the source rocks. The research technology roadmap is shown in Figure 3.

The organic carbon content evaluation criteria adopted in this study are derived from the previous classification of organic carbon content of Upper Paleozoic mudstone source rocks in the central Ordos Basin, focusing on the organic carbon content of carboniferous and Permian coal-bearing strata (Ratnayake, 2024; Li, 2015; Chen et al., 2023).

The coal samples primarily originate from the Benxi Formation and the Taiyuan Formation, with total organic carbon (TOC) ranging from 60.34% to 85.15%, and an average value of 68.31%. The carbonaceous mudstones are distributed across the Shanxi, Benxi, and Taiyuan formations, with TOC ranging from 9.12% to 36.8% and an average of 18.79%. In the study area, the organic carbon content of 122 mudstone samples from the Upper Paleozoic varies between 0.04% and 8.41%, with an average of 1.87%, indicating a good hydrocarbon generation capacity (Figure 4).

Analysis of the organic carbon content of source rocks in the Upper Paleozoic of the Ordos Basin shows that the organic carbon indices of mudstones from the Carboniferous-Permian are generally above the moderate hydrocarbon source rock threshold of 0.6%, reaching the standards for moderate source rocks. Additionally, approximately half of the samples meet the criteria for good source rocks, reflecting a favorable hydrocarbon generation potential. Among them, the organic matter content in the mudstone source rocks of the Shihezi Formation ranges from 0.04% to 1.54%, with an average of 0.31%, indicating that most are non-source rocks. The organic carbon content in the mudstone source rocks of the Shanxi Formation ranges from 0.10% to 8.41%, with an average of 1.64%, categorizing them as poor to good source rocks. The organic carbon content in the mudstone source rocks of the Taiyuan Formation varies from 0.36% to 8.21%, averaging 3.45%, placing them in the good to very good source rock category. Finally, the organic carbon content of the mudstone source rocks in the Benxi Formation ranges from 0.16% to 8.13%, with an average of 3.18%, also categorizing them as good to very good source rocks (Table 2).

Based on the analysis of organic carbon content in the Upper Paleozoic source rocks of the Yanan Gas Field, it is evident that the organic carbon indices of the Carboniferous to Permian strata generally exceed the moderate hydrocarbon source rock threshold of 0.6%, consistently meeting the criteria for moderate source rocks. Specifically, most samples from the Shihezi Formation are classified as non-source rocks, while the mudstone source rocks from the Taiyuan and Benxi Formations exhibit exceptionally high organic carbon content, reaching the levels of very good source rocks, indicating that this stratigraphic sequence possesses excellent hydrocarbon generation potential. Additionally, the presence of numerous coal seams and carbonaceous mudstones within the study area further highlights their hydrocarbon generation capabilities (Figure 4).

Furthermore, according to the contour map of organic carbon content in the mudstones of the Upper Paleozoic in the Yanan Gas Field (Figure 5), the organic carbon values in the study area display a general trend of lower values in the south and higher values in the north. Only in the Ganquan-Fuxian region does the organic carbon content fall below 0.5%, while most areas exceed the threshold of 1.0% for good source rocks. In the central and northern regions of the study area, organic carbon content typically reaches above 3.0%.

Due to the high maturity to over-maturity stage of the Upper Paleozoic coal-measure source rocks in the study area, most of the residual soluble organic matter and thermally degradable organic matter have escaped. Consequently, the content of chloroform bitumen “A” is generally low and cannot accurately indicate the organic matter abundance in this region; it serves merely as a reference for comprehensive evaluation.

