A total of 25 coal samples were collected from the Ordos Basin and Tarim Basin. Ten coal samples are selected from C₂y, C₂j, C₂b, P₁t and P₁s of Carboniferous to Permian in Ordos Basin, which mainly develops alluvial fan, delta, marshes and tidal flat sedimentary environment [17] [18] [19] [20] . Another coal samples are from T₃h, T₃t, J₁y, J₂k and J₂q of Triassic to Jurassic, where sedimentary environment is fan delta system [19] [20] . Nineteen samples were selected for vitrinite reflectance and whole rock maceral analysis especially for this research. All coal samples were crushed into fine powder and analyzed for their contents of total organic carbon (TOC) and rock pyrolysis.

The measurement of reflectance and maceral were determined by Leica MPV-SP photomicroscope. Before the measurement, the samples were crushed to a maximum size of 1-mm. While at least 30 points were counted in every sample, then the type index (T_i) is used to calculate kerogen type indicators.

TI = ( sapropelic × 1 00 + exinite × 5 0 + vitrinite × ( − 75 ) + inertinite × ( − 1 00 ) ) / 100

Fortotal organic carbon (TOC) and rock pyrolysis, all the coal samples need to be crush into fine powder and analyzed. However, before TOC determinations, all samples were removed by HCl treatment. The total carbon content was determined by LECO CS-2000 induction furnace. Pyrolysis was determined by Rock Eval VI made in China.

Bitumen “A” was extracted by the Soxhlet extraction for 3 days with a dichloromethane/methanol mixture (93:7 v/v). Then, the extracted bitumens were fractionated into saturated, aromatic hydrocarbons, NSOs (nitrogen, sulfur and oxygen) and asphaltenes using open column chromatography filled silica gel and the alumina. The study mainly analyzes the saturate and aromatic hydrocarbons fraction. The model of gas chromatography-mass spectrometry is HP-GC 6980/5873 MSD. And the HP-6890 GC is equipped with a HP-5MS fused silica capillary column (30 m × 0.25 mm × 0.125 μm). Helium is used as the carrier gas with a rate of 1.0 ml/min. The injector temperature is 300˚C. The GC temperature is programmed to start at 50˚C for 1 min; the temperature will increase to 310˚C at a rate of 3˚C/min with a final hold of 18 min. The scanning range is 50 - 550 amu.

The results of rock pyrolysis and R_o determination showed that the maturity range of coal sample from the kuqa Depression in Tarim Basiniswide. t_max value range from 430˚C to 529˚C. R_o ranges from 0.51% to 1.63% with an average value of 0.72%. In contrast the maturity of the coal samples from the Ordos Basin is relatively high. The average of R_o is 0.99%. In the whole rock maceral, T_i is an index to judge the type of parent material. The T_i index can be calculated using the relative percentage content of vitrinite, exinite, inertinite and so on. T_i values in the selected coal sample is −68.48 - −37.88, which reveals that all coal samples are III kerogen (Table 1).

Note: T_i = (100a + 75b + 50c + (−75)d + (−100)e)/100, a, b, c, d and e respectively represent nonfixiform organic matter, alginate, exinite, vitrinite and inertinite.

In this study, the complete distribution of sterane series compounds was detected in the coal sample from Kuqa Depression and Ordos Basin. When the t_max value is about 440˚C (R_o = 0.7%), the distribution of C₂₇ and C₂₉ regular sterane is consistent with the typical coal measures hydrocarbon source rocks with C₂₉ regular sterane accounting for absolute advantage. It presents asymmetrical “V” type or the “L” type (Figure 1(a)). With the thermal evolution degree increasing, the typical coal sample of C₂₉ regular sterane relative content advantage has disappeared. On the contrary, the relative content of C₂₇ regular sterane has a tendency of increasing (Figure 1(b)). The distributions of different maturity sterane on the same section area in Ordos basin revealed that the relative content of C₂₇, C₂₉ regular sterane are consistent with typical coal measures hydrocarbon source rock under the condition of low maturity (t_max value 441˚C, R_o values 0.66%). In other words, the C₂₉ regular sterane has absolute predominance. However, with the increase of maturity, when the t_max value reaches 499˚C, C₂₇ regular sterane predominates, which is similar to Figure 2(b). This phenomenon is consistent with previous studies, when thermal evolution degree is higher; the C₂₇ regular sterane relative content will gradually increase with the increasing maturation [21] . Although the samples from the same section, the distribution of C₂₇ sterane and C₂₉ sterane is different in different maturity. However, the distribution of C₂₇ sterane and C₂₉ steraneis consistency are in different parts of the section under the condition of similar maturity. Compared C₂₇ sterane and C₂₉ sterane, C₂₈ sterane has no obvious change in the same profile under the condition of different maturity. That is to say, the distribution of sterane cannot be used to distinguish the hydrocarbon source rocks in the same area under the condition of different maturity.

