Basis of Identification, Type of Syngenetic Assemblage, and Pattern of Development of Coal and Oil Shale in the Tanshan Area of the Eastern Liupanshan Basin, China

Abstract

The Yan’an Formation in the Liupanshan Basin hosts substantial coal and oil shale resources. However, coal and oil shale often exhibit different types of associated or syngenetic combinations, which makes it difficult to recognize coal and oil shales, and research on the patterns of development of coal and oil shales is lacking. In this study, field outcrop, core, logging, and analytical data are comprehensively utilized to describe the characteristics of coal and oil shale, classify their syngenetic combinations, and establish a developmental model. Analytical results from the Tanshan area reveal that coal exhibits a lower density and higher oil content than oil shale. Specifically, coal shows oil contents ranging from 7.22% to 13.10% and ash contents of 8.25–35.66%, whereas oil shale displays lower oil contents (3.88–6.98%) and significantly higher ash contents (42.28–80.79%). The oil and ash contents of both coal and oil shale in the Tanshan area show a negative correlation, though this correlation is significantly stronger in coal than in oil shale. In long-range gamma-ray and resistivity logs, coal exhibits substantially higher values compared to oil shale, whereas in density logs, oil shale shows greater values than coal. Acoustic time difference logging reveals marginally higher values for coal than for oil shale, though the difference is minimal. There are five combination types between coal and oil shale in this area. The oil shale formed in a warm, humid, highly reducing lacustrine environment within relatively deep-water bodies, while coal developed in swampy shallow-water environments; both derive organic matter from higher plants. Variations in depositional settings and environmental conditions resulted in five distinct combination types of coal and oil shale.

1. Introduction

With the increasing global shortage of oil and other conventional energy sources, oil shale has gained attention as an unconventional oil and gas resource. Its abundant resources, relatively easy exploitation, diverse applications, and mature processing technologies have made it a focal point in unconventional energy research [1,2,3]. Notably, it is regarded as one of the most promising new energy sources for partially replacing petroleum [4,5,6].

The Liupanshan Basin is jointly located in the Ordos Block, the Alashan Block, and the Qiliann–Qingling Fold Belt (see Figure 1) and is a small Mesozoic–Cenozoic and Cenozoic terrestrial lake basin developed on the metamorphic crystalline basement and the Galilean Fold basement [7,8,9]. Due to its special tectonic setting and the presence of two sets of hydrocarbon source rocks, the Liupanshan Basin has become an important area for oil and gas exploration in northern China. Currently, hydrocarbon exploration in the Liupanshan Basin primarily focuses on the Cretaceous system in the middle and western parts of the basin. Numerous scholars have conducted studies on various aspects, including basin tectonic evolution, hydrocarbon source rocks and petroleum systems, hydrocarbon potential, reservoir characteristics, and organic matter types via path investigations and seismic, drilling, and electromagnetic methods and have made many breakthroughs [10,11,12,13,14,15,16,17]. However, the Jurassic system, which also has exploration prospects, remains poorly understood, and the system has been relatively understudied. Some scholars have conducted preliminary studies on Jurassic hydrocarbon source rocks. Notably, Bai et al. reported that abundant oil shale resources, in addition to coal, are present in the Jurassic system in the Tanshan area in the eastern part of the Liupanshan Basin [18]. The oil shale is a medium-quality, siliceous-ash, low-sulfur, and coal-associated oil shale which formed primarily in aquatic environments with abundant water inputs and high water levels. The depositional environment was a warm, humid, anoxic, terrestrial freshwater lacustrine and swamp environment, and the type of oil shale generated was humic muddy humus [19,20,21]. The sediment source of the lower Yan’an Formation in this area was not only the basal sedimentary strata but also the Early Palaeozoic granite along the western margin of the Liupanshan Basin, Nanhuashan–Xihuashan, and Changcheng System’s Haiyuan Group, with the tectonic background representing a continental island arc environment formed by land–land collision [21,22]. Hai et al. conducted a study on Jurassic rocks in the Pengyang area, which is located at the southeastern margin of the Liupanshan Basin. They investigated the depositional environment of the Jurassic Yan’an Formation in this region and concluded that swamps, depressions, and meandering river deposits predominantly developed during the Yan’an Formation’s depositional period. Their findings indicate that widespread lakes did not form in this area during this geological interval [23].

