Effect of Ancient Salinity on the Distribution and Composition of Tricyclic Terpane in Hydrocarbon Source Rocks in the Mahu Depression

Abstract

Ma2 and Ma3 hydrocarbon source rock samples from the Fengcheng Formation in well Maye 1, Mahu Depression, Junggar Basin, were studied using conventional geochemical analysis methods and saturated hydrocarbon gas chromatography–mass spectrometry. The distribution patterns, abundance, relative content, and ratios of different carbon compounds of tricyclic terpane in hydrocarbon source rocks from fresh-to-mildly-saline (type I), moderately saline (type II), and saline (type III) water environments significantly differed. The C₂₈–C₂₉TT/C₃₀H and C₁₉–C₂₉TT/C₃₀H ratios were the lowest in the type I hydrocarbon source rock. The relative ratios of C₂₃TT/C₂₁TT, C₂₅TT/C₂₄TT, C₂₈TT/C₂₆TT, (C₂₃–C₂₆TT)/(C₁₉–C₂₂TT), and (C₂₈–C₂₉TT)/(C₁₉–C₂₂TT) gradually increased with the increase in the salinity of the hydrocarbon source rock. The percentage of low-carbon tricyclic terpanes gradually decreased to 28%, whereas those of the medium- and high-carbon tricyclic terpanes increased to 52% and 20%, respectively. The differences in triterpane types of different hydrocarbon source rocks were mainly controlled by the depositional environment. The primary factor that controlled the distribution pattern; relative abundance, especially the high carbon tricyclic terpane content; and differences in the relative ratio of different carbon compounds in different hydrocarbon source rocks was the salinity of the ancient waterbody during deposition.

1. Introduction

Tricyclic terpane compounds are widely distributed in geological bodies, including hydrocarbon source rocks and crude oil, and are important components of saturated hydrocarbon biomarker compounds. Anders et al. first identified C₂₀–C₂₅ tricyclic terpanes from the Green River shale in the United States; subsequently, Seifert et al. further confirmed the widespread presence of tricyclic terpane compounds in the pyrolysis products of hydrocarbon source rocks and biodegraded crude oils [1,2,3]. Moldowan et al. first detected long-chain tricyclic terpanes in crude oil, with carbon numbers reaching up to 45; a decade later, Grande used GC/MS/MS analysis techniques to detect complete C₂₉–C₅₄ long-chain tricyclic terpanes from samples of hydrocarbon source rocks and crude oil from different depositional environments, implying that these compounds contain rich geological and geochemical information in geological bodies [4,5]. Therefore, the relative abundance of tricyclic terpane compounds and the ratio of compounds with different carbon numbers can be used to explore organic matter sources, thermal maturity, depositional environments, and the degree of biodegradation of crude oil [6,7,8,9,10,11,12,13,14,15,16,17,18].

Disagreement persists regarding the precursors of tricyclic terpane compounds. The main two viewpoints are as follows: First, it is believed that the extraction products of tricyclic terpanes from salt lake mudstones and marine carbonate rock hydrocarbon source rocks, as well as the biological sources of tricyclic terpanes in crude oil, are bacteria or algae; the high abundance of tricyclic terpanes is generally associated with rocks rich in Tasmanites algae from high paleolatitudes, indicating their origin from these algae [10,14,19,20]. Second, it is believed that hydrocarbon source rocks and crude oil in the highly overmature stage are relatively enriched in tricyclic terpane compounds. This means that the higher the maturity is, the greater the tricyclic terpane content. Absolute concentration and relative content changes in tricyclic terpane compounds in terrestrial organic matter, such as lignite deposited under weak oxidation–weak reduction conditions, as well as in high-temperature and high-pressure thermal simulation products of strongly reducing saltwater lake hydrocarbon source rocks, revealed the enrichment of tricyclic terpane compounds. The ratio of tricyclic terpanes to hopanes significantly increases with increasing temperature [21]. In addition, the distribution pattern and relative abundance of tricyclic terpanes with different carbon numbers in different environmental hydrocarbon source rocks differ [22,23,24,25,26].

