Maturity effects on steranes of source rocks in Lishu fault depression, Songliao Basin

Chuanjun Yi · Min Zhang · Li Teng

Abstract Geochemical analysis results of 30 source rock samples from Lishu fault depression in Songliao Basin show that samples are compositionally similar and represent a uniform organic facies. Differential distribution of steranes among the samples is due to rank, with vitrinite reflectances (Ro) ranging from 0.65 to 1.61%. In the maturity stage (Ro = 0.65-1.20%), the absolute concentrations of steranes increase, which is affected by kerogen degradation, whereas the pyrolysis of high mature source(Ro = 1.20-1.61%) show a decrease in the abundance of steranes. Simultaneously, the parameters of steranes vary greatly with maturity. Decrease of prognane/regular sterane value with the increase of maturity suggests that pregnane is not a product of regular steranes pyrolysis. The ratios of C27/C29 regular sterane gradually increase, while the value of C29-20S/(20S + 20R) and αββ/(αββ + ααα) decrease at high maturity stage, which is associated with the difference in the thermostability of steranes involved. This reversed trend can be used to determine the high-maturity stage of source rocks. Whereas the values of rearranged steranes/regular steranes in source rocks show an apparent positive correlation with maturity, it, therefore, appears to be particularly useful for maturity assessment at elevated levels.

Keywords Thermal effect · Steranes · Genetic mechanism · Lacustrine source rocks · Maturity

1 Introduction

Steranes, referring to a class of tetracyclic compound (also called steroid nucleus) with an alkyl side chain, in which an angular methyl exists both in A/B ring and C/D rings, occur ubiquitously in petroleum and hydrocarbon source rocks as an important composition part of biological markers(Burlinggame et al. 1965; Seifert and Moldowan 1986).Steranes have complex quaternary ring structure and rich geochemical information, which is of great significance in evaluating the sedimentary environment, input type and maturity of organic matter in crude oil and hydrocarbon source rocks (Mackenzie et al. 1980; Shang et al. 1982;Wang et al. 1983). However, the abnormal distribution and parameters of steranes occurring in hydrocarbon source rocks and crude oils have been widely reported in recent years. Moldowan (1989) found that C27, C28, and C29regular steranes displayed an inverted ‘‘L’’ type distribution dominated by C29regular steranes in the hydrous pyrolysis experiment of Triassic immature source rocks(Monte Prena). There was a pronounced shift towards the C27and C28regular steranes with increasing maturity until the C27, C28, and C29regular steranes showing an asymmetric ‘‘V’’ type distribution. Subsequently, through the study on absolute concentrations of biomarkers in Carboniferous coals from western Pennsylvania and eastern Ohio, Dzou et al. (1995) found that the absolute amount of C29regular sterane was initially high in coals of low maturity, but decreased rapidly in the Rorange of 0.6-1.0%and then remained relatively constant at high maturities,whereas the absolute concentration of C27was uniform through the entire maturity range, resulting in C29/C27sterane ratios that essentially followed the trend of decreasing C29steranes. Zhang et al. (2002) discovered that the content of C27regular steranes increased with increasing thermal maturity through the analysis of geochemical characteristics of the Jurassic and Triassic cores and coal samples of outcrops in Kuqa Depression of Tarim Basin, especially at late mature stage. Sun et al. (2011)conducted a simulation experiment on peat and coal, which indicated that the relative content of C29regular steranes increased with simulated temperature increase and then remained constant after simulated temperature up to 350 °C, while that of C27and C28regular steranes had an increasing trend through the entire maturity range.

In addition, Lewan et al. (1986) found that the isomerization parameters of sterane reverses with increasing temperature or heating time in the pyrolysis experiment.Xia and Luo (1995) conducted thermal simulation experiments on Tertiary lignite in Nanning Basin. they found that the parameters of steranes changed repeatedly with the increase of temperature, from large to small and then to large. Chen et al. (1997) pointed out that the maturity parameters C29-20S/(20S + 20R) and αββ/(ααα + αββ)showed obvious inversion at the high and over mature stages when they studied the characteristics of bitumen biomarkers in natural gas reservoirs. Nevertheless, it should be noted that according to Yang et al. (2018), the C27/C29sterane ratios in the colas from Kuqa depression and Ordos Basin also changed significantly with thermal maturity on the research of the thermal effect on distributions of regular steranes, increasing with enhanced Rovalues or Tmaxvalues. This phenomenon further illustrates that the thermal maturation has some connections with the distributions of regular steranes. In this study, we studied the variation of sterane parameters at different thermal evolution stages.