Oil shale in the Tanshan area is interspersed with coal or as the top or bottom plate of coal seams [20]. The existence of oil shale has long been neglected in the process of coal exploration, and it has been directly researched and developed as a type of coal, resulting in a great waste of resources. However, the formation conditions of coal and oil shale are quite different, and their coexistence requires a unique geological environment; however, research in this area is relatively limited. In view of this, in this study, the basis for identifying coal and oil shale in the Tanshan area is summarized in detail through low-temperature dry distillation, whole-rock mineral X-ray diffraction analyses, borehole core observations, and logging; on this basis, the types of syngenetic combinations of coal and oil shale are clarified, and the method of oil shale development is further explored to provide a scientific basis for the exploration and development of oil shale in the area.

2. Geological Setting

The Tanshan area is located in the eastern part of the Liupanshan Basin in the Tanshan Bulge, close to the Yaoshan Bulge and the Tongxin Depression to the north, connected with the Guyuan Depression in the west, and adjacent to the Ordos Block of the North China Plate in the east (see Figure 1). The deep tectonics are active in the study area and its surroundings, being manifested by deep faults that spread in a nearly north–south direction, such as the Qingshuihe Great Fault on the west side of the Tanshan Bulge, the Qingtongxia–Guyuan Deep Fault on the east side of the bulge, and the development of a series of faults and folds with different orientations and natures in the intervals between the deep faults, which are mostly hidden underneath the Cenozoic and Cretaceous systems [20,22,24].

The study area is covered mostly by loess, with only sporadic bedrock outcrops in the southeastern, southwestern, and central deep-cut valleys (see Figure 2). According to drilling data, the stratigraphy of the study area consists mainly of Precambrian, Middle Jurassic, Palaeocene, Neocene, and Quaternary strata, and each stratum is in angular unconformity contact [20], among which the Middle Jurassic Yan’an Formation is the main ore-containing stratum with coal and oil shale in this area. The Yan’an Formation is a sandstone–conglomerate–mudstone assemblage with large variations in thickness and lithology, and the sedimentary facies is primarily fluvial, with local lake and swamp facies in the section [22]. Owing to their different lithologies and depositional environments, coal and oil shales can be divided into four lithological sections, and coal and oil shales are distributed in each lithological section but are concentrated mainly in the first and second lithological sections.

In the study area, anticlines and synclines are intermittently distributed, and the large-scale folds include mainly the Qinjiayao anticline, Huaichaliang syncline, Caozi anticline, Xiaoxiwan syncline, Guwan anticline, Dangjiagou syncline, and Yushuwan anticline. The axial direction of the folds is nearly north–south. On the fold axis and between the two wings, a series of nearly north–south-spreading faults are present, and the larger faults are F1, DF1, DF2, DF3, F21, F18, and F4, which are mostly high-angle reverse faults (see Figure 2).

3. Datasets and Methodology

This study analyzed the industrial indicators of oil shale and the chemical properties of coal in the Tanshan area based on data from field outcrops, core observations, logging curves, and geochemical tests. The specific experiments were conducted by Shaanxi Coalfield Geological Laboratory Testing Co., Ltd., China, using standardized methodologies: the oil content of oil shale was determined by the aluminum retort method [25], ash content by high-temperature combustion [26], total sulfur content by coulometric titration [27], volatile matter by muffle furnace pyrolysis [26], and calorific value by oxygen bomb calorimetry [28]. Four oil shale samples and two coal samples were analyzed by whole-rock mineral X-ray diffraction (XRD) at the Earth Science Laboratory of Cheung Kong University, using a SmartLab SE X-ray diffractometer.

4. Results

4.1. Coal and Oil Shale Characteristics

4.1.1. Coal Seam Characteristics

The coal-bearing stratum in the Tanshan area is the Middle Jurassic Yan’an Formation, which contains more than 30 seams of coal (including coal lines) and 16 numbered seams; most of the more stable and recoverable seams are number 4 seams (coal 14, coal 16, coal 17, and coal 19), the locally recoverable unstable seams are number 5 seams (coal 10, coal 11, coal 12, coal 15, and coal 21), and the rest are extremely unstable seams with occasional recovery points (coal 2, coal 6, coal 8, coal 18, coal 22, coal 23, and coal 24), and the average thickness of recoverable seams is 17.83 m. The coefficient of recoverable coal is 4.51%. The recoverable coal seams are distributed in all four lithological sections of the Yan’an Formation, but they are concentrated mainly in the lower parts of the first and second lithological sections (see

Table 1. Coal-bearing characteristics of various lithological sections of the Middle Jurassic Yan’an Formation in the Tanshan area.