2. Geological Background

The Junggar Basin is a typical large overlapping oil and gas basin and the Mahu Depression is a sublevel negative structural unit in the central depression of the basin. Located in the northwestern part of the Junggar Basin, the Mahu Depression extends from the Wuxia Fault Zone to the north of the Basong Uplift in the south, borders the Kepai Fault Zone and Zhongguai Uplift to the west, and extends to the Xiayan Uplift and Yingxi Depression in the east. With an area of ~5000 km², it is a hydrocarbon-generating depression with the highest oil and gas enrichment levels in the Junggar Basin. During the Early-to-Middle Permian, Chara and Zailiysky Alatau were continuously thrusted and overthrusted into the basin under the intense collision between the Siberian Plate and Junggar Block, resulting in an overall asymmetric fold–thrust belt structure in the Permian of the Mahu Depression. Controlled by ancient topography and lithofacies paleogeography, the Permian sedimentary environment in the Mahu Depression is diverse and the stratigraphy is complete. The Fengcheng Formation in the Lower Permian exhibits sedimentary characteristics of “thick and steep in the northwest, thin and gentle in the southeast” in plan view, with a thickness of up to 1800 m in the depocenter. It can be divided into three vertical segments, with the main development of fine-grained sedimentary rocks in lacustrine and shallow-to-deep lake low-energy environments, including dark gray to light black mudstone, argillaceous dolomite, and dolomitic siltstone. Occasional volcanic-eruption-related constructions can be found in the lower-to-middle parts of the first segment of the Fengcheng Formation in northern China. Near the basin margin, orogenic belts and foreland coarse clastic sedimentation of the fan delta are predominant. During the deposition of the Fengcheng Formation, the lake level in the Mahu Depression fluctuated frequently and the depression had the highest salinity in the Permian saline lake basin of the Junggar Basin (Figure 1).

The burial depth of the Maye 1 well in the Fengcheng Formation, which is 439.5 m thick, ranges from 4494.4 to 4933.9 m. The first segment of the Fengcheng Formation (Feng 1) has a thickness of 98.1 m, with the bottom consisting of tuff and fine sandstone and the top containing alkaline minerals. The second segment of the Fengcheng Formation (Feng 2) has a thickness of 218.4 m. This segment is mainly composed of mudstone and dolomitic mudstone, with well-developed alkaline minerals. The third segment of the Fengcheng Formation (Feng 3) has a thickness of 123 m and predominantly consists of mudstone and dolomitic mudstone, with fine sandstone at the top. Vertical lithological changes indicate that the Mahu Depression experienced overall environmental transitions of lake level lowering, evaporative alkaline lake basin formation, and lake level rise during the deposition of the Fengcheng Formation [27].

3. Samples and Analysis

3.1. Samples

The Maye 1 well is located on the northern slope of the Mahu Depression and contains well-developed hydrocarbon source rocks in the second and third members of the Permian Fengcheng Formation. In this study, 223 mudstone samples were systematically collected, including 87 from the third member and 136 from the second member of the Fengcheng Formation. After completing the organic carbon and rock pyrolysis analyses, 39 representative samples were selected for further analyses, such as the determination of the vitrinite reflectance, chloroform asphaltene “A” extraction, compound separation, and saturated hydrocarbon GC-MS testing.

3.2. Analytical Conditions

The conditions for saturated hydrocarbon GC-MS analysis: An HP GC 6890/5973MSD mass spectrometer was used. The chromatographic column was an HP-5MS flexible quartz capillary column (30 m × 0.25 mm × 0.25 μm) with splitless injection, a pulse pressure of 15 psi, injector temperature of 300 °C, helium as the carrier gas, and flow rate of 1 mL/min. The heating program was as follows: the temperature was initially held at 50 °C for 2 min, ramped up to 310 °C at a rate of 3 °C/min, and held for 18 min. The electron ionization (EI) method was used with an ionization energy of 70 eV.