Songliao Basin is a continental petroliferous basin rich in oil and gas, there are abundant lacustrine source rocks in the basin. Based on the summarization of predecessor’s studies, they have found abundant steranes in crude oil and lacustrine source rocks with quite a different composition and distribution. What’s more, the formation and distribution of steranes in this region were studied in terms of organic matter sources and sedimentary environment(Chen 2012; Zhang 2014; Jiang and Zhang 2015), they discussed that there was a general positive relation between different diahopanes and regular steranes. Thermal action is one of the important factors affecting steranes in crude oil and source rocks, but there are few systematic reports in the existing literature of the Songliao Basin. This paper systematically investigate the organic geochemistry of typical lacustrine mudstones in Songliao Basin to further discuss the composition and distribution of steranes and study the effects of thermal action on steranes, at the same time, the application range of sterane parameters was analyzed, which may provide some great significance for a better understanding of controlling factors of steranes.

2 Samples and experiments

2.1 Samples

The Songliao Basin is a significant petroleum-producing province, with a total area of about 2.6 × 105km2. Thirty source rock samples from Early Jurassic-Cretaceous selected from the Lishu fault depression located in the southeast uplifted zone of the Songliao Basin (Fig. 1), the detailed stratigraphic informations of selected samples are shown in Table 1. Previous research reported that the sedimentary facies of the Lishu Fault Depression changed from shore-shallow lake to shallow lake, to semi-deep and deep lake, and finally to shallow lake with water depth from shallow to deep to shallow (Chen 2012; Li et al.2013). Affected by the continuous development of faults,the sedimentary environment is mainly dominated by deltaic facies and lacustrine clastic deposits, which is broad in the area and deep in thickness. It mainly presents the characteristics of continental deposits with lakes as the center and the surrounding water system carrying sediments to the lacustrine basin. Feng (2008) and Huang(2004) have described the detailed evolution and sedimentary sequence stratigraphy of the Songliao Basin.Briefly, there are significant hydrocarbon source rocks strata including the Huoshiling Formation (J3h and K1h),Shahezi Formation (K1sh), Yingcheng Formation (K1yc),Denglouku Formation (K1d), and Quantou Formation(K1q) in this study. General information regarding the source rock samples are provided in Table 1.

Fig. 1 Tectonic location Lishu fault depression, Songliao Basin

Table 1 Parameters used for source-rock assessment of samples studied

2.2 Experiments

All samples were collected for vitrinite reflectance (Ro)analysis especially for this research. The measurement of reflectance was performed on polished resin-embedded whole-rock blocks with a Leica MPV3 photomicroscope by counting at least 30 points.

Bitumen extractions were performed on the powdered samples using a Soxhlet apparatus for 72 h with an azeotropic mixture of dichloromethane/methanol (93:7),then separated into saturated hydrocarbon, aromatic hydrocarbon and NSOs(nitrogen, sulfur, and oxygen), and asphaltenes by liquid column chromatography. Saturated hydrocarbons were analyzed using an HP 5873 mass spectrometer coupled to an HP6890 GC chromatograph equipped with HP-5MS fused silica capillary column of 30 m in length, 0.25 mm in internal diameter, and 0.25 μm of film thickness. The GC column temperature program was 50 °C for 1 min to 100 °C at a rate of 20 °C/min, then from 100 to 315 °C at a rate of 3 °C/min and with a final hold of 18 min. Helium was used as a carrier gas flowing at a rate of 1.0 ml/min and the gasification chamber was kept at 300 °C. the ionization source operated at 70 eV. The scanning range is 50-550 amu. The identifications of biomarker are obtained through, saturated hydrocarbon ratios,as well as the relative sterane and triterpane abundance,were calculated using the integrated peak areas for the relevant ionm/z191 andm/z217 spectrogram.