Formation	Average Formation Thickness/m	Average Coal Seam Thickness/m	Average Minable Coal Seam Thickness/m	Coal Seam Coefficient/%	Minable Coal Seam Coefficient/%	Reservoir Number
J2y4	31.75	1.81	-	5.70	-	coal 2,
J2y3	117.78	8.14	5.47	6.91	4.64	coal 6, coal 8, coal 10, coal 11, coal 12
J2y2	85.30	3.66	3.84	4.29	4.50	coal 14, coal 15
J2y1	160.79	13.07	8.52	8.51	5.30	coal 16, coal 17, coal 18, coal 19, coal 21, coal 22, coal 23, coal 24

Note: The data in the table are from the [29].

). The coal quality of the recoverable seams is mainly low-ash, medium-sulfur, low-phosphorus, and medium-high-calorific-value coal. The coal types are mainly long-flame coal to weakly caking coal.

4.1.2. Oil Shale Characteristics

There are 15 layers of oil shale in the Tanshan area, with a cumulative thickness of 18.76 m, an average oil content of 4.94%, an average ash content of 61.25%, an average calorific value of 10.85 kJ/g, and an average total sulfur content of 0.57% (see

Table 2. Main industrial parameters of the oil shale in the Tanshan area.

Serial Number	Formation	Reservoir Number	Depth/m	Oil-Bearing Ratio/%	Ash/%	Calorific Value/(kJ/g)	Total Sulfur/%	Volatile Matter/%
1	J2y4	oil 1	1.10	4.50	42.28	17.77	0.51	43.55
2	J2y3	oil 2-1	0.86	4.11	74.49	5.33	0.15	68.97
3		oil 2	1.52	4.40	61.35	10.81	0.24	46.18
4		oil 3-1	1.29	4.13	80.79	3.33	0.20	72.32
5		oil 3	1.36	6.98	65.23	5.81	0.43	52.47
6		oil 3-2	0.93	3.88	62.55	10.66	0.42	49.72
7		oil 3-3	1.55	4.50	68.67	8.93	0.79	52.52
8	J2y2	oil 4-1	0.70	5.30	62.40	11.07	0.42	48.91
9		oil 4	2.32	5.31	61.76	11.26	0.48	50.33
10		oil 4-2	0.77	4.40	60.73	11.86	0.94	46.52
11		oil 5	1.56	6.04	58.13	12.33	0.75	50.22
12	J2y1	oil 6-1	0.89	4.30	59.69	11.22	1.60	49.11
13		oil 6	1.99	5.28	56.73	13.30	0.51	48.11
14		oil 7	1.14	5.79	50.49	15.37	0.72	44.01
15		oil 8	0.78	5.20	53.41	13.74	0.43	44.23
Total/Average			18.76(Total)	4.94	61.25	10.85	0.57	51.14

), of which oil 4 and oil 6 are the most stable (see Figure 3). Oil 4 is located in the second lithological section of the Yan’an Formation, with a maximum thickness of 3.37 m (well GY2), a minimum thickness of 0.88 m (four boreholes are oil-solidified), an average thickness of 2.32 m, an average oil content of 5.31%, and an average ash content of 61.76%, indicating a medium-quality oil shale; oil 6 is located in the first lithological section of the Yan’an Formation, with a maximum thickness of 2.37 m (three boreholes are oil-solidified), a minimum thickness of 1.39 m (four boreholes are oil-solidified), an average thickness of 1.39 m (four boreholes are oil-solidified), and an average thickness of 1.39 m. Oil 6 is located in the first lithological section of the Yan’an Formation, with a maximum thickness of 2.37 m (in oil-solidified borehole 3), a minimum thickness of 1.39 m (in oil-solidified borehole 4), and an average thickness of 1.99 m, with an average oil content of 5.28% and an average ash content of 56.73%, which indicates medium quality. The TOC (total organic carbon) content of oil shale ranges from 13.93% to 41.38%, with an average value of 27.87%. The kerogen types are predominantly Type III (humic) and secondarily Type II₂ (sapropelic-humic) [30]. The oil shale is often interbedded with coal or appears as the top or bottom plates of coal seams (see Figure 3).

