Petrology and hydrocarbon significance of the coaly source rocks from the Pinghu Formation in the Xihu Sag, East China Sea Shelf Basin

The Xihu Sag in the East China Sea Shelf Basin contains abundant oil and gas reserves and is a focus for hydrocarbon exploration and development. Source rocks are mainly coals and coal-measures mudstones in the Paleogene Pinghu and Huagang formations. Samples from the Pinghu Formation in the Xihu Sag were collected for petrology, total organic carbon, and Rock-Eval analysis for the purpose of investigating macerals component and their contributions to hydrocarbon generation potential. The coaly source rocks from the Pinghu Formation are dominated by vitrinite (average 86.18%) but have an obviously elevated content of liptinite (average 12.59%) and a much lower amount of inertinite (average 1.23%). Liptinite of the samples is mainly composed of resinite, with a small amount of cutinite, sporinite and alginate in descending order. TOC values are 37.55%–65.58% (average 49.16%). Effective HI values are 167–281 mg HC/g TOC (average 223.5 mg HC/g TOC), suggesting the organic matter is type II kerogen. Relatively high HI values and macerals components suggest that the coaly source rocks can generate both oil and gas. Although the liptinite in the coaly source rocks has a content lower than vitrinite values, it makes a significant contribution to both total hydrocarbon and liquid hydrocarbon generation. The contributions of vitrinite, liptinite and inertinite to the total hydrocarbon generation approximately are 63.21%, 36.46% and 0.33%, respectively. The contributions of vitrinite and liptinite to the liquid hydrocarbon generation are approximately 40.95% and 59.05%, respectively. These results demonstrate that the coaly source rocks are dominated by vitrinite macerals with a relatively higher content of liptinite macerals, especially resinite, and these source rocks are more prone to both total hydrocarbon and liquid hydrocarbon generation. Paleogene coaly source rocks from other parts of the world should be considered for their oil-prone nature.


Introduction
Coals can be one of the most important source rocks for oil and gas (Ayinla et al., 2017;Petersen et al., 2010;Snowdon, 1991). Coaly rocks as sources for oil and gas have been found in a number of oil basins worldwide, including the Jurassic of the Turpan-Hami Basin (Huang et al., 1991;Shao et al., 2003;Wu et al., 2016;Zhang et al., 1997), the Late Cretaceous to Eocene of the Gippsland Basin (Smith and Cook, 1984), and the Cretaceous-Cenozoic of the Taranaki Basins (Sykes et al., 2014). Coal macerals constitute the basic units of the coaly source rocks for hydrocarbon generation, and the liptinite macerals are regarded as the most important sources of liquid hydrocarbons in these oilprone coals (Sykes and Snowdon, 2002;Wilkins and George, 2002). Subsequent studies have shown that the liptinite group is not the only, nor necessarily the most important, source of liquid hydrocarbons in certain coals (Sykes and Snowdon, 2002;Wilkins and George, 2002). Hydrogen-rich vitrinite can also generate certain amounts of liquid hydrocarbons (Killops et al., 1998;Smith and Cook, 1984;Sykes and Snowdon, 2002). Despite numerous viewpoints proposed by different researchers, there are still many unanswered questions about the hydrocarbon generation potential of coal macerals as coals with high hydrogen-rich maceral contents are rarely seen in geological history. More studies need to be conducted on coals with high hydrogen-rich maceral contents, especially liptinite-rich coals, for their hydrocarbon generation potential.
The Xihu Sag in the East China Sea Shelf Basin (ECSSB) contains abundant hydrocarbon resources dominated by natural gas and condensate oil (Huang and Ye, 2010;Sua´rez-Ruiz et al., 2012;Xu et al., 2003Xu et al., , 2017. Oil and gas fields, such as those from Chunxiao, Pinghu, Tianwaitian, and Kongqueting, are currently being developed. A series of petroliferous structures have been discovered, which include two oil and gas fields with reserves of up to a hundred million tons oil equivalent. Oil-source correlation has confirmed that the hydrocarbons in the Xihu Sag are derived from Paleogene coals and coaly mudstones (Fu, 1994;Li et al., 1995;Qian et al., 2012;Xu et al., 2017;Wei et al., 2013Wei et al., , 2019. Previous studies for the sources rocks in the Xihu Sag are mainly concentrated on their depositional environments (Yi et al., 2018;Zhou et al., 2016), accumulation and distribution (Tian et al., 2019;Yu, 2019), and geochemistry (Zhu et al., 2012), and only a few studies are dealing with the coal petrology and the relationships between the maceral types and the hydrocarbon generation. Although the liptinite macerals were found to have relatively higher contents in the coals of the Xihu Sag than other coal basins (Li et al., 1995(Li et al., , 1997, the maceral types and compositions of the Paleogene coals in the Xihu Sag and their hydrocarbon generation potentials need to be assessed more carefully. For examples, what types of liptinite macerals were dominant in the coals, and in addition to liptinite macerals, are the vitrinite macerals also of hydrocarbon generation potentials? Detailed characterization of the coaly source rocks is of importance to hydrocarbon exploration in the Xihu Sag as the unique maceral compositions of the Paleogene coals may be associated with a different hydrocarbon generation capacity. Therefore, a coal petrology study is especially crucial for recognition of the oil-prone properties of the different macerals in the coals, and the assessment of the hydrocarbon resources in the Xihu Sag.
This study will provide a detailed assessment of the petrology and hydrocarbon generation of the coaly source rocks in the Pinghu Formation from the Xihu Sag, and quantitatively determine the contributions to oil and gas of major maceral groups.

