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Paleovegetational Reconstruction and Implications on Formation of Oil Shale and Coal in the Lower Cretaceous Laoheishan Basin (NE China): Evidence from Palynology and Terpenoid Biomarkers

: In some cases, the oil shale deposited in shallow lakes may be genetically associated with the coal-bearing successions. Although paleovegetation is an important controlling factor for the formation of oil shale- and coal-bearing successions, few studies have focused on their joint characterization. In this study, a total of twenty-one oil shale and coal samples were collected from the upper member of the Lower Cretaceous Muling Formation (K 1 ml 2 ) in the Laoheishan Basin, and investigated for their bulk geochemical, maceral, palynological, and terpenoid biomarker characteristics, in order to reconstruct the paleovegetation and reveal its inﬂuence on the formation of oil shale and coal. The K 1 ml 2 is subdivided into lower, middle, and upper units. The studied oil shale samples from the lower and upper units display a high ash yield (A d ), low total organic carbon (TOC) and sulfur (S) contents, and limited hydrocarbon generation potential. The studied coal samples from the middle unit are characterized by low A d , and high TOC and low S values, and show signiﬁcant hydrocarbon generation potential. The paleovegetation during the formation of the lower unit was dominated by mire vegetation, such as shrubs (e.g., Lygodiaceae, Schizaeaceae), tree ferns (e.g., Dicksoniaceae/Cyatheaceae), and coniferous trees (e.g., Podocarpaceae). In the middle unit interval, the paleovegetation was represented by highland vegetation (Pinaceae and Araucariaceae) and peat-forming coniferous plants (e.g., Podocarpaceae, Cupressaceae/Taxodiaceae). Various vegetation, such as herbs (e.g., Osmundaceae), shrubs (e.g., Schizaeaceae), and coniferous trees (e.g., Podocarpaceae) was prosperous during the upper unit interval. Coniferous trees could provide abundant hydrogen-rich materials (e.g., resins) to the mire/lake, which may elevate the hydrogen content in peat/lake sediments, and ﬁnally result in higher hydrocarbon generation potential in the coal than in the oil shale. Therefore, the inﬂuence of paleovegetation on the formation of oil shale and coal should be fully considered when studying oil shale- and coal-bearing successions. The results also provide guidance for further exploration studies on oil shale and coal in northeast China.


Geological Setting
The Laoheishan Basin is an intermontane basin situated in the Xingkai Block, northeast China ( Figure 1A,B). The basement surrounding the basin comprises Precambrian metamorphic rocks and Meso-Cenozoic volcanic rocks [30]. The basin covers an area of approximately 400 km 2 and is filled with the Lower Cretaceous Muling Formation, Dongshan Formation, and Neogene Chuandishan Formation ( Figure 1C,D). A thick basalt layer of the Neogene Chuandishan Formation is extensively developed in the central and southeastern parts of the basin. As a result, the Lower Cretaceous strata is only exposed in the northwest ( Figure 1C).
The Lower Cretaceous basin fill is dominated by the Muling Formation, with the thickness decreasing southeastwards ( Figure 1D). The Muling Formation can be further divided into two members: the lower member (K 1 ml 1 ) consists of conglomerate interbedded with sandstone and mudstone, and the upper member (K 1 ml 2 ) mainly comprises sandstone, siltstone, mudstone, coal, and oil shale, with limited conglomerate (Figure 2A). Therefore, the K 1 ml 2 was selected as the target interval for this study.
Based on the lithological variation, the K 1 ml 2 is further divided into lower, middle, and upper units ( Figure 2B). The lower unit (from the bottom to 294 m) consists of fine sandstone and siltstone interbedded with oil shale. The middle unit (294 to 201 m) comprises conglomerate, and coarse to medium sandstone, developing several coal and oil shale layers. The upper unit (201 m to the top) is mainly composed of coarse to medium sandstone interbedded with siltstone and oil shale. Fossils of ferns are widely found in the lower and upper units, whereas possible gymnosperms (probably conifer remains) have been observed in the middle unit ( Figure 2C-H).