In general, aquatic organisms are rich in C₂₇ααα20R sterane, while terrigenous higher plants are rich in C₂₉ααα20R sterane. That view has been generally accepted [22] . The relationship between the relative content C₂₇, C₂₈, C₂₉ regular sterane and the vitrinite reflectance R_o (%) in Kuqa Depression in Tarim Basin and Ordos Basin reveals that the thermal action has influence to the C₂₇, C₂₈ and C₂₉ regular sterane. When the R_o (%) value is less than 0.7%, the coal samples are still accounts for absolute advantage for C₂₉ regular sterane. With the higher evolution degree, the relative content of C₂₉ regular sterane gradually diminishes, while the C₂₇ regular sterane increases. This leads to “invert” phenomenon that the content of C₂₇ regular sterane is greater than the content of C₂₉ regular sterane with the R_o (%) values increasing continually. In the stage of mature to high mature, the relative content of C₂₈ regular sterane seems to be no obvious change. Thus, in the high and over mature evolutionary stages, when using the C₂₇, C₂₈ and C₂₉ regular sterane to judge parent material types needs to be careful. Otherwise, the conclusions from the distribution of regular sterane may be inconsistent with the practical point in the natural profile.

Previous studies illustrated the change rule of the C₂₇, C₂₈ and C₂₉ regulars terane compounds with the increasing maturity in simulation experiments. The change is quite outstanding especially in high or over maturity [14] [15] [16] . To further exploring the relationship between the sterane C₂₇, C₂₈ and C₂₉ regular sterane and the thermal action, the relationship between C₂₇/C₂₉ regular sterane ratio and maturity parameters of R_o (%) and t_max (˚C) were discussed firstly (Figure 3). When R_o value is 0.7% - 1.5%, the C₂₇/C₂₉ regular sterane ratio has a good linear relationship with R_o in the Kuqa Depression and the Ordos Basin (Figure 4(a)). Mean while, the C₂₇/C₂₉ regular sterane ratio and t_max (<500˚C) is a linear relationship, which is similar to the R_o (%). That may be the thermal effect on the demethylation of C₂₉ sterane obviously in the evolution stage of mature to high-mature [17] [23] .

The coal samples are at low mature to over mature stage with t_max ranging from 432˚C to 529˚C. The maturity parameters of saturated hydrocarbons have reached equilibrium value; so it cannot effectively represent the maturity of samples. However, compared with the maturity parameters of saturated hydrocarbon, the maturity parameters of aromatic hydrocarbon could judge the high maturity samples. In addition to the commonly used methyl phenanthrene index MPI [24] [25] , Chakhmakhchev et al. put forward dimethyl-benzothiophene maturity parameters. The works of the relationship between R_o and a dimethyl-benzothiophene ratio parameter have done a lot of. And the conversion formula of R_o and dimethyl-benzothiophene ratio has been put forward [26] [27] . The ratio of three methylnaphthalene TMNr = 2,3-TMN/(1,2,5-TMN + 2,3,6-TMN) were put forward [28] . And when the ratio of TMNr is greater than 0.6, it represents the high maturity [29] . In the aromatic compounds, many evaluation parameters can be used to evaluate the maturity. This present paper chose naphthalene, fluorene series compounds for research.

The C₂₇/C₂₉ regular sterane ratios and aromatic maturity parameters DMDBT and TMNr also have a good linear relationship. And the relationship is similar to t_max and R_o (Figure 5). That is to say the thermal of hydrocarbon source rocks has distinct effect on the relative concentration distribution of regular sterane C₂₇, C₂₈ and C₂₉ regular sterane. So, in the same research area, the C₂₇/C₂₉ regular sterane ratio can be used to determine hydrocarbon source rock maturity, especially in mature-high and over mature stage of evolution.

In order to demonstrate the thermal effect on sterane distribution rules, the thermal simulation test of a coal sample had been done. Six temperature points, 200˚C, 250˚C, 300˚C, 350˚C, 400˚C, 450˚C, were chosen in the process of experiment. The results show that C₂₇/C₂₉ regular sterane ratio increases with the simulative temperature rising (Figure 6). This further elucidated that the influence of the distribution of C₂₇ and C₂₉ regular sterane in the sample is objective, especially, in the high-mature stage (thermal simulative temperature >350˚C). The relationship of the absolute concentration of αααC₂₇, C₂₉ regular sterane and pyrolysis temperature are shown in Figure 6(b). The absolute concentration of C₂₉ regular sterane decrease is faster than C₂₇ sterane in low mature stage (thermal simulative temperature <350˚C). But when the temperature is greater than 350˚C, the C₂₇ regular sterane is increasing faster than C₂₉ regular sterane. This phenomenon proves that the C₂₇/C₂₉ regular sterane ratio and the degree of thermal evolution have a certain correlation.

Structures determine properties in organic chemistry. It is because of the diversity of steranes compounds configuration change. Therefore, steranes compounds have high practical value in researching sources, thermal evolution and sedimentary environment of organic matter. In general, the longer chain of carbon alkane molecules is, the more will be easily broken. So the naphthene is steadier than side chain alkanes, and the more branched alkanes is prone to cracking reaction [30] . The C₂₉ regular sterane adds an ethyl in C-24 place compared with the C₂₇ regular sterane. Owing to hydrocarbons with different carbon number needing different chemical bond dissociation energy to fracture, it leads to the variation of C₂₇/C₂₉ regular sterane ratio with the increasing thermal maturity (Table 2).