4. Results and Discussion

4.1. Geochemical Characteristics of Hydrocarbon Source Rocks

4.1.1. Organic Matter Abundance

The abundance of organic matter is an indicator for evaluating source rocks and is one of the most important factors in determining the hydrocarbon generation potential of source rocks. The total organic carbon (TOC) content of the Fengcheng Formation, Fensanduan Member (P₁f₃), in the Maye 1 well in the Mahu Depression ranged from 0.11% to 2.85%, with an average value of 0.58%. The hydrocarbon generation potential ranged from 0.12 to 12.84 mg/g, with an average value of 2.14 mg/g, and the chloroform bitumen “A” content mostly varied from 0.04% to 0.92%, with an average value of 0.26%. The TOC value (136 mudstone samples) of the Feng’erd Member (P₁f₂) in the Maye 1 well ranged from 0.41% to 2.40%, with an average value of 0.96%. The hydrocarbon generation potential ranged from 2.02 to 15.41 mg/g, with an average value of 4.86 mg/g, and the chloroform bitumen “A” content varied from 0.05% to 1.34%, with an average value of 0.28%. Based on the TOC content, hydrocarbon generation potential, and chloroform bitumen “A” content, the organic matter abundance of the Fengcheng Formation, that is, the Fensanduan and Feng’erd members, in the Maye 1 well in the Mahu Depression was overall at a medium-to-good level in terms of source rocks. Figure 2 reveals a positive correlation between TOC and the hydrocarbon generation potential (Pg) in the Fengcheng Formation, that is, in the Fensanduan and Feng’erd members, in the Maye 1 well. The organic carbon frequency distribution statistics show that 40% of the mudstone samples in the Fensanduan Member had a TOC content of <0.5%, whereas >60% of the mudstone samples in the Feng’erd Member had a TOC concentration of >0.5%. This indicates that the mudstone in the Feng’erd Member had a higher organic matter abundance (Figure 2).

4.1.2. Organic Matter Type and Thermal Evolution Degree

The diagram showing the correlation between the hydrogen index (HI) and T_max of 223 mudstone samples from the Maye 1 well in the Fengcheng Formation (Figure 3) indicates that the main kerogen type of the source rock in the Fensanduan Member (P₁f₃) was type II kerogen, with a small amount of type III kerogen. The main kerogen type of the source rock in the Feng’erd Member (P₁f₂) was type II₁ kerogen, followed by type II₂ kerogen and a small amount of type I kerogen, indicating that the organic matter types of the Fensanduan and Feng’erd members in the Maye 1 well were similar and both were mainly composed of type II kerogen.

The maturity of organic matter is a key factor controlling the generation of oil and gas. The abundance and type of organic matter in source rocks are the material bases for oil and gas generation, representing only the quality of the material basis for generating oil and gas, which is not equivalent to the generation of oil and gas. Significant hydrocarbon generation begins only when the organic matter reaches a certain degree of thermal evolution. Based on the analysis of vitrinite reflectance and pyrolysis data of the Fengcheng Formation in this well, the distribution range of Ro values for mudstone samples from the Fensanduan Member ranged from 0.71% to 0.92%, with the majority of T_max values varying between 430 and 457 °C and an average value of 440 °C. The distribution range of Ro values for mudstone samples from the Feng’erd Member ranged from 0.72% to 0.97%, with the majority of T_max values varying between 431 and 451 °C and an average value of 441 °C. Therefore, it can be inferred that the organic matter thermal evolution of the source rocks in the Fengcheng Formation of the Maye 1 well was in a mature stage and the maturity levels of the source rocks in both the Fensanduan and Feng’erd members were similar (Figure 4).

4.2. Tricyclic Terpane Composition of Source Rocks

Gammaceranes are a class of compounds commonly found in source rocks and crude oil. Their content often characterizes the stratification of waterbodies in marine and non-marine source rock depositional environments because of salinity differences. In other words, an increase in the water salinity during source rock deposition leads to an increase in the gamma alkane index [10]. Therefore, gammaceranes are used as markers to reflect the salinity depositional environment. The depositional environment of organic matter can be semi-quantitatively determined using the gamma-alkane index. Wang et al. [28] proposed that gamma-alkane indexes ranging from 0.78 to 1.45, 0.82 to 0.30, and 0.30 to 0.08 indicate brackish, semi-brackish, and freshwater-to-slightly-brackish lake facies, respectively. Gammaceranes were abundant in the source rocks of the Maye 1 well in the Mahu Depression. The respective gamma-alkane index ranged from 0.06 to 1.39 (

Table 1. Geochemical parameters of source rock of Fengcheng Formation from the Maye 1 well.