Geological setting
The Xihu Sag, which is located in the eastern depression belt of the ECSSB, is a Meso-Cenozoic superimposed sedimentary basin. It covers an area of about 5.18 Â 10 4 km 2 and is elongated in a NNE direction. It is surrounded by the Diaoyu Island to the east, the Haijiao, Hupijiao and Eastern Yushan Uplifts to the west, and by the Fujiang and Northern Diaoyu Sag to the north and south respectively ( Figure 1). Controlled by two NNE and NW directions fault systems, the Xihu Sag can be subdivided into five sub-units, and from the west to the east they comprise the West Slope Belt, the West Sub-Sag, the Central Anticlinal Belts, the East Sub-Sag, and the East Fault Belt (Figure 1). Together with the basins in eastern China, the Xihu Sag has undergone three stages of tectonic evolution, namely, rifting from the Paleocene to the Middle Eocene, compression from the Late Eocene to the Oligocene, and inversion from the Miocene to the Holocene ( Figure 2) (Guo et al., 2015;Li and Zhu, 1992;Zhao et al., 2016).
Cenozoic deposits in the Xihu Sag include nine formations, namely, in ascending order the Bajiaoting, Baoshi, Pinghu, Huagang, Longjing, Yuquan, Liulang, Santan and Donghai formations ( Figure 2). Unconformities are located at the top of the Baoshi, Pinghu, Huagang, and Liulang formations. The target strata of this research from the Pinghu Formation are mainly composed of fine sandstone, siltstone and mudstone intercalated with a number of thin coals. The Pinghu Formation is late Eocene in age and was deposited in tidal flat and deltaic environments (Li and Zhu, 1992;Wei et al., 2013;Zhou et al., 2016). The Paleogene coal beds in the Xihu Sag are characterized by small thickness of single seams (generally less than 2 m) but a large accumulative thickness. The accumulative thickness of the coals in the Pinghu Formation is up to 65 m.

Samples
Thirty-three samples from well-cuttings with very fine grains (mostly <2 mm) were collected from the Pinghu Formation ( Figure 2) from the A-1, A-2 and A-3 wells in the Western Slope Belts, and the B-1 and B-2 wells in the Western Sub-Sag ( Figure 1). Sample depths range from 3518 m to 4775 m. Preprocessing was necessary before conducting the experiments as the original samples were covered by drilling mud. Well-cuttings were cleaned with the deionized water three times and then air dried in natural conditions. Subsequently, coaly cuttings were picked by hands from the mixture of the coal, coaly shale, and mudstone in the well-cuttings. Finally, samples were crushed into about 1 mm fragments, and then the sample was divided into two parts, one for coal petrology analysis and another for organic geochemistry analysis.