Samples and Methods
Twenty-one samples, comprising 10 oil shales and 11 coals, were collected from the Well N1, which is located in the central basin ( Figure 1C). The core samples were fresh and the sampling depth is illustrated in Figure 2B. The macroscopical photos of oil shale and coal are provided in Figure 2I-L. All samples were evaluated by ash yield (dry basis; A d ), total organic carbon (TOC), total sulfur (S), Rock-Eval pyrolysis, and maceral biomarker analyses. Two coal samples were selected for vitrinite reflectance (Ro) measurement. Twelve samples, comprising 7 oil shales and 5 coals, were chosen for palynological identification and statistics.
Ash yield was determined following the ASTM Standard D3174-12 [31]. Total organic carbon (TOC) and S contents were analyzed using a Leco CS-230 instrument after pretreatment of samples with HCl, in order to remove carbonate. Pyrolysis was determined using a Rock-Eval 6 instrument. The hydrocarbon generated from kerogen (S 2 ) was normalized to TOC to characterize the hydrogen index (HI = S 2 /TOC*100), and the temperature of maximum generation (T max ) served as a maturity indicator. These parameters were calculated according to [32]. All of the above analyses were conducted in the Key Laboratory for Oil Shale and Paragenetic Minerals of Jilin Province (Jilin University, Changchun, China).
For palynological investigation, samples were prepared following a standard procedure using HCl and HF acids [33]. No oxidative reagents or ultra-sonication were used during the sample preparation. The residues were sieved (10 µm) and mounted on slides with polyvinyl alcohol and Canada Balsam. The sections were closely observed using an Olympus BX51 biological microscope with transmitted light. Approximately 200 grains of spores and pollen were counted in each slide to provide the spore-pollen assemblage. The palynological investigation was conducted in the Research Center of Paleontology & Stratigraphy (Jilin University, China).
Samples for maceral analysis were firstly crushed to a maximum size of 1 mm. The granular samples were then mixed with Canada Balsam and polished for subsequent microscopic observation. A Leica MPV microscope with white and fluorescent lights, equipped with a 50× objective, was applied. At least 500 points were counted in each slide. The maceral contents refer to volume percentages on a mineral matter-free basis (vol. %, mmf). The Ro of coal samples was determined using a Leica MPV microscope in reflected white light. At least 50 vitrinite (telovitrinite) grains were counted for each slide. Standard materials, including sapphire (0.59% reflectance) and gadolinium gallium garnet (1.72% reflectance) were used for calibration. Both maceral analysis and Ro measurement were conducted in the Key Laboratory for Oil Shale and Paragenetic Minerals of Jilin Province (Jilin University, China).
Samples for biomarker analysis were firstly extracted by dichloromethane in a Dionex ASE 200 accelerated solvent extractor for about 1 h to obtain the extractable organic matter (EOM). The EOM was then separated into asphaltenes, NSO compounds, and saturated and aromatic hydrocarbon fractions using centrifugation and medium pressure liquid chromatography (with a Köhnen-Willsch instrument [34]). The saturated and aromatic hydrocarbon fractions were analyzed by a gas chromatograph equipped with a 30 m × 0.25 mm DB-5MS fused silica column (0.25 µm film thickness) and coupled to a ThermoFisher ISQ quadrupole mass spectrometer (GC-MS). The oven temperature was programmed from 70 • C to 300 • C at 4 • C/min, followed by an isothermal phase of 15 min. Absolute concentrations of compounds in the saturated and aromatic hydrocarbon fractions were calculated in comparison to the peak area of an internal standard (deuterated n-tetracosane and 1,1 -binaphthyl for saturated and aromatic fractions, respectively). The concentrations were normalized to the TOC content. The biomarker analysis was completed in the Department of Applied Geosciences and Geophysics (Montanuniversität Leoben, Leoben, Austria).