4.2. Basis for the Identification of Coal and Oil Shale

4.2.1. Physical Characteristics

The coal seams in the Tanshan area have a low degree of metamorphism and are generally long-flame coals, with black and brown–black colors; brown–black striations; banded, linear, laminated, and blocky structures; bituminous and greasy lusters; low hardness; primarily crumbly and powdery textures; jagged fractures; and relatively large amounts of pyrite films in the sedimentary layers and fractures (see Figure 4a–c). The oil shale is lighter in color, mainly gray–black and dark gray, larger in texture, denser, with more jagged or shell-like fractures, darker with mostly asphalt lusters, and mainly blocky laminations (see Figure 4b–e).

4.2.2. Main Industrial Indicators

The results for the chemical properties of coal in the Tanshan area are shown in

Table 3. Chemical properties of major coal seams in the Tanshan area.

Serial Number	Seam Number	Tar Yield/%	Ash/%	Calorific Value/(kJ/g)	Total Sulfur/%	Volatile Matter/%
1	coal 6	10.20	30.01	22.13	0.96	45.11
2	coal 10	11.90	21.28	25.50	0.85	45.02
3	coal 11	13.10	8.25	27.99	0.79	44.37
4	coal 12	9.97	24.17	24.23	0.87	43.48
5	coal 14	10.20	20.55	25.88	0.85	44.30
6	coal 16	9.60	35.66	20.53	0.56	44.00
7	coal 17	9.30	32.01	22.11	1.19	42.34
8	coal 19	7.22	28.45	23.21	0.87	41.50
9	coal 21	9.25	25.81	23.86	0.67	41.91
Average value		10.08	25.13	23.94	0.85	43.56

Note: The data in the table are average values for different coal seams.

. To determine the difference between the oil shale and coal, the correlations between the chemical properties of coal and the main industrial indicators of oil shale were analyzed separately (see Figure 5).

The results of the study reveal that the oil contents of coal seams in the Tanshan area range from 7.22% to 13.10%, with an average of 10.08%, whereas the oil content of oil shale is generally lower than that of coal seams, with contents ranging from 3.88% to 6.98% and an average of 4.94%. The ash contents of the coal seams are all lower than 40%, ranging from 8.25% to 35.66%, with an average of 25.13%, whereas the ash contents of the oil shale are all greater than 40%, with values ranging from 42.28% to 80.79% and an average of 61.25%. The oil content (tar yield) and ash content of both coal and oil shale in the Tanshan area are negatively correlated, but the correlation with coal is significantly stronger than that with oil shale (see Figure 5a); the tar yield and heat generation of coal are obviously positively correlated, whereas there is no obvious relationship between the oil content and heat generation of oil shale (see Figure 5b). The ash and heat contents of coal and oil shale both exhibit obvious negative correlations, and the correlation coefficients are relatively similar (see Figure 5c); the volatile matter and heat contents of oil shale are obviously negatively correlated (see Figure 5d) and obviously positively correlated with the ash content (see Figure 5e); and there was little relationship between the volatile matter and heat and ash contents of coal (see Figure 5d,e). The oil content (tar yield) of both coal and oil shale have an insignificant relationship with total sulfur, but the range of values in total sulfur in oil shale is significantly greater than that in coal, and the total sulfur content of coal is more concentrated (see Figure 5f). In addition, the oil content and heat generation of coal are significantly greater than those of oil shale (see Figure 5a–d); the volatile fraction of oil shale is greater than that of coal, and the range of values is greater than that of coal, whereas the volatile fraction of coal is more concentrated (see Figure 5d,e).

4.2.3. Mineral Composition

The results of XRD of the samples are shown in

Table 4. Quantitative analytical results of X-ray diffraction of whole rock minerals of coal and oil shale in the Tanshan area.