Depth/m	Lithology	Layer	1	2	3	4	5	6	7	8	9	10	Type
4580.85	Mudstone	P1f3	0.13	1.05	0.48	0.39	1.00	0.89	0.84	1.47	0.56	1.17	Type I source rock
4585.59	Mudstone	P1f3	0.11	1.14	0.48	0.40	1.08	0.97	0.96	1.54	0.62	1.12
4590.70	Mudstone	P1f3	0.12	1.19	0.48	0.39	0.91	0.91	0.80	1.31	0.45	1.38
4595.61	Mudstone	P1f3	0.06	1.26	0.47	0.36	0.94	0.83	0.99	1.39	0.60	0.92
4599.68	Mudstone	P1f3	0.11	1.04	0.48	0.35	0.96	0.84	0.77	1.31	0.46	1.14
4601.96	Mudstone	P1f3	0.11	1.16	0.46	0.36	0.99	0.82	0.75	1.34	0.44	1.14
4605.10	Mudstone	P1f3	0.11	0.84	0.47	0.40	1.03	0.84	0.65	1.26	0.41	1.23
4609.34	Mudstone	P1f3	0.16	0.93	0.47	0.41	1.08	0.68	0.93	1.46	0.61	2.34
4613.67	Mudstone	P1f3	0.23	0.92	0.47	0.38	0.99	0.88	0.82	1.25	0.45	2.04
4631.20	Mudstone	P1f2	0.25	0.81	0.47	0.36	1.18	1.13	0.90	1.56	0.51	2.44
4639.47	Mudstone	P1f2	0.15	0.94	0.46	0.34	1.01	0.97	0.89	1.38	0.47	1.82
4646.64	Mudstone	P1f2	0.26	0.93	0.47	0.33	1.04	0.88	0.81	1.32	0.48	1.68
4660.71	Mudstone	P1f2	0.19	0.92	0.48	0.35	1.20	1.02	0.90	1.41	0.39	1.78
4669.49	Mudstone	P1f2	0.30	0.85	0.50	0.35	1.11	0.94	0.90	1.38	0.45	2.24
4678.02	Mudstone	P1f2	0.20	0.91	0.48	0.36	1.17	1.07	1.00	1.50	0.50	2.33
4683.82	Mudstone	P1f2	0.31	0.80	0.48	0.35	1.43	1.17	0.91	1.65	0.51	2.30	Type II source rock
4687.45	Mudstone	P1f2	0.30	0.76	0.48	0.36	1.43	1.18	1.16	1.78	0.69	2.23
4693.94	Mudstone	P1f2	0.45	0.64	0.48	0.34	1.42	1.11	1.16	1.62	0.57	2.04
4704.14	Mudstone	P1f2	0.40	0.65	0.47	0.35	1.37	1.03	0.99	1.60	0.48	2.07
4708.80	Mudstone	P1f2	0.64	0.63	0.47	0.33	1.46	1.03	1.12	1.49	0.50	2.02
4710.54	Mudstone	P1f2	0.33	0.65	0.48	0.36	1.27	1.01	0.95	1.51	0.43	3.24
4713.61	Mudstone	P1f2	0.69	0.60	0.48	0.33	1.49	1.06	1.10	1.60	0.55	1.85
4719.36	Mudstone	P1f2	0.68	0.61	0.48	0.33	1.62	1.18	1.17	1.77	0.73	1.71
4721.97	Mudstone	P1f2	0.39	0.68	0.48	0.35	1.37	1.07	1.06	1.63	0.56	2.68
4725.59	Mudstone	P1f2	0.46	0.62	0.47	0.34	1.51	1.16	1.21	1.86	0.77	2.13
4729.64	Mudstone	P1f2	0.52	0.55	0.48	0.34	1.40	1.06	1.23	1.61	0.70	1.71
4734.34	Mudstone	P1f2	0.69	0.56	0.48	0.33	1.56	1.10	1.23	1.61	0.60	2.01
4742.28	Mudstone	P1f2	0.54	0.55	0.48	0.35	1.39	1.04	1.09	1.61	0.52	2.67
4759.58	Mudstone	P1f2	0.64	0.61	0.48	0.35	1.38	1.06	1.07	1.51	0.48	2.53
4770.86	Mudstone	P1f2	0.54	0.71	0.47	0.36	1.32	1.05	1.05	1.60	0.49	3.13
4789.14	Mudstone	P1f2	0.97	0.74	0.47	0.34	1.35	1.12	1.04	1.68	0.57	2.77	Type III source rock
4792.80	Mudstone	P1f2	1.00	0.73	0.48	0.34	1.38	1.12	1.13	1.82	0.71	2.57
4794.96	Mudstone	P1f2	1.04	0.80	0.48	0.34	1.30	1.06	1.15	1.62	0.58	3.11
4796.75	Mudstone	P1f2	1.04	0.82	0.46	0.34	1.31	1.04	1.09	1.71	0.61	3.31
4797.49	Mudstone	P1f2	1.04	0.83	0.46	0.36	1.23	1.05	1.12	1.66	0.64	2.93
4799.57	Mudstone	P1f2	1.35	0.77	0.48	0.35	1.33	1.01	1.27	1.68	0.66	3.49
4813.17	Mudstone	P1f2	1.39	0.89	0.46	0.35	1.24	0.97	1.17	1.64	0.64	4.29
4816.27	Mudstone	P1f2	1.00	0.77	0.47	0.36	1.81	1.10	1.40	2.62	1.27	3.18
4836.18	Mudstone	P1f2	0.91	0.76	0.46	0.37	1.54	1.15	1.20	2.28	0.99	3.22