Ash Yield and Bulk Geochemistry
Ash yield (dry basis; A d ) is an important factor for classifying oil shale and coal in the Laoheishan Basin [27]. In this study, the A d values of oil shale range from 45.4 to 61.0 wt. % with an average of 51.8 wt. %, which is higher than those of coal (13.6 to 35.6 wt. %, avg. 25.5 wt. %; Figure 3A). The TOC values of oil shale vary between 5.2 and 21.8 wt. % (avg. 12.8 wt. %), lower than those of coal (33.1 to 63.7 wt. %, avg. 48.8 wt. %; Figure 3B). Most oil shale and coal contain S contents less than 0.5 wt. %; only two coals exhibit S contents around 1.0 wt. % ( Figure 3C). The S 2 values are characterized by lower values in oil shale (8.2 to 79.9 mg/g, avg. 34.1 mg/g) and higher values in coal (137.7 to 304.2 mg/g, avg. 202.9 mg/g; Figure 3D). A positive correlation occurred between S 2 values and TOC contents in both oil shale and coal (r 2 = 0.86; Supplementary Figure S1). The HI values range from 140 to 367 mg/g TOC (avg. 248 mg/g TOC) and 304 to 589 mg/g TOC (avg. 417 mg/g TOC) in oil shale and coal, respectively ( Figure 3E), suggesting type II-III kerogen (Supplementary Figure S2). The T max values vary between 421 and 429°C in oil shale and coal ( Figure 3F). Overall, the oil shale characterized by high A d , and low values of TOC, S, S 2 , and HI, was mainly developed in the lower and upper units, whereas coal with low A d and S contents, and high values of TOC, S 2 , and HI, primarily occurred in the middle unit.

Ash Yield and Bulk Geochemistry
Ash yield (dry basis; Ad) is an important factor for classifying oil shale and coal in the Laoheishan Basin [27]. In this study, the Ad values of oil shale range from 45.4 to 61.0 wt. % with an average of 51.8 wt. %, which is higher than those of coal (13.6 to 35.6 wt. %, avg. 25.5 wt. %; Figure 3A). The TOC values of oil shale vary between 5.2 and 21.8 wt. % (avg. 12.8 wt. %), lower than those of coal (33.1 to 63.7 wt. %, avg. 48.8 wt. %; Figure 3B). Most oil shale and coal contain S contents less than 0.5 wt. %; only two coals exhibit S contents around 1.0 wt. % ( Figure 3C). The S2 values are characterized by lower values in oil shale (8.2 to 79.9 mg/g, avg. 34.1 mg/g) and higher values in coal (137.7 to 304.2 mg/g, avg. 202.9 mg/g; Figure 3D). A positive correlation occurred between S2 values and TOC contents in both oil shale and coal (r 2 = 0.86; Supplementary Figure S1). The HI values range from 140 to 367 mg/g TOC (avg. 248 mg/g TOC) and 304 to 589 mg/g TOC (avg. 417 mg/g TOC) in oil shale and coal, respectively ( Figure 3E), suggesting type II-III kerogen (Supplementary Figure S2). The Tmax values vary between 421 and 429 ℃ in oil shale and coal ( Figure 3F). Overall, the oil shale characterized by high Ad, and low values of TOC, S, S2, and HI, was mainly developed in the lower and upper units, whereas coal with low Ad and S contents, and high values of TOC, S2, and HI, primarily occurred in the middle unit.

Maceral Composition and Vitrinite Reflectance (Ro)
Vitrinite was the predominant maceral in oil shale and coal, ranging from 50.

Maceral Composition and Vitrinite Reflectance (Ro)
Vitrinite was the predominant maceral in oil shale and coal, ranging from 50.  Figure 4C-E). Inertinite contents were low in oil shale and coal, with percentages of <1.5 vol. %. Fusinite, including pyrofusinite and degradofusinite, was the commonly observed inertinite, generally revealing high reflectance ( Figure 4F). Pyrite was rarely observed in the oil shale and coal, which is consistent with the low sulfur contents (<1.2 wt. %; Figure 3C). The measured Ro of coal samples was in the range of 0.43−0.44% (Supplementary Table S1).
Energies 2021, 14, x FOR PEER REVIEW 6 of 22 nated by sporinite, resinite, cutinite, and fluorinite, all of which displayed high fluorescence under the microscope ( Figure 4C-E). Inertinite contents were low in oil shale and coal, with percentages of <1.5 vol. %. Fusinite, including pyrofusinite and degradofusinite, was the commonly observed inertinite, generally revealing high reflectance ( Figure 4F). Pyrite was rarely observed in the oil shale and coal, which is consistent with the low sulfur contents (<1.2 wt. %; Figure 3C). The measured Ro of coal samples was in the range of 0.43−0.44% (Supplementary Table S1).