Sample Number	Sample Name	Mineral Content/%
		Quartz	K-Feldspar	Plagioclase	Calcite	Dolomite	Siderite	Magnesite	Pyrite	Hematite	Analcime	Clay Minerals
TY-1	oil shale	26.4	1.4	1.0	0.1	-	-	0.4	2.2	-	5.0	63.6
TY-2	oil shale	46.7	2.7	1.3	-	0.8	0.6	-	-	-	-	47.9
TY-3	oil shale	51.1	2.3	1.5	-	-	11.6	-	-	-	-	33.5
TY-4	oil shale	52.1	1.9	0.9	-	0.5	1.2	-	-	-	-	43.3
Average		44.1	2.1	1.2	-	-	-	-	-	-	-	47.1
TM-1	coal	29.6	2.2	-	0.8	-	-	-	-	1.5	-	65.9
TM-2	coal	32.9	-	1.3	-	-	0.4	-	0.4	-	-	64.9
Average		31.3	2.2	1.3	-	-	-	-	-	-	-	65.4

. The oil shale samples in the Tanshan area are composed of mainly clay minerals, with an average content of 47.1%, followed by quartz, potassium feldspar, and plagioclase, with average contents of 44.1%, 2.1%, and 1.2% respectively; individual oil shale samples also contain rhodochrosite, dolomite, pyrite, calcite, and galena. For example, the content of rhodochrosite in the TY-3 oil shale sample is as high as 11.6%, and that of galena in the TY-1 oil shale sample is as high as 5.0%. The mineral composition of the coal is basically similar to that of the oil shale, with little difference, and coal is also mainly composed of clay minerals, with an average of 65.4%, which is slightly greater than that of the oil shale (see Figure 6). This is followed by quartz, potassium feldspar, and plagioclase feldspar, with averages of 31.3%, 2.2%, and 1.3%, respectively, and a relatively low quartz content (see Figure 6). Small amounts of hematite, calcite, rhodochrosite, and pyrite, etc., were also observed.

4.2.4. Logging Curves

The long-range gamma, resistivity, acoustic time difference, and density logging curves of the oil shale in the Tanshan area differ more obviously from those of coalbeds [20]. The characteristics of the logging curves of the GY1 borehole clearly reveal that in long-range gamma and resistivity logging, the coalbed is obviously larger than the oil shale, and the curves all show jump or stepped patterns. In density logging, the oil shale is larger than the coalbed, and the curve is relatively smooth but also shows a jump pattern. In acoustic time difference logging, the curve is smooth, the coal exhibits larger values than the oil shale, and the difference between the two is relatively small (see Figure 7). Previous studies have shown that long-range gamma energy is related mainly to the contents of radioactive U, Th, K, and other elements in a formation, and radioactive element contents are positively correlated with the organic matter content; organic matter is not conductive, and the higher the content of organic matter, the lower the conductivity, while the resistivity increases [31]. The total organic carbon (TOC) content of each recoverable coal seam in the Tanshan area is significantly greater than that of the oil shale, and that of the tar yield is also greater, with an average of 10.08% (see Table 3), which is significantly greater than the oil content of the oil shale; thus, the coal seams have larger values than the oil shale in the long-range gamma and resistivity logs. Typically, the acoustic time difference between mudstone and shale decreases with increasing burial depth, and when rocks contain oil, gas, or organic matter, the acoustic time difference increases with increasing organic matter content, so the acoustic time difference values of coal are greater than those of oil shale. However, the acoustic time difference is strongly affected by the mineral composition, mud content, and degree of compaction between rock particles [31]. The density refers mainly to the bulk (skeleton) density of the rock, and the density of organic matter is relatively small, close to 1.0 g/cm³. When organic matter replaces the rock skeleton, the density of the rock decreases, so the coal in the Tanshan area has a relatively low density.

5. Discussion

5.1. Types of Coal and Oil Shale Symbiotic Combinations

The lithologies of the tops and bottoms of oil shales in the oil shale boreholes in the Tanshan area were determined (see

Table 5. Lithological characteristics of the top and bottom surfaces of the oil shale in the Tanshan area.