Analytical procedures
For coal petrology analysis, crushed samples were prepared as polished coal blocks for the analysis of vitrinite reflectance (Ro) and coal maceral compositions, following the GBT6948-2008 and GBT8899-1998 standards. Vitrinite reflectance measurements were undertaken at the School of Resources and Environment, Henan Polytechnic University, using a Zeiss Axioskop 40 microscope and MSPUV-VIS2000 microphotometer under oilimmersion white reflect light, with 500 times magnification, measuring at least 50 values for each sample. Values of Ro presented in this study are all average values of random reflectance of collotelinite. Maceral composition statistics were conducted at the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing, using a Leica DM2700P microscope and Leica EL6000 fluorescent external light source in oilimmersion under both white reflected light and blue-light irradiation. Samples were investigated at 500 times magnification, measuring at least 500 values for every sample.
Total organic carbon (TOC) measurement and Rock-Eval analysis were conducted at the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing. The TOC measurement was performed using a LECO WR-112 instrument following the GB/T19145-2003 guidelines. Firstly, the samples were milled into 200 mesh and the inorganic carbon was removed by dilute hydrochloric acid. Then, the samples were burned to a high-temperature in oxygen conditions to convert all the organic carbon into carbon dioxide, which was tested with a thermal conductivity detector. Pyrolysis analysis was conducted using an OGE-II instrument following the GB/T18602-2001 guidelines. Samples were heated to 600 C in a helium atmosphere to obtain free hydrocarbon (S 1 ), pyrolysis hydrocarbon (S 2 ), and Tmax (Espitali e et al., 1977). Here S 1 represents free and volatile hydrocarbons which are liberated from rocks at 300 C, S 2 represents hydrocarbons generated from kerogen decomposition that arises during the heating progressive from 300 C to 600 C, and Tmax represents the temperature at which the maximum amount of S2 is generated (Espitali e et al., 1977). Other parameters such as hydrocarbon generation potential (S1þS2), hydrogen index (HI¼S2/TOC) (Espitali e et al., 1977), bitumen index (BI¼S1/TOC) (Killops et al., 1998), and quantity index (QI¼(S1þS2)/TOC) were calculated from the pyrolysis analysis data and TOC values (Pepper and Corvi, 1995a).

Calculation of the original TOC and effective HI
The original TOC is defined as the organic matter content in source rocks before the hydrocarbon is extensively expulsed. TOC values of source rocks vary with its maturity (Pang et al., 2014;Tissot and Welte, 1978) and increase with an increase of maturity for coaly source rocks (Lu et al., 2003;Zhang and Peng, 2011). Therefore, it is necessary to calculate the original organic matter content for the high-over mature source rocks. A series of correction coefficients of coal with different organic matter type, maturity, and TOC ranges were provided in the correction chart by Zhang and Peng (2011)

Ro (%)
0.5-0.6 0.6-0.7 0.7-0.8 0.8-0.9 0.9-1.0 1.0-1.1 1. In the study of New Zealand Cretaceous-Cenozoic humic coals, Sykes and Snowdon (2002) concluded that an increasing trend in HI occurs prior to oil expulsion. They presented a simple graphical plot for adjusting the measured HI values to the effective HI values, which is near to the onset of oil expulsion. The Xihu coals are similar to Cretaceous-Cenozoic humic coals from New Zealand (see Sykes and Snowdon, 2002;Sykes et al., 2004Sykes et al., , 2014 both in the coal petrology and bulk organic geochemistry, so it is suitable to gain effective HI values in this way. Firstly, we plotted the Rank (Sr) (see Suggate, 2002) and the measured HI of the coal samples in their diagram. Then, we adjusted the measured HI values of our samples to the effective HI values, which is equivalent to the inferred oil expulsion position according to the maturity pathway of the coal ( Figure 3). Finally, the effective HI values are gained by reading the corresponding HI values in their diagram.