Palynology
A total of 53 palynomorph taxa (30 spores and 23 pollen taxa) were identified at a species or genus level in oil shale and coal. Selective spores and pollen taxa are shown in Figures 5 and 6, respectively. The preservation of palynomorphs was good, revealing no signs of post depositional degradation [35]. The quantitative distribution patterns of selective spores and pollen are illustrated in Figures 7 and 8, respectively. Distinctive sporepollen assemblages were present in three units. The lower unit was characterized by high spore (avg. 74.1%) and low pollen (avg. 25

Palynology
A total of 53 palynomorph taxa (30 spores and 23 pollen taxa) were identified at a species or genus level in oil shale and coal. Selective spores and pollen taxa are shown in Figures 5 and 6, respectively. The preservation of palynomorphs was good, revealing no signs of post depositional degradation [35]. The quantitative distribution patterns of selective spores and pollen are illustrated in Figures 7 and 8, respectively. Distinctive sporepollen assemblages were present in three units. The lower unit was characterized by high spore (avg. 74.1%) and low pollen (avg. 25

Maturity and Hydrocarbon Generation Potential
The Tmax values of oil shale and coal varied between 421 and 429 ℃, revealing an immature character. This interpretation is further supported by the range of measured Ro (0.43−0.44 %; Supplementary Table S1) and the yellow to orange color of the palynomorphs (thermal alteration index 1 to 2; after [36]). According to the modified HI-Tmax diagram in [37], the studied oil shale sample plots into the area which classified as mixed gas-and oil-prone to gas-prone, whereas the coal samples were classified as oil-prone ( Figure 13). This result was further supported by the higher oil yield (determined by Fischer Assay Procedure) in coal (8.2 wt. % to 14.1 wt. %) than in oil shale (3.6 wt. % to 7.2 wt. %) from the Laoheishan Basin [27]. Considering the rank-related increase in HI of lowrank coals, the studied coal samples exceed the minimum HI of 300 mg/g TOC required for oil generation [38] when their thermal maturity reaches the onset of oil expulsion (''effective HI'' in [37]). In addition, because pyrite was rarely observed in the studied oil shale and coal [29], the influence of pyrite on S2 during pyrolysis could be excluded.

Maturity and Hydrocarbon Generation Potential
The T max values of oil shale and coal varied between 421 and 429°C, revealing an immature character. This interpretation is further supported by the range of measured Ro (0.43−0.44 %; Supplementary Table S1) and the yellow to orange color of the palynomorphs (thermal alteration index 1 to 2; after [36]). According to the modified HI-T max diagram in [37], the studied oil shale sample plots into the area which classified as mixed gas-and oil-prone to gas-prone, whereas the coal samples were classified as oil-prone ( Figure 13). This result was further supported by the higher oil yield (determined by Fischer Assay Procedure) in coal (8.2 wt. % to 14.1 wt. %) than in oil shale (3.6 wt. % to 7.2 wt. %) from the Laoheishan Basin [27]. Considering the rank-related increase in HI of low-rank coals, the studied coal samples exceed the minimum HI of 300 mg/g TOC required for oil generation [38] when their thermal maturity reaches the onset of oil expulsion ("effective HI" in [37]). In addition, because pyrite was rarely observed in the studied oil shale and coal [29], the influence of pyrite on S 2 during pyrolysis could be excluded. Energies 2021, 14, x FOR PEER REVIEW 13 of 22 Figure 13. Modified HI-Tmax diagram highlighting the increase in HI prior to the onset of oil expulsion, after [37]. The classification of kerogen quality is from [39].