Ore Vein Serial Number	GY1		GY2		GY3		GY4
	Topographic Lithology	Base Surface Lithology	Topographic Lithology	Base Surface Lithology	Topographic Lithology	Base Surface Lithology	Topographic Lithology	Base Surface Lithology
Oil 1	/	/	/	/	/	/	carbonaceous mudstone (geology)	siltstone
Oil 2-1	/	/	shale	shale	/	/	coal 6	siltstone
Oil 2	/	/	shale	siltstone	/	/	/	/
Oil 3-1	/	/	siltstone	siltstone	siltstone	coal 12	/	/
Oil 3	/	/	shale	siltstone	coal 12	coal 12	/	/
Oil 3-2	/	/	/	/	siltstone	siltstone	/	/
Oil 3-3	/	/	/	/	siltstone	siltstone	/	/
Oil 4-1	/	/	/	/	/	/	siltstone	coal seam (line)
Oil 4	coal 14	shale	coal 14	siltstone	shale	coal 14	shale	siltstone
Oil 4-2	shale	siltstone	/	/	/	/	/	/
Oil 5	/	/	siltstone	siltstone	/	/	shale	shale
Oil 6-1	/	/	/	/	/	/	siltstone	siltstone
Oil 6	siltstone	siltstone	siltstone	shale	coal 16	shale	siltstone	siltstone
Oil 7	shale	shale	/	/	siltstone	siltstone	/	/
Oil 8	/	/	shale	conglomerates	/	/	/	/

Note: “/” means that the layer of oil shale was not observed in the borehole.

), and the lithologies of the tops of the oil shale are mainly mudstone, siltstone, siltstone, coal, and fine sandstone; for example, oil 4 is the number 14 coal seam at the tops of boreholes GY1 and GY2 (see Figure 7), but it becomes mudstone at the tops of boreholes GY3 and GY4. Oil 6 is siltstone at the tops of boreholes GY2 and GY4. Oil 6 is a silty mudstone at the top of drill hole GY1, a medium sandstone at the top of drill hole GY3, and the number 16 coal seam at the top of drill hole GY3. The lithologies of the bottom surface of the oil shale are mainly mudstone, siltstone, fine sandstone, coal, and siltstone, such as oil 6, which is mudstone at the bottom surface of the GY2 and GY3 drill holes; siltstone at the bottom surface of drill hole GY4; and fine sandstone at the bottom surface of drill hole GY1. Oil 4 is mudstone at the bottom surface of drill hole GY1 (see Figure 7), siltstone at the bottom surface of drill hole GY2, and the number 14 coal seam at the bottom surface of drill hole GY3. The individual oil shale is fine sandstone. The individual oil shale is mudstone at the bottom surface of drill hole GY1 (see Figure 7), siltstone at the top surface of drill hole GY2, and the number 16 coal seam at the top surface of drill hole GY3. The individual oil shale is fine sandstone; individual oil shale formations have conglomerate base surfaces, such as in oil 8 (see Table 5).

On the basis of the relationships of the lithological assemblages between oil shale and coal, mudstone, siltstone, etc., with emphasis on the characteristics of coal assemblages, the types of syngenetic assemblages of coal–oil shale in the Tanshan area are classified into the following five types (see Figure 8): (1) coal seam/oil shale (C–OS); (2) oil shale/coal seam (OS–C); (3) oil shale/coal seam/oil shale (OS–C–OS); (4) coal seam/oil shale/coal seam (C–OS–OS–C); and (5) oil shale/other sediment/coal seam (OS–F/S–C), with the other sediments, mainly fine sandstone, mudstone, and silty mudstone. The existence and development of these five different types of syngenetic assemblages reflect the special types of geological conditions in the area and the regularity and variability of their evolution and indicate that both coal and oil shale had their own geological conditions during formation, and the differences between them are relatively large.

5.2. Patterns of Oil Shale Development

The differences between coal and oil shale in terms of physical properties, industrial indices, mineral compositions, and well logging curves primarily stem from variations in their original organic matter sources, depositional environments, and sedimentation mechanisms. Generally, both coal and oil shale formation require relatively stable tectonic settings, warm-humid climates, abundant organic matter supply, moderate water depth, and limited terrigenous clastic input. Their material sources include both higher plants and lower plants. The distinctions lie in the following aspects: Coal typically forms in shallow submerged swamp environments, whereas oil shale develops in environments with greater variations in water depth, most crucially requiring stable water column stratification in lakes. Additionally, coal-forming materials are dominated by higher plants primarily through terrigenous clastic input, while oil shale’s material sources mainly consist of lower plants with predominantly chemical precipitation origins [32,33,34].