Coal petrology
Vitrinite reflectance. Vitrinite reflectance values of the 33 samples range between 0.55% and 1.28% (Table 2). The values of Ro in the West Sub-Sag vary between 0.76% and 1.28% with an average of 0.99%, and those in the west slope belts range between 0.55% and 1.05% with an average of 0.70%.
Maceral types. Macerals of the vitrinite, inertinite and liptinite groups can be found in the studied samples, although the inertinite group is absent in some samples. The vitrinite group includes telinite, collotelinite, corpogelinite, gelinite, collodetrinite and vitrodetrinite. Telinite is characterized by more or less recognizable cell walls of plant tissue. The cell lumens are usually closed by swollen cell wall, or filled with resinite ( Figure 4(a)), gelinite and minerals, with few of them being empty. Collotelinite is characterized by a smooth surface and poorly preserved plant cell structure (Figure 4(b) and (f)). A gradual transition Figure 3. Cross-plots of HI vs. Rank (Sr) from the Paleogene coaly source rocks in the Xihu Sag (base map modified after Sykes and Snowdown, 2002). Note: Blue dashed lines are the maturity pathway of the coals, and triangle symbols represent effective HI of the corresponding samples. Green dashed lines represent gasprone, gas-and oil-prone, and oil-prone boundaries. Shadow area is the New Zealand coal band. from collotelinite to telinite may occur occasionally. Corpogelinite, which occurs in a spherical or oval shape, occurs as single bodies or grouped together (Figure 4(b) and (c)), and has a slightly higher reflectance than other vitrinite components. Collodetrinite is characterized by its mottled surface and an unclear boundary with collotelinite, often binding other coal components (Figure 4(c) and (f)). Vitrodetrinite represents the fragments of vitrinite with a diameter less than 10 lm, and commonly cemented by collodetrinite (Figure 4(d)). The inertinite group consists mainly of fusinite, semifusinite, and funginite, with a small amount of macrinite and micrinite, while inertodetrinite is rarely detected. Reflectance values of the inertinite group are distinctly higher than those of the vitrinite and liptinite groups. Plant cell walls are well preserved in fusinite and semifusinite (Figure 4(e)). Sometimes, fusinite and semifusinite are broken into arc-shaped fragments because of the pressure (Figure 4(e)). The shape of funginite is rounded to oval, with clear boundaries, and often occurs separately (Figure 4(f)), in which both fungal spores and sclerotia can be found in the samples (Figure 4(f)).
The liptinite group is distinguished from other maceral groups by its lower reflectance (Pickel et al., 2017), and mainly includes resinite, cutinite, sporinite, and alginite. Exsudatinite is intruded into empty spaces, such as fissures, cracks and other cavities, and is of a slightly lower fluorescence and intensity than the surrounded source macerals. Bituminite is also found in the Xihu coals, which has no specific or distinct morphology, and can occur as fine granular groundmass, laminae, irregular streaks, wisps, displaying various intensities of fluorescence or no fluorescence. Fluorescence intensity of liptinite macerals decreases with maturity increase ( Figure 6). As shown in Figure 6, liptinite macerals in these photos are mostly resinite with the fluorescence from greenish-yellow ( Figure 6 (b)) to brown (Figure 6(d)), and to almost disappeared (Figure 6(f)).

Bulk organic geochemistry
Results of the TOC and Rock-Eval analysis are shown in Table 3. TOC values of the samples vary between 37.55 and 65.58%, with an average of 49.16%. The values of S1,   Table 3. It is shown that values of the measured TOC are from 37.55% to 65.58%, being about 3-8% higher than the original TOC values which range from 34.55% to 58.04%. Corrected effective HI values of the samples are 167-281 mg HC/g TOC, averaging 223.5 mg HC/g TOC. The effective HI is elevated by 0-99 mg HC/g TOC, with an average increase of 33 mg HC/g TOC.  Note: S1: Free hydrocarbon content (mg HC/g rock). S2: Remaining hydrocarbon generative potential (mg HC/g rock).

Organic matter abundance
Organic matter abundance in this research is mainly characterized by TOC and Rock-Eval pyrolysis data. Relatively high values of the TOC (37.55%-65.58%, average 49.16%) of the samples provide foundations for hydrocarbon generation. HI is an important parameter to evaluate the organic matter abundance (Pepper and Corvi, 1995b;Tissot and Welte, 1984). Pepper and Corvi (1995b) proposed that a HI of 200 mg HC/g TOC is the minimum requisite for oil expulsion. Peters et al. (2005) suggested that HI values >300 mg HC/g TOC are oil-prone, values of 200-300 mg HC/g TOC are gasand oil-prone, and values of 50-200 mg HC/g TOC are gas-prone (Peters et al., 2005). On this basis, the coaly sources rocks from the Pinghu Formation in the Xihu Sag, whose effective HI values are 167-281 mg HC/g TOC (averaged 223.5 mg HC/g TOC), are gasprone to gas-and oil-prone, and most are gas-and oil-prone (Figure 3).