The absolute concentration of steranes was quantified using 5α-androstane as an internal standard. Within the linear range detected by the instrument, the detected peak area is proportional to the mass of the compound. Therefore, a certain amount of standard samples are added before sample detection, and the absolute concentration of the compound is calculated according to the ratio relation between the standard samples and the measured peak area of standard samples. The absolute concentration of steranes can be calculated as following formula:

where Ccis the absolute concentration of the compound,Scis the peak area of the compound, Wsis the standard sample mass used;Ssis the peak area of the standard sample tested, WAorTis the quantity of chloroform asphalt‘‘A’’ for the analysis of GC-MS or total organic carbon.

3 Results and discussion

3.1 Basic geochemical characteristics

3.1.1 Bulk compositions and properties of source rocks

Hydrocarbon source rock evaluation is the most basic work and research in the exploration practice, which directly determines the exploration direction and prospect of oil and gas. Source-rock properties of source rocks from the Songliao Basin are researched in the study for the sake of detecting the organic abundance, hydrocarbon potential of the organic matter and its thermal maturity. TOC is used to evaluate source rock generative potential. the TOC values of lacustrine mudstones are in the range of 0.34-2.09 wt%,most of which more than 0.4 wt% (Table 1). The TI is a useful geochemical indicator of organic matter source.Xiong et al. (2001) improved the calculation method of Ti index in order to more accurately indicate the type of organic matter. According to following formula:

a, b, c, d, e respectively represent the percentage of implied amorphous organic matter in mineral asphalt matrix, alginite, exinite, vitrinite, and inertinite, TI range from - 3.60 and 16.50, and indicates that the Early Jurassic-Cretaceous source rocks in the Lishu fault depression are type III kerogens. In addition, the relationship between pyrolysis peak temperature (Tmax) and hydrogen index (HI) of hydrocarbon source rocks in Lishu fault depression further indicate lacustrine mudstone are type III kerogens (Table 1 and Fig. 2). Tmaxvalues (411-551 °C) and Ro(0.65-1.61%) in the Songliao Basin illustrate that these samples are mature to high-mature (Table 1).

Fig. 2 The relationship between pyrolysis peak temperature (Tmax)and hydrogen index (HI) of hydrocarbon source rocks in Lishu fault depression, Songliao Basin

Fig. 3 Representative mass chromatograms (m/z 191)showing the distribution of triterpanes from source rocks in Lishu fault depression, Songliao Basin

3.1.2 Molecular composition and distribution characteristics

Various types of depositional environments have shown that C23TT is often dominant in marine or lacustrine oils or source rocks while C19and C20TT are more abundant in oils or source rocks of terrestrial origin (Peters and Moldowan 1993; Aquino Neto et al. 1983). Tricyclic terpene(TT) and hopane biomarkers are identified through characteristic mass chromatograms and quantified by intergration of peak areas using them/z191 chromatogram. The geochemical characteristics of saturated hydrocarbons in source rock samples are very similar, all lacustrine source rock samples contain relative low amount of tricyclic terpenes with the predominance of C23tricyclic terpene and show a general normal distribution with C19-C26carbon number (Fig. 3). In addition to tricyclic terpane, the samples also have a significant amount of C24TeT, which are relatively high concentrations in oils and hydrocarbon source rocks with terrigenous input (Philp and Gilbert 1986). The source rock samples under investigation have relatively high tricyclic terpanes compare to C24TeT and low values of C24TeT to C26TT (C24TeT/C26-TT = 0.42-0.65), indicating that the organic matter of the mudstone samples may primarily originate from algae and microorganism. As shown in Table 2, the Pr/Ph (pristane/phytane) ratios for lacustrine mudstone from the Songliao Basin range from 0.29 to 0.72, Pr/nC17and Ph/nC18are 0.41-0.75, 0.47-1.24, respectively, the gammacerane index (G/C30hopane) of most samples less than 0.3,which indicate lacustrine mudstone deposited in reducing and brackish-water environment. Additionally, the m/z 191 mass chromatogram shows that pentacyclic triterpanes have a complete distribution pattern with all of the C27-C35pentacyclic triterpanes present. Furthermore, C30hopane is the dominant pentacyclic titerpane (tricyclic terpenes/hopanes = 0.22-0.75), there is a wide range of Ts/Tm from 0.44 to 1.77 corresponding to thermal maturity. The extended hopanes or homohopanes are dominated by C31homohopane and generally decreasing toward the C35homohopane, meanwhile, the C31-22S/(22S + 22R) are inthe range of 0.54-0.62 (Table 2). On the whole, the sedimentary environment of hydrocarbon source rocks samples in the study area are a brackish water environment under the reduction condition as well as the biomarker combination features of source rocks are consistent with that of typical lacustrine source rocks.