Furthermore, the development of oil shale is jointly constrained by multiple factors such as paleoclimate, paleoproductivity, and preservation conditions. Different symbiotic association types reflect unique geological conditions and the regularity–variability of their evolutionary processes. The Chemical Index of Alteration (CIA) and trace element analyses indicate that the oil shale in the Tanshan area formed in a warm-humid, oxygen-deficient continental freshwater lacustrine–swamp environment [20]. Building on these insights and integrating the symbiotic relationship between coal and oil shale in the Tanshan area (see Figure 9), the developmental model of oil shale was systematically explored.

Under warm and humid climatic conditions, close to the sedimentation center location, the water body was deep, and stratification of the water body occurred, with the upper part of the water body having a high oxygen content, high productivity, and development of herbaceous plants and algal microorganisms; the lower part of the water body was in a long-term strongly reducing environment, which inhibited the oxidative decomposition of organic matter and provided better conditions for the preservation of organic matter, resulting in the formation of oil shale with a high oil content. As the water body shallowed, it changed from a lake environment to a swamp environment, at which point terrestrial higher plants developed, peat depressions further developed, and thicker coal seams eventually formed. Thus, the C–OS syngenetic assemblage resulted vertically. Coal seams began to develop in the bog area, and as precipitation increased or the sedimentary center migrated, the water body became deeper, and oil shale developed, resulting in an OS–C syngenetic assemblage from this environment. In the depositional center, a short-lived depressional environment occurred, after which the water body became deeper again, resulting in an OS–C–OS syngenetic assemblage. In areas where swampy depositional environments predominated, seasonal precipitation caused the water body to deepen and then turn swampy again, resulting in the C–OS–C syngenetic assemblage. In areas where oil shale developed, the deposition of silt and fine sand occurred when the river carried more land-sourced coarser sediments into the lake, and the development of oil shale continued with the disappearance of the river or the deepening of the water body, thus forming an OS–F/S–C syngenetic assemblage.

6. Conclusions

The Yan’an Formation in Tanshan area of the Liupanshan Basin develops multiple sets of coal and oil shale, with the coal predominantly classified as long-flame coal, characterized by ash content below 40%. The oil shale comprises 15 layers with a cumulative thickness of 18.76 m, exhibiting average values of 4.94% oil yield, 61.25% ash content, 10.85 kJ/g calorific value, 0.57% total sulfur content, and 27.87% total organic carbon (TOC) content, alongside kerogen predominantly classified as Type III (humic). Coal and oil shale exhibit significant differences in industrial indices, mineral composition, and logging responses. Both coal tar yield and oil shale oil yield display negative correlations with ash content, though the correlation is markedly stronger in coal. Additionally, a distinct positive correlation exists between coal tar yield and calorific value, whereas no significant relationship was observed between oil shale oil yield and calorific value. In terms of logging characteristics, coal demonstrates significantly higher long-spaced gamma-ray and resistivity values compared to oil shale, while its density is notably lower.

In the Tanshan area, there are five genetic combination types between coal seams and oil shale: coal seam/oil shale symbiotic combination (C-OS), oil shale/coal seam symbiotic combination (OS-C), oil shale/coal seam/oil shale symbiotic combination (OS-C-OS), coal seam/oil shale/coal seam symbiotic combination (C-OS-C), and oil shale/other sedimentary deposit/coal seam symbiotic combination (OS-F/S-C). These distinct symbiotic combinations reflect the region’s unique geological conditions and demonstrate both the regularity and variability in their evolutionary processes.

The combination type of oil shale and coal and the organic matter enrichment in oil shale are significantly affected by the ancient depositional environment, such as the ancient climate, ancient water depth, ancient productivity and preservation conditions, etc. Under warm and humid climatic conditions, close to the location of the depositional center, the water body is deeper, and there is a stratification of the water body, with the upper part of the water body having a high content of oxygen, high productivity, and the relative development of herbaceous plants and algal microorganisms and the lower part of the body being in a long-term strong reducing environment, which inhibits the oxidative decomposition of organic matter and provides better preservation conditions for organic matter, thus forming oil shale with a higher oil content.