Organic matter type
Organic matter is typically distinguished into three main genetic types, namely types I, II and III (Peters and Cassa, 1994;Tissot et al., 1974). Type I displays a very high potential for liquid hydrocarbon generation, while the oil potential of type III is only moderate although it may still generate abundant gas at greater depths, and type II can generate considerably both oil and gas (Vandenbroucke and Largeau, 2007). Classification of organic matter type is mainly based on Rock-Eval pyrolysis data, using the discrimination diagram of the HI versus Tmax plot provided by Espitali e et al. (1977). Pyrolysis data from coals in the Pinghu Formation, including HI and Tmax, demonstrate that the analyzed samples generally plot in the zone of type II (Figure 7), which is different from our general cognition that the terrestrial plants are prone to being type III organic matter. This may be attributed to high contents of liptinite and dominance of perhydrous collodetrinite of the vitrinite.
The organic matter type of source rocks can also be classified by optical microscopic evaluation methods, using the oil production index (OP) and hydrocarbon production index (HP) based on the statistical results from macerals (Jin and Xiao, 1990;Xiao, 1992). The calculated values of OP and HP of the samples are listed in Table 2. The OP values vary between 106 and 258, which demonstrates a dominance of type II 2 organic matter, and only four samples from the total of 33 analyzed fall into the type III range. The HP values vary between 248 and 405, which reveals the dominance of type II 2 organic matter too, with even one sample falling into the type II 1 organic matter.
Overall, kerogens of the coaly source rocks from the Pinghu Formation in the Xihu Sag are dominated by type II 2 organic matter, indicating that the coaly sources rocks in this geological context can generate not only gas but also considerable levels of oil. The type II organic matter further confirms the uniqueness of the Pinghu coals.

Organic matter maturity
Vitrinite reflectance is a good indicator of coal rank (Stach And Murchison, 1982;Wang et al., 2019). The vitrinite reflectance of the samples, ranging between 0.55 and 1.28%, corresponds to bituminous coals (ISO 11760, 2005), of which the West sub-Sag (B-1 and B-2) has a slightly higher coal rank (bituminous C to bituminous B) than that in West Slope Belts (A-1, A-3, and A-2) whose coal rank mostly ranges from bituminous D to bituminous C. The reason for this difference is because of a deeper burial depth and higher geothermal gradient in the western sub-Sag than those in the West Slope Belts (Tong et al., 2009).
Occurrence of exsudatinite and bituminite in the Xihu coals indicates that the liquid hydrocarbon was generated from liptinite and perhydrous vitrinite macerals. Threshold for oil expulsion is indicated by the peak in QI (Sykes and Snowdon, 2002). The onset of oil expulsion for the coaly source rocks in the Xihu Sag is at the Rank (Sr) between 11 and 12.5 (Figure 8), corresponding to 430-440 C of Tmax and 0.7-0.85% of Ro values. These values are highly consistent with those of the Cretaceous-Cenozoic humic coals in New Zealand (Sykes and Snowdon, 2002). The threshold of the oil generation for the coaly source rocks in the Xihu Sag is at about 0.55% of the Ro value (Li et al., 1997;Zhang, 2017). As shown in Figure 9, all the analyzed samples have reached the threshold of oil generation, and more than one-third of the samples have reached the threshold of oil expulsion (Figures 8 and 9). Therefore, the coaly rocks in the Xihu Sag are the effective source rocks and could provide abundant hydrocarbon for oil and gas fields.