The chloroform bitumen “A” content in the Upper Paleozoic source rocks of the study area is relatively low, ranging from 0.0002% to 0.0023%. Notably, the chloroform bitumen “A” content in coal samples is higher than that in mudstone samples, with values ranging from 0.0018% to 0.017%. Except for two mudstone samples from the Benxi Formation, the chloroform bitumen “A” content in other mudstone source rocks is significantly lower than in coal samples. The average content of chloroform bitumen “A” in Upper Paleozoic mudstones is 0.00117%, with values ranging from 0.0002% to 0.023%, showing a gradual increase from shallow to deep layers. Specifically, the chloroform bitumen “A” content in the Shihezi Formation mudstones ranges from 0.0005% to 0.0021%, with an average of 0.0009%; in the Shanxi Formation, it ranges from 0.0002% to 0.0037%, with an average of 0.0014%; in the Taiyuan Formation, it ranges from 0.0004% to 0.0046%, with an average of 0.0017%. The Benxi Formation has only two mudstone samples, with chloroform bitumen “A” content ranging from 0.0193% to 0.0230% (see Table 3).

The minimum lower limit for the hydrocarbon generation potential of non-source rocks in coal-measure source rocks is set at 0.5 mg/g (Wang and Yin, 2019). Similar to chloroform bitumen, for source rocks with a maturity level higher than high maturity, the hydrocarbon generation potential serves only as a reference for comprehensive evaluation according to relevant standards.

In the study area, the hydrocarbon generation potential (S1 + S2) of the Upper Paleozoic coal-measure source rock samples is generally low, except for the coal samples. Specifically, the S1 + S2 values for coal samples range from 0.90 mg/g to 8.047 mg/g, with an average of 2.78 mg/g. In contrast, the S₁ + S₂ values for mudstone range from 0.01 mg/g to 4.22 mg/g, with an average of 0.291 mg/g.

For the Shihezi Formation mudstone, the S₁ + S₂ values range from 0.02 mg/g to 0.52 mg/g, with an average of 0.152 mg/g. The Shanxi Formation mudstone shows S₁ + S₂ values ranging from 0.01 mg/g to 1.49 mg/g, averaging 0.204 mg/g. The Taiyuan Formation mudstone has S₁ + S₂ values between 0.08 mg/g and 4.22 mg/g, with an average of 0.693 mg/g. Finally, the Benxi Formation mudstone exhibits S₁ + S₂ values ranging from 0.06 mg/g to 0.51 mg/g, averaging 0.212 mg/g (see Table 4).

A total of 213 samples of mudstone and coal were analyzed for the identification of kerogen, with the relative percentage distribution of its four types presented in Table 5. Among the samples, there are 109 instances of Type III kerogen, 62 instances of Type II₂ kerogen, 47 instances of Type II₁ kerogen, and 12 instances of Type I kerogen.

In the study area, the kerogen types extracted from coal samples are primarily Type III kerogen, with vitrinite content exceeding 80%. In the mudstone samples, Type III kerogen is predominant, with the main composition being vitrinite, while the contents of exinite and inertinite are relatively low. However, in the Upper Paleozoic mudstone source rocks, not only is there a significant presence of Type III kerogen, but there is also a certain proportion of Type II₁ and II₂ kerogen, while Type I kerogen is relatively scarce (Thul, 2012) (Figure 6 and Table 6).

Based on the contour map of organic matter type indices for the Upper Paleozoic source rocks in the Yanan Gas Field (Figure 7), it is observed that the organic matter type index exhibits a pattern of higher values in the central area and lower values towards the edges. According to the classification standards for organic matter type indices, areas with indices less than 20 are predominantly composed of humic kerogen, while those with indices ranging from 20 to 40 mainly consist of humic-sapropelic kerogen. In regions with indices greater than 40, sapropelic-humic kerogen is the dominant type.

From the analysis of Figure 7, it can be concluded that the overall kerogen in the study area is primarily of humic type. In the central region of the study area, the kerogen is mainly composed of humic-sapropelic and sapropelic-humic types. Additionally, both the western and eastern edges of the study area feature the development of humic-type kerogen.

By projecting the H/C atomic ratio and O/C atomic ratio onto the “Van Krevelen” diagram (see Figure 8), it can be observed that most of the sample points from the study area fall on both sides of the Type III kerogen region, while a small portion of samples is located within the Type II kerogen area. This indicates that the organic matter type of the Permian source rocks in the Yanan Gas Field generally leans towards gas generation. However, there are notable differences across various regions.