Note: 1—Ga/C₃₀H; 2—Pr/Ph; 3—C₂₉ααα20S regular sterane; 4—C₂₇-regular sterane/C₂₉-regular sterane; 5—C₂₃TT/C₂₁TT; 6—C₂₅TT/C₂₄TT; 7—C₂₈TT/C₂₆TT; 8—C₂₃–C₂₆TT/C₁₉–C₂₂TT; 9—C₂₈–C₂₉TT/C₁₉–C₂₂TT; 10—C₁₉–C₂₉TT/C₃₀H.

), reflecting significant changes in the ancient water salinity during the deposition of source rocks in the Maye 1 well in the Mahu Depression. From the bottom to the top of the stratigraphic section, the lower part of the Feng’erd Member corresponded to a saline water depositional environment and type I source rock, the middle part corresponded to a semi-brackish water depositional environment and type II source rock, and the upper part of the Feng’erd Member and the lower part of the Fensanduan Member corresponded to a freshwater-to-slightly-brackish water depositional environment and type III source rock.

4.2.1. Distribution of the Tricyclic Terpane Carbon Number

The abundance and distribution patterns of tricyclic terpane compounds with different carbon numbers often reflect the organic matter sources and depositional environments of hydrocarbon generation. Generally, in humic-type source rocks (with land-derived higher plants as the main hydrocarbon source), C₁₉TT dominates the tricyclic terpanes, with gradually decreasing contents of C₁₉TT, C₂₀TT, and C₂₁TT showing a stepwise distribution and a relatively low content of C₂₃TT [29,30,31,32,33,34,35,36,37,38]. In freshwater lacustrine source rocks, C₂₁TT tends to be dominant in tricyclic terpanes and the C₂₃TT content is significantly higher than the C₂₁TT concentration [31]. In saline and marine source rocks, C₂₃TT is dominant, with a very low content of C₁₉TT [5].

Figure 5 shows the distribution of tricyclic terpanes in different types of source rocks in the Maye 1 well. Generally, the C₁₉–C₂₉ tricyclic terpane series compounds (except for C₂₇TT) exhibited a relatively complete distribution in the source rock samples from different depositional environments, which was mainly manifested as the predominant peak of C₂₃TT. The distribution of low-carbon compounds was C₂₄TT > C₂₅TT > C₂₆TT, whereas high-carbon C₂₈TT and C₂₉TT were relatively abundant. In comparison, the type I source rocks had the highest C₂₀TT and C₂₁TT contents, followed by the type II source rocks. The lowest C₂₀TT and C₂₁TT contents were observed in the type III source rocks. In addition, the gammacerane content in source rocks with different ancient water salinities significantly differed, with a high gamma-alkane content in source rocks deposited in saline water environments. The latter was higher than that of C₃₀ hopanes. The second most abundant source rocks were deposited in semi-saline water environments, with a gammacerane content similar to that of C₃₁ hopanes. The lowest gammacerane content was identified in source rocks deposited in fresh-to-slightly-brackish water environments.