Paleovegetational Reconstruction
The palynofloral assemblage of coal-bearing sequences in variable geological ages is an important parameter for paleovegetational reconstruction [35,[40][41][42][43][44][45][46][47][48][49]. Nevertheless, the sole usage of palynological data could be problematic due to short-and long-distance transportation by surface water and wind, respectively, or positions of the sampled coal seam in the precursor paleomires and/or the quantity of sporomorphs that was produced by mother plants. Therefore, the palynological data should be supported by maceral and biomarker compositions of the studied samples. The palynofloras of the K1ml2 interval in the Laoheishan Basin comprise palynoflora originating from various plant groups (e.g., ferns and conifers). Information on the botanical affinity, vegetation, and humidity-aridity types of the spores and pollen are provided in Table 1. Six principal groups were recognized based on the compilation, namely, ferns, sphenopsids, bryophytes, lycopods, conifers, and seed ferns. The parent plants can be classified into four vegetation types, i.e., herbs, shrubs, tree ferns, and coniferous trees. In addition, most of the parent plants belong to the humidity-aridity types of phreatophyte and mesophyte, suggesting a humid to semi-humid paleoclimate prevailed during the K1ml2 interval.
Five typical palynofloral provinces have been classified in China during the early Cretaceous, namely, the Disacciatrileti-Cicatricosisporites Province in the north, the Classopollis-Schizaeoisporites Province in the southeast, the Dicheiropollis Province in the Tibet-Tarim region, the Araucariacites-Callialasporites Province in the southern Tibet, and a wide transitional zone [50]. The northern Disacciatrileti-Cicatricosisporites Province generally contains abundant and diverse bisaccate conifer pollen grains (e.g., Pinaceae and Podocarpaceae) and ferns (especially Cicatricosisporites), whereas Classopollis and Schizaeoisporites are rare [51]. This region has been considered as having a warm-humid paleoclimate during the early Cretaceous [50]. The palynoflora of the K1ml2 interval in the study area was dominated by highland-related bisaccate coniferous pollen (e.g., Pinuspollenites, Podocarpidites, and Protoconiferus) and fern spores (e.g., Concavissimisporites, Cicatricosisporites, and Osmundacidites), which is similar to the palynoflora of the northern Disacciatrileti-Cicatricosisporites province.
Based on the identified palynomorph assemblage (Figures 7 and 8), three types of palynofloras were classified into the studied interval, corresponding to three units. In the lower unit interval, the paleovegetation was dominated by shrubs, including Lygodiaceae (Concavissimisporites and Cardioangulina) and Schizaeaceae (Cicatricosisporites), accompa- Figure 13. Modified HI-T max diagram highlighting the increase in HI prior to the onset of oil expulsion, after [37]. The classification of kerogen quality is from [39].

Paleovegetational Reconstruction
The palynofloral assemblage of coal-bearing sequences in variable geological ages is an important parameter for paleovegetational reconstruction [35,[40][41][42][43][44][45][46][47][48][49]. Nevertheless, the sole usage of palynological data could be problematic due to short-and long-distance transportation by surface water and wind, respectively, or positions of the sampled coal seam in the precursor paleomires and/or the quantity of sporomorphs that was produced by mother plants. Therefore, the palynological data should be supported by maceral and biomarker compositions of the studied samples. The palynofloras of the K 1 ml 2 interval in the Laoheishan Basin comprise palynoflora originating from various plant groups (e.g., ferns and conifers). Information on the botanical affinity, vegetation, and humidity-aridity types of the spores and pollen are provided in Table 1. Six principal groups were recognized based on the compilation, namely, ferns, sphenopsids, bryophytes, lycopods, conifers, and seed ferns. The parent plants can be classified into four vegetation types, i.e., herbs, shrubs, tree ferns, and coniferous trees. In addition, most of the parent plants belong to the humidity-aridity types of phreatophyte and mesophyte, suggesting a humid to semi-humid paleoclimate prevailed during the K 1 ml 2 interval.
Five typical palynofloral provinces have been classified in China during the early Cretaceous, namely, the Disacciatrileti-Cicatricosisporites Province in the north, the Classopollis-Schizaeoisporites Province in the southeast, the Dicheiropollis Province in the Tibet-Tarim region, the Araucariacites-Callialasporites Province in the southern Tibet, and a wide transitional zone [50]. The northern Disacciatrileti-Cicatricosisporites Province generally contains abundant and diverse bisaccate conifer pollen grains (e.g., Pinaceae and Podocarpaceae) and ferns (especially Cicatricosisporites), whereas Classopollis and Schizaeoisporites are rare [51]. This region has been considered as having a warm-humid paleoclimate during the early Cretaceous [50]. The palynoflora of the K 1 ml 2 interval in the study area was dominated by highland-related bisaccate coniferous pollen (e.g., Pinuspollenites, Podocarpidites, and Protoconiferus) and fern spores (e.g., Concavissimisporites, Cicatricosisporites, and Osmundacidites), which is similar to the palynoflora of the northern Disacciatrileti-Cicatricosisporites province. Based on the identified palynomorph assemblage (Figures 7 and 8), three types of palynofloras were classified into the studied interval, corresponding to three units. In the lower unit interval, the paleovegetation was dominated by shrubs, including Lygodiaceae (Concavissimisporites and Cardioangulina) and Schizaeaceae (Cicatricosisporites), accompanied by tree ferns and coniferous trees, which were represented by Dicksoniaceae/Cyatheaceae (Cyathidites) and Podocarpaceae (Podocarpidites), respectively. The paleovegetation during the middle unit interval was characterized by highland vegetation and peat-forming coniferous plants, including Pinaceae (Pinuspollenites, Cedripites), Araucariaceae (Araucariacutes), Podocarpaceae (Podocarpidites), and Cupressaceae/Taxodiaceae (Protoconiferus). In addition, shrubs also occurred during the middle unit, mainly consisting of Lygodiaceae (Concavissimisporites) and Schizaeaceae (Cicatricosisporites). Different vegetation types, including herbs, shrubs, and coniferous trees, flourished during the upper unit interval. The herbs and shrubs were represented by Osmundaceae (Osmundacidites and Baculatisporites) and Schizaeaceae (Cicatricosisporites), respectively. In addition, the coniferous trees mainly comprised highland vegetation and peat-forming coniferous trees, such as Pinaceae (Pinuspollenites) and Podocarpaceae (Podocarpidites).
The formation of fernanes is considered to be associated with fernenol, which is widespread in vascular plants, especially ferns [67,68]. However, some of the fernane type compounds may also have originated from bacteria, as indicated by the co-occurrence of fern-7-ene and methanogenic biomarkers in modern sediments from an anoxic environment in Antarctica [69,70]. In the present case, there is no doubt that the precursors of fernanes were ferns, because abundant spores were found through the studied interval ( Figure 6). As illustrated in Figures 10-12, low concentrations of sesquiterpenoids and diterpenoids, and a high concentration of fernanes, are present in the lower and upper units, probably indicating a paleovegetation dominated by ferns. In contrast, high concentrations of sesquiterpenoids and diterpenoids, together with low concentrations of fernanes, were observed in the middle unit, probably suggesting a paleovegetation characterized by coniferous trees, such as Pinaceae, Podocarpaceae, Cupressaceae, Taxodiaceae, and Araucariaceae. The conclusions are in good agreement with the palynological results.