The analysis suggests that the kerogen types favoring gas generation can be attributed to two main factors: first, the samples have relatively high maturity levels; second, most of these samples originate from fluvial facies, which possess poorer organic matter quality, thus naturally categorizing them within the humic kerogen range.

The data presented in Figure 8 indicates that the O/C and H/C atomic ratios of most samples from the study area are located on both sides of the Type III and Type II kerogen regions, suggesting that the organic matter type of the Permian source rocks in the Yanan Gas Field generally leans towards gas generation. However, variations exist across different regions.

The analysis identifies two primary reasons for the kerogen types favoring gas generation: first, the samples exhibit relatively high maturity levels; second, these samples predominantly originate from fluvial facies deposits, which have poorer organic matter quality, thus classifying them within the humic kerogen range.

In the Upper Paleozoic coal source rock samples from the Yanan gas field, the reflectance (Ro) values of 5 coal samples range from 1.95% to 3.28%, with an average value of 2.67%, indicating that they are in the over mature gas stage. The Ro values for 30 mudstone samples from the Shihezi Formation range from 1.48% to 2.25%, with an average value of 2.01%. The Ro values for 65 mudstone samples from the Shanxi Formation range from 1.66% to 2.78%, with an average value of 2.19%, also indicating the over mature gas stage. The Ro values for 5 mudstone samples from the Taiyuan Formation range from 1.88% to 2.82%, with an average value of 2.22%. The Ro values for 4 mudstone samples from the Benxi Formation range from 1.98% to 2.88%, with an average value of 2.31%. Overall, these samples exhibit characteristics indicative of the over mature gas stage (see Table 7). These data suggest that the coal source rocks in the Upper Paleozoic of the Yanan gas field are generally in the over mature gas stage, reflecting a high degree of thermal evolution of the organic matter in this area.

In the central Ordos Basin, the planar characteristics of the reflectance (Ro) of vitrinite in the Upper Paleozoic coal source rocks indicate that most areas in the study zone have Ro values greater than 2.0%, generally reaching the over mature gas stage (see Figure 9). The planar distribution of Ro values shows a certain regularity, with the lowest maturity of the source rocks in the central area of the study zone at 1.7%, gradually increasing towards the northwestern and southwestern edges, reaching a maximum of 3.3%. Overall, the organic matter maturity of the Upper Paleozoic coal source rocks in the Yanan gas field is relatively high, generally in the over mature gas stage, indicating that this area is well-developed for Upper Paleozoic source rocks.

According to the analysis results of the Tmax values of core samples from the Upper Paleozoic coal source rocks in the Yanan gas field, all five tested coal samples in the study area are in the over mature stage, with pyrolysis peak temperatures ranging from 561°C to 572°C, with an average value of 568°C. Among the 119 shale samples, the minimum Tmax value is 377°C, the maximum is 587°C, and the average is 531°C. Specifically, the Tmax values for the shale samples of the Shihezi Formation have a minimum of 419°C, a maximum of 564°C, and an average of 516°C; the Shanshi Formation shale samples have a minimum of 377°C, a maximum of 587°C, and an average of 529°C; the Taiyuan Formation shale samples have a minimum of 486°C, a maximum of 580°C, and an average of 552°C; the Benxi Formation shale samples have a minimum of 491°C, a maximum of 573°C, and an average of 538°C (Table 8).

When the maximum pyrolysis peak temperature exceeds 490°C, it indicates that the organic matter has entered the over mature stage. In the coal source rock samples from the Yanan gas field, 86% of the samples have maximum pyrolysis peak temperatures exceeding 490°C, indicating that the coal source rocks of the Shihezi Formation, Shanshi Formation, Taiyuan Formation, and Benxi Formation have overall entered the over mature stage (see Figure 10).