Table 2 Partial biomarker parameters of source rocks in the Lishu fault depression

Steranes, including pregnane (C21, C22), regular steranes (C27-C29), rearranged steranes (C27, C29) and methyl steranes, are detected in typical lacustrine source rocks in Songliao Basin. The composition and distribution of steranes show apparently distinct characteristics in different maturity stages which is associated with a wide range of Rovalues (0.65-1.61%) of source rock samples. Figure 4 shows the mass chromatogram of sterane m/z 217 from lacustrine source rock samples in the Songliao Basin.It’s obvious that sterane distribution patterns in source rocks can be classified into three types, Groups I, II, III.Group I distribute in low maturity stage (Ro-= 0.65-0.84%) with abundant pregnane, (C21+ C22)/regular steranes are in the range of 0.09-0.19 while(C21+ C22)/C29ααα20R are 0.73-1.55, the regular steranes are characterized by a ‘‘V’’ pattern distribution of C27＞C29＞C28which is consistent with typical lacustrine source rock, demonstrating that the primary organic matter in source rocks is mainly derived from lower aquatic organisms. Group II distributes in maturity stage with Rofrom 0.86 to 1.32%. The distribution characteristics of steranes in source rocks have changed greatly, the relative pregnane content decreased, (C21+ C22)/regular steranes are in the range of 0.04-0.11 while (C21+ C22)/C29ααα20R are 0.36-0.97 (＜1), The distribution of C27-C29regular steranes changed from the typical asymmetric‘‘V’’ type of lacustrine source rocks to the inverted ‘‘L’’type. In addition, Group III is in high maturity stage with Rofrom 1.37 to 1.61%. The C27αααR-C28αααR-C29αααR steranes in source rocks exhibit asymmetric ‘‘V’’ pattern where the content of C27αααR sterane is higher than that of the C29αααR sterane, which is similar to that in Group I, whereas the pregnane content is relatively low,(C21+ C22)/regular steranes are in the range of 0.04-0.11 less than Group I and Group II, and (C21+ C22)/C29-ααα20R are 0.40-0.97 (＜1) (Table 4). According to above analysis result, steranes anomaly distribution mainly is related to the maturity of source rocks under the similar sedimentary environment and organic matter sources of source rocks in the Lishu fault depression of songliao basin. Namely, the different distribution patterns of steranes depend on the degree of thermal evolution.

Fig. 4 Mass chromatograms (m/z 217) showing the distributions of steranes from source rocks in Lishu fault depression, Songliao Basin