Paleovegetation Influences the Formation of Oil Shale and Coal
The paleoenvironment and paleovegetation during the formation of oil shale and coal in the Laoheishan Basin are illustrated in Figure 14. Oil shale was deposited in shallow lakes with freshwater, as indicated by low sulfur contents (avg. 0.12 wt. %; Figure 3C). Based on the results of palynology and terpenoid biomarkers, the paleovegetation during the formation of oil shale mainly comprised ferns (e.g., Cicatricosisporites, Osmundacidites, and Cyathidites) and sphenopsids (e.g., Concavissimisporites) on the lake shore, and the high-lands in the adjacent regions of the study area hosted coniferous forest (mainly Pinaceae; Figure 14A). Ferns and sphenopsids living on the lake shore may have allowed a large number of spores to enter the lake (transported by surface water and/or wind-blown), and therefore be preserved in the oil shale [52]. In contrast, coal was formed in freshwater low-lying mire, as suggested by high ash yields (avg. 25.5 wt. %) and low sulfur contents (avg. 0.41 wt. %) [71]. The paleovegetation during the formation of coal was composed of highland vegetation and peat-forming coniferous plants, such as Pinaceae, Araucariaceae, Podocarpaceae, Cupressaceae, and Taxodiaceae, accompanied by sphenopsids (e.g., Concavissimisporites) and ferns (e.g., Cicatricosisporites; Figure 14B). Although pollen could be carried a long distance by wind, surface water, and/or insects, a large amount of pollen can still be "in situ" buried due to the flourishing of coniferous trees in the mire (e.g., Podocarpaceae, Cupressaceae, and Taxodiaceae) [72]. In addition, some kinds of coniferous trees, such as Podocarpaceae and Cupressaceae, could produce a large number of resins [73][74][75][76]. These resins would be transported into the lake or mire by surface water and/or wind, thus contributing to the formation of hydrogen-rich material (e.g., resinite) in oil shale and coal, and finally resulting in relatively high hydrocarbon generation potential. Because coniferous trees (including highland and peat-forming conifers) flourished in greater numbers in/near the mire than the lake shore ( Figure 12), higher hydrocarbon generation potential was commonly present in the coal than in the oil shale. The results reveal the significance of terrestrial organic matter in the formation of excellent (coaly-) source rocks, which has been previously proven by the oil-source correlation results from Southeast Asia, Australia, and Northwest China [77,78]. generation potential. Because coniferous trees (including highland and peat-forming conifers) flourished in greater numbers in/near the mire than the lake shore ( Figure 12), higher hydrocarbon generation potential was commonly present in the coal than in the oil shale. The results reveal the significance of terrestrial organic matter in the formation of excellent (coaly-) source rocks, which has been previously proven by the oil-source correlation results from Southeast Asia, Australia, and Northwest China [77,78].