4.2.2. Distribution of Different Carbon Number Ratios of Tricyclic Terpanes

The relative ratios (parameters) of different carbon compounds within tricyclic terpanes in the source rocks and crude oil reveal the organic matter source, depositional environment, degree of thermal evolution, and extent of biodegradation of the crude oil. The C₂₃TT/C₂₁TT ratio reflects differences in the salinity level of a waterbody during source rock deposition; generally, the more saline the water is, the more enriched the C₂₃ tricyclic terpane is [10,39]. Considering the characteristics of the organic matter type (mainly type II kerogen), with a similar thermal evolution degree (both mature stage source rocks) but significantly different ancient water salinity of the source rock samples in this study, the correlations between the C₂₃TT/C₂₁TT, C₂₅TT/C₂₄TT, and C₂₈TT/C₂₆TT ratios, between the ratio of mid-carbon-number (C₂₃–C₂₆TT) to low-carbon-number (C₁₉–C₂₂TT) compounds, and between the ratio of high-carbon-number (C₂₈–C₂₉TT) to low-carbon-number (C₁₉–C₂₂TT) compounds were studied (Figure 6). Figure 6 reveals strong positive correlations between the C₂₃TT/C₂₁TT, C₂₅TT/C₂₄TT, and C₂₈TT/C₂₆TT ratios of different types of source rocks, the ratio of mid-carbon-number (C₂₃–C₂₆TT) to low-carbon-number (C₁₉–C₂₂TT) compounds, and the ratio of high-carbon-number (C₂₈–C₂₉TT) to low-carbon-number (C₁₉–C₂₂TT) compounds. These ratios increased with the increase in the water salinity during source rock deposition. Samples of freshwater-to-slightly-brackish source rocks (type I) were clustered in one area, whereas it was difficult to distinguish samples of semi-brackish (type II) and saline (type III) source rocks, which were clustered in the same area.

4.3. Correlations between Tricyclic Terpane Compounds and Other Biomarker Parameters

4.3.1. C₂₉ Sterane 20S/(20S + 20R)

Farrimond et al. revealed significant changes in the mutual transformation and relative ratios of different stereoisomers of tricyclic terpanes with an increasing degree of thermal evolution in the geological thermal evolution profile [11]. The organic matter in the source rock samples in this study was in the peak oil/gas generation stage of maturity (Ro ranging from 0.71% to 0.97%). Therefore, the C₂₉ sterane 20S/(20S + 20R) maturity parameter can be utilized to represent the maturity biomarker maturity and explore the effect of maturity on the distribution of tricyclic terpanes (Figure 7). Figure 7 shows that the ranges of the C₂₉ sterane 20S/(20S + 20R) maturity parameter were narrow due to the relatively small span of maturity of source rocks in the study area, ranging from 0.46 to 0.50. This indicates that the ratios of tricyclic terpanes of different carbon numbers, that is, C₂₃TT/C₂₁TT, C₂₅TT/C₂₄TT, C₂₃–C₂₆TT/C₁₉–C₂₂TT, and C₂₈–C₂₉TT/C₁₉–C₂₂TT, were less affected by the maturity.

4.3.2. C₂₇ Regular Sterane/C₂₉ Regular Sterane

Huang et al. [32] and Meinschein et al. [33] studied the abundance and relative compositions of C₂₇, C₂₈, and C₂₉ sterols in various modern sediments, such as various types of organisms, and the Gulf of Mexico. They pointed out that the C₂₇ regular sterane is mainly of marine origin and the C₂₉ regular sterane is derived from higher plants. Therefore, the use of the distribution of C₂₇ and C₂₉ regular steranes to study the input of the source materials was proposed. The predominant kerogen type in the source rocks studied in this work was type II, indicating that the primary biological sources of these source rocks were mainly lower aquatic organisms and land-derived higher plants, as manifested in the narrow ranges of the C₂₇ regular sterane/C₂₉ regular sterane ratio (0.33 to 0.41; Table 1). This implies that the ratios of tricyclic terpanes with different carbon numbers, that is, C₂₃TT/C₂₁TT, C₂₅TT/C₂₄TT, C₂₃–C₂₆TT/C₁₉–C₂₂TT, and C₂₈–C₂₉TT/C₁₉–C₂₂TT, were less affected by the source material (Figure 8).

5. Discussion

As mentioned earlier, the distribution and relative ratios of tricyclic terpanes in the different types of source rocks in the study area significantly differed. However, it is difficult to distinguish between these differences using the maturity parameter of the biological marker C₂₉ sterane 20S/(20S + 20R) and the parameters representing the sources of lower aquatic organisms and land-derived higher plants, that is, C₂₇ regular sterane/C₂₉ regular sterane. This implies that the redox conditions and paleosalinity of the ancient depositional environment of source rocks may have had a significant effect on the distribution patterns, relative ratios, and abundance of tricyclic terpanes in the source rocks.