3.2 Maturation effects on absolute concentrations of steranes

The absolute concentrations of compounds is an important index to measure the content of compounds in source rocks. Furthermore, the change of absolute concentrations can clearly reveal the formation and evolution of steranes in different evolution stages. As shown in Fig. 5 and 6, the absolute concentrations of steranes in μg/mg of bitumen and in μg/mg TOC show the same trend. At the same time,the absolute concentrations of steranes (in μg/mg of bitumen and in μg/mg TOC) in source rocks of Lishu fault depression can be roughly divided into two stages with the change of maturity: Ro: 0.65-1.20% and 1.20-1.61%(Table 3, Figs. 5, 6), corresponding to the maturity and high maturity stages of the evolution of source rocks,respectively. In the maturity stage (Ro= 0.65-1.20%), the absolute concentrations of pregnane, homopregnane, and C27-C29regular steranes increase with maturity, suggesting that this stage is favorable for the formation of pregnane and C27-C29regular steranes. In other words, pregnane and C27-C29regular steranes gradually enriched in source rocks with the degradation of kerogen, leading to an increase in absolute concentrations and a peak at Ro= 1.2%. In the high maturity stage with Rovalues from 1.20 to 1.61%, the absolute concentrations of pregnane and C27-C29regular steranes substantially decline. Thermal cracking is the main factor to promote the transformation of organic matter in this stage. At high temperature, steranes content rapidly decrease at high maturity stage, steranes C-C bonds are broken and cracked into hydrocarbons with low molecular weight. In contrast, the absolute concentrations of rearranged steranes in source rocks were significantly different from that of pregnane and regular steranes, which show a substantial increase with maturity at Rovalue 0.80%. The result reveals that the main formation stage of rearranged steranes is the maturity to high maturity stage,meanwhile, the increase of absolute concentrations of rearranged steranes may be related to the rearrangement and transformation of regular steranes in the high evolution stage.

Table 3 The absolute concentration of steranes in source rocks from the Lishu fault depression

3.3 Effects of maturity on sterane parameters

3.3.1 Pregnane (C21, C22)

The pregnane series, belong to short-chain steranes occur ubiquitously in petroleum and source rocks, including pregnane (C21) and homopregnane (C22). It’s universally believed that the genesis of pregnane series is complex and there are primary and secondary formation pathways. Primary pregnane is mainly formed by the diagenetic evolution of biohormone pregnols and pregnone, while secondary pregnane is the product of thermal evolution(Huang 1984). There are two main opinions on the formation mechanism of secondary pregnane: one is that pregnane is formed by the side chain breaking of regular sterane under thermal action (Huang 1984), and the other holds that pregnane originate from the degradation of kerogen organic matter during hydrocarbon generation(Xia and Luo 1995; Liu et al. 2007). The absolute concentration variation tendency of pregnane and homopregnane in lacustrine source rocks of Songliao Basin are more consistent with the hydrocarbon generation model of kerogen. Although pregnane is abundantly produced in the mature stage, them/z217 mass chromatograms of source rocks exhibit that pregnane shows a gradually decreasing trend (Fig. 4). Furthermore, it is inferred from Fig. 7a, b that the ratios of (C21+ C22)/regular sterane and(C21+ C22)/C29ααα20R show a sharp decrease withincreasing maturity. As shown in Figs. 5 and 6, the absolute concentrations of pregnane as well as homopregnane increased less than that of other steranes in same stage,resulting in a decrease of (C21+ C22)/regular steranes and(C21+ C22)/C29ααα20R. Thus, this study shows that pregnane parameters have a limited potential to reflect thermal maturity (Table 4).

Table 4 Sterane parameters of source rocks from the Lishu fault depression

Fig. 5 Plots showing the variations in absolute concentration of steranes (in μg/mg of bitumen) with Ro from source rocks in Lishu fault depression, Songliao Basin

Fig. 6 Plots showing the variations in absolute concentration of steranes (in μg/mg TOC) with Ro from source rocks in Lishu fault depression,Songliao Basin

Fig. 7 Plots showing the parameters of steranes with Ro from source rocks in Lishu fault depression, Songliao Basin

3.3.2 Regular steranes

Regular steranes are the main body of steranes, which are named according to their structures that contain a methyl at C-10 and C-13 position and five asymmetric carbon atoms(C-5, C-14, C-17, C-20, and C-24). Regular steranes widely distributed in the geosphere mainly consist of C27,C28, and C29ααα(20S + 20R) and αββ(20S + 20R). In general, the relative abundance and composition of C27,C28, and C29steranes can be used to indicate the organic matter source. It’s generally believed that dominance of C27sterols (steranes) mainly derived from algae, while the C29sterols (steranes) are more typically associated with land plants (Philp 1985; Peters and Moldowan 1993;Huang and Meinschein 1979). Meanwhile, the ratios of different stereoisomers of regular steranes can also indicate maturity. The distribution of regular steranes isomers are mainly affected by thermal effect, with the increase of maturity, the 20R configuration changed to 20