Conclusions
Paleovegetational reconstruction and its implications for the formation of oil shale and coal in the Lower Cretaceous Laoheishan Basin were investigated in detail for the first time. Based on the lithological variation, three units were outlined in the studied interval (K1ml2, the upper member of the Lower Cretaceous Muling Formation). Oil shale was mainly developed in the lower and upper units, which were characterized by high Ad (avg. 51.8 wt. %), and low values of TOC (avg. 12.8 wt. %), S (avg. 0.12 wt. %), S2 (avg. 34.1 mg/g), and HI (avg. 248 mg/g TOC). In contrast, coal primarily occurred in the middle unit, exhibiting low Ad (avg. 25.5 wt. %) and S (avg. 0.41 wt. %), and high values of TOC (avg. 48.8 wt. %), S2 (avg. 202.9 mg/g), and HI (avg. 417 mg/g TOC). Both oil shale and coal revealed an immature character, as evidenced by the Tmax values (421-429 ℃), Ro (0.43−0.44 %), and yellow to orange color of the palynomorphs (thermal alteration index 1 to 2). The palynofloral assemblage and terpenoid biomarker concentrations (sesquiterpenoids, diterpenoids, and fernanes) reflect the existence of various forms of paleoveg-

Conclusions
Paleovegetational reconstruction and its implications for the formation of oil shale and coal in the Lower Cretaceous Laoheishan Basin were investigated in detail for the first time. Based on the lithological variation, three units were outlined in the studied interval (K 1 ml 2 , the upper member of the Lower Cretaceous Muling Formation). Oil shale was mainly developed in the lower and upper units, which were characterized by high A d (avg. 51.8 wt. %), and low values of TOC (avg. 12.8 wt. %), S (avg. 0.12 wt. %), S 2 (avg. 34.1 mg/g), and HI (avg. 248 mg/g TOC). In contrast, coal primarily occurred in the middle unit, exhibiting low A d (avg. 25.5 wt. %) and S (avg. 0.41 wt. %), and high values of TOC (avg. 48.8 wt. %), S 2 (avg. 202.9 mg/g), and HI (avg. 417 mg/g TOC).
Both oil shale and coal revealed an immature character, as evidenced by the T max values (421-429°C), Ro (0.43−0.44 %), and yellow to orange color of the palynomorphs (thermal alteration index 1 to 2). The palynofloral assemblage and terpenoid biomarker concentrations (sesquiterpenoids, diterpenoids, and fernanes) reflect the existence of various forms of paleovegetation during deposition of the three units. In the lower unit interval, the paleovegetation was represented by shrubs (Lygodiaceae and Schizaeaceae), accompanied by tree ferns (Dicksoniaceae/Cyatheaceae) and coniferous trees (Podocarpaceae). During the formation of the middle unit, the paleovegetation was dominated by highland vegetation and peat-forming coniferous trees, including Pinaceae, Araucariaceae, Podocarpaceae, and Cupressaceae/Taxodiaceae. The paleovegetation in the upper unit interval mainly comprised herbs (Osmundaceae) and shrubs (Schizaeaceae), followed by coniferous trees (Pinaceae and Podocarpaceae). The hydrogen-rich materials (e.g., resin) produced by coniferous trees may be responsible for higher hydrocarbon generation potential in the coal than the oil shale. Overall, our results support the suggestion that the paleovegetation has an influence on the formation of oil shale and coal. In addition, this study provides crucial information for further exploration studies on oil shale and coal in northeast China.

Supplementary Materials:
The following are available online at https://www.mdpi.com/article/10 .3390/en14154704/s1, Figure S1: Cross-plot of total organic carbon (TOC) versus the hydrocarbon generated from kerogen (S 2 ) of oil shale and coal samples, Figure S2: Cross-plot of the temperature of maximum generation (Tmax) versus hydrogen index (HI) of oil shale and coal samples, Table S1: Ash yield, bulk geochemistry, maceral composition, and vitrinite reflectance (Ro) of the studied oil shale and coal.