Hydrocarbon generation potentials of different macerals
Liptinite macerals have the highest hydrogen content, volatile yield, hydrocarbon yield, in comparison to those of the vitrinite and inertinite as they contain compounds of mainly aliphatic nature (Tissot, 1984). As a consequence, liptinite macerals are the main sources for oil in coal (Guo and Bustin, 1998;Han, 1996;Scott, 2002;Tissot, 1984;Wilkins and George, 2002). Previous research has shown that macerals with visible fluorescence can generate liquid hydrocarbons (Xiao and Jin, 1991). Liptinite macerals of the coaly source rocks of the Pinghu Formation show a different degree of visible fluorescence (Figures 5 and 6). Snowdon (1991) considered that as little as 10% of hydrogen-rich maceral (liptinite) is needed for the source rocks to be capable of generating commercial oil. The Paleogene coals from the Xihu Sag have liptinite contents ranging from 4.69 to 22.05%, with an  Sykes and Snowdown, 2002). Note: The inferred thresholds for oil expulsion is indicated by the peak in QI at Rank (Sr) 11-12.5. average of 12.46%, and on this basis should be able to generate commercial value oil. Collodetrinite of the samples with low Ro (generally lower than 0.9%) also shows weakly visible fluorescence. In consideration of the fluorescence of collodetrinite macerals, it can be concluded that the coals in the Pinghu Formation are characterized by a dominance of hydrogen-rich vitrinite, and abundant liptinite dominated by resinite. Therefore, not only the liptinite but also some collodetrinite in the coaly source rocks from the Pinghu Formation in the Xihu Sag can generate liquid hydrocarbon.
It has become apparent that not all macerals are of the same quality for petroleum generation (Guo and Bustin, 1998;Scott, 2002;Snowdon, 1991;Wilkins and George, 2002). Relationships between maceral compositions and hydrocarbon generation potentials can reveal the capacity of hydrocarbon generation of different macerals. For the coaly source rocks in Pinghu Formation, the S1þS2 values are positively correlated with the contents of liptinite, and negatively correlated with the contents of vitrinite ( Figure 10). It should be noted that the positive or negative correlations represent the hydrocarbon Figure 10. Relationships between S1þS2 and maceral groups of the Paleogene coaly source rocks in Xihu Sag. generation capacity of maceral groups are above or below that of the averaged level of the whole coal.
Quantitative analysis of the hydrocarbon generation of different maceral groups has been undertaken by many researchers (e.g., Ao et al., 2011;Guo and Bustin, 1998;Liu et al., 2004). Ao et al. (2011) concluded that the hydrocarbon generation potential of liptinite is 4 times that of vitrinite and 10 times that of inertinite. On this basis, the contributions of vitrinite, liptinite, and inertinite to hydrocarbon generation potential for the coaly source rocks from the Pinghu Formation in the Xihu Sag would be approximately 63.21%, 36.46%, and 0.33%, respectively. Liu et al. (2004) suggested that the liquid hydrocarbon generation rate of liptinite is 5-15 times that of the vitrinite group. According to this ratio and neglecting the contribution of inertinite to liquid hydrocarbon generation, contributions of vitrinite and liptinite to liquid hydrocarbon generation potential for the Paleogene coaly source rocks in the Xihu Sag would be 31.61-58.1% (average 40.95%) and 41.9-69.39% (average 59.05) respectively. Therefore, the liptinite makes an important contribution to hydrocarbon generation even though its content is low.

Oil-prone coal implication
Oil-prone coals are extensively documented since Hedberg (1968) proposed the link between oil and terrigenous organic matter. Worldwide, oil-prone coals are mostly preserved in Eocene and Oligocene basins, such as those in China (Huang et al., 1991;Shao et al., 2003;Wang et al., 2018;Zhang et al., 1997Zhang et al., , 2019, Indonesia (Panggabeans, 1991), Australia (Smith and Cook, 1984), and New Zealand (Sykes and Snowdon, 2002;Sykes et al., 2014). Maceral components of oil-prone coal in the oil and gas fields of the world are summarized in Table 4. The oil-prone coals generally have a relatively high content of liptinite (mostly >10%), including the Turpan-Hami, Junggar, and Santanghu basins in China (Dai et al., 2000), the Barito Basin in Indonesia (Panggabeans, 1991), and the coals in the Gippsland Basin (Smith and Cook, 1984) although these have a relatively low content of liptinite and inertinite. Compared to the coal-derived hydrocarbon fields, the coaly source rocks of the Pinghu Formation in the Xihu Sag, whose liptinite content >10% and inertinite content are very low, should have the potential to generate commercial oil and gas.
The coals of the Pinghu Formation are not only typical for the higher liptinite group contents, but also typical for the higher contents of resinite. This is different from other oil-prone coals such as those form the Junggar and Santanghu basins whose liptinite groups macerals are dominated by cutinite and sporinite (Yao et al., 1997). Conifers are an important source of resinite (Han, 1996;Pickel et al., 2017). During the Eocene, the sporopollen of terrestrial plants in the Xihu Sag was dominated by conifer plants form the families Pinaceae, Cupressaceae from the families Taxodiaceae, and some ferns from the families Lygodiaceae and Polypodiaceae (Wu, 2014). The conifer plants make a significant contribution to the maceral components of the high contents of resinite (Han, 1996;Pickel et al., 2017). Therefore, the coals originate from arborecent conifers and can generate commercial oil.
During the Paleogene, the global climate changed from greenhouse to icehouse (Zachos et al., 2001). As a result, floras gradually changed from tropical plants to temperate forests dominated by conifers (Crouch and Brinkhuis, 2005;Tuo and Liu, 2003). Thus, the coals in the Paleogene, especially formed in the period near to the First Oligocene Glacial (Oi-1), should be paid attention for further study for its hydrocarbon potentials. Table 4. Coal petrology properties of some hydrocarbon fields generated from coals.