The pristane/phytane ratio (Pr/Ph) is an effective parameter reflecting redox conditions. Generally, Pr/Ph ≤ 0.8 indicates a reducing, high-saline, or carbonate sedimentary environment; 0.8 < Pr/Ph ≤ 3.0 indicates a weakly oxidizing–weakly reducing environment; and Pr/Ph > 3.0 indicates an oxidizing environment with strong terrigenous input [34]. The relative abundance of different carbon compounds in tricyclic terpanes in the source rocks often serves as an indicator of the depositional environment. For example, C₁₉TT and C₂₀TT mainly originate from higher plants, whereas C₂₃ tricyclic terpane mainly originates from algae. Their relative contents in marine and brackish lacustrine environments are significantly higher than those in terrestrial freshwater environments and they typically serve as biomarkers for marine and lacustrine environments. Different source rocks in the Maye 1 well exhibited varying Pr/Ph ratios. The Pr/Ph ratios of samples from the type I source rocks from freshwater-to-slightly-brackish environments were greater than 0.8, ranging from 0.81 to 1.16, with an average value of 0.99. This indicates that these samples were deposited in an oxidizing–reducing environment. Except for a few samples from types II and III source rocks in brackish-to-saline environments, the Pr/Ph ratios of other samples were less than 0.8, with an overall distribution between 0.55 and 0.77 and an average value of 0.70, indicating that these samples were deposited in a reducing environment (Table 1). Figure 9 was prepared to further explore the correlations between Pr/Ph and the ratios of tricyclic terpanes with different carbon numbers in different types of source rocks. This figure reveals a relatively good separation between the type I and types II and III source rocks; however, the samples from the types II and III source rocks were mixed, making it difficult to differentiate them.

As previously stated, the ratio of tricyclic terpanes to 17α (H)-hopanes (TT/C₃₀H) is commonly used to characterize the thermal evolution degree and source of organic matter. Peters et al. confirmed that the TT/C₃₀H ratio gradually increases with increasing pyrolysis temperature during the hydrothermal process of Monterey shale, which may be due to the rapid migration of tricyclic terpanes or the strong affinity of 17α (H)-hopanes to the rock matrix [35]. In fact, because of the different precursors of tricyclic terpanes and hopanes, considerable differences exist in the content and TT/C₃₀H ratio of tricyclic terpanes in source rocks and crude oil from different source rocks or even the same source rock but different lithofacies. This difference can be used to characterize the maturity of the source rocks and crude oil and to distinguish the genetic types of petroleum in different sedimentary environments [10]. The ratios of tricyclic terpanes to 17α (H)-hopanes (C₁₉–C₂₉TT/C₃₀H) in different types of source rock samples in this study exhibited a strong positive correlation with the gammacerane index (Figure 10a), with the types I, II, and III source rock distributed in different regions. The TT/C₃₀H ratio of the type I source rock samples from freshwater to slightly brackish environments was the lowest, ranging from 0.92 to 2.44, with an average of 1.65. The type II source rock from brackish water environments had a higher TT/C₃₀H ratio, ranging from 1.71 to 3.24, with an average of 2.29. The type III source rock from saline water environments had the highest TT/C₃₀H ratio, ranging from 2.57 to 4.29, with an average of 3.21. This indicates that the TT/C₃₀H ratio gradually increased with the increase in the salinity of the ancient waterbody. Furthermore, the ratio of tricyclic terpanes with high carbon number (C₂₈–C₂₉TT) to C₃₀ hopanes strongly and positively correlated with the gammacerane index for different types of source rocks (Figure 10b). In other words, saline environments were more conducive to the formation and enrichment of tricyclic terpanes in the source rocks of this area. As early as 1993, the results of studies on the characteristics of tricyclic terpane compounds in different sedimentary environments in Brazilian source rocks and crude oil showed that these compounds are more abundant in source rocks and crude oil extracted from saline and marine carbonate source rocks, indicating certain salinity conditions for their bioprecursors [5]. The results of a study of the geochemical characteristics of Permian source rocks on the northwestern margin of the Junggar Basin by Chen et al. showed that mature Fengcheng Formation source rocks, whether they belong to evaporitic, reducing saline lake sedimentary carbonates or strongly reducing deep lacustrine sedimentary mudstone, have a high abundance of tricyclic terpanes originating from locally deposited algae and bacteria [14].

In addition to influencing the abundance of tricyclic terpanes, the salinity of ancient waterbodies also affects the ratios of tricyclic terpanes with different carbon numbers. Figure 11 shows the ratio between the gammacerane index and single-carbon tricyclic terpenes (C₂₃TT/C₂₁TT and C₂₈TT/C₂₆TT) in different types of source rocks, as well as the ratio between high-, medium-, and low-carbon tricyclic terpenes, showing a strong overall positive correlation between these parameters. These three types of source rocks are distributed across different regions. In combination with the relative content and relative ratios (

Table 2. Relative ratio and percentage content of tricyclic terpanes of different types of source rock from Maye 1 well.

Sedimentary Environment	Tricyclic Terpane Ratios					Relative Percentage Content of Tricyclic Terpanes (%)			Type
	1	2	3	4	5	6	7	8
Freshwater-to-slightly-brackish water environment	0.91~1.20 1.05	0.68~1.13 0.91	0.65~1.00 0.86	1.25~1.56 1.39	0.39~0.62 0.49	32~37 35	46~51 48	14~20 17	Type I source rock
Moderate-salinity environment	1.27~1.62 1.43	1.01~1.18 1.09	0.91~1.23 1.10	1.49~1.86 1.63	0.43~0.77 0.57	27~34 31	49~52 51	15~21 18	Type II source rock
Saline environment	1.23~1.81 1.39	0.97~1.15 1.07	1.04~1.40 1.17	1.62~2.62 1.86	0.57~1.27 0.74	20~31 28	50~54 52	18~26 20	Type III source rock

Note: 1—C₂₃TT/C₂₁TT; 2—C₂₅TT/C₂₄TT; 3—C₂₈TT/C₂₆TT; 4—(C₂₃–C₂₆TT)/(C₁₉–C₂₂TT); 5—(C₂₈–C₂₉TT)/(C₁₉–C₂₂TT); 6—(C₁₉–C₂₂TT)/(C₁₉–C₂₉TT); 7—(C₂₃–C₂₆TT)/(C₁₉–C₂₉TT); 8—(C₂₈–C₂₉TT)/(C₁₉–C₂₉TT).

) of tricyclic terpanes with different carbon numbers (low, medium, and high), these results reveal that the relative ratios of C₂₃TT/C₂₁TT, C₂₅TT/C₂₄TT, C₂₈TT/C₂₆TT, (C₂₃–C₂₆TT)/(C₁₉–C₂₂TT), and (C₂₈–C₂₉TT)/(C₁₉–C₂₂TT) were the smallest in the type I source rocks from freshwater-to-slightly-brackish environments, with average values of 1.05, 0.91, 0.86, 1.39, and 0.49, respectively. The percentages of low-, medium-, and high-carbon tricyclic terpanes were 35%, 48%, and 17%, respectively. As the salinity of the source rock water gradually increased, the source rock type transitioned from type I to types II and III. The relative ratios of C₂₃TT/C₂₁TT, C₂₅TT/C₂₄TT, C₂₈TT/C₂₆TT, (C₂₃–C₂₆TT)/(C₁₉–C₂₂TT), and (C₂₈–C₂₉TT)/(C₁₉–C₂₂TT) also gradually increased, with average values of 1.39, 1.07, 1.17, 1.86, and 0.74, respectively. The percentage of low-carbon tricyclic terpanes as a proportion of the total tricyclic terpanes gradually decreased to 28%, whereas the percentage of medium- and high-carbon tricyclic terpanes gradually increased to 52% and 20%, respectively.

In summary, the distribution pattern and relative abundance, especially those of tricyclic terpanes with a high carbon number, in the Fengcheng Formation source rock of the Maye 1 well in the Mahu Depression were mainly controlled by the depositional environment. The relative differences in the tricyclic terpane contents of different carbon compounds were mainly controlled by the salinity of the ancient waterbody, which represented the main controlling factor of the differences in the tricyclic terpanes of different types of source rocks.

6. Conclusions

(1) The Fengcheng Formation source rock contained a complete series of C₁₉–C₂₉ tricyclic terpane compounds, with C₂₃TT as the main peak. The lower-carbon-number compounds C₁₉–C₂₁TT showed a gradually increasing trend, while the middle-carbon-number compounds C₂₄–C₂₆TT showed a gradually decreasing trend.

(2) There were significant differences in the distribution pattern, abundance, and relative percentage content of tricyclic terpanes, with the type I source rocks having the lowest contents of the high- and middle-carbon-number tricyclic terpanes and the highest contents of the lower-carbon-number tricyclic terpanes. The opposite distribution pattern was observed in the type II and type III source rocks compared with the type I source rocks.

(3) The differences in tricyclic terpanes of different types of source rocks in the study area were influenced to a lesser extent by the thermal maturity of organic matter and the biological source of the parent material and were primarily controlled by the depositional environment variation, especially for the salinity change in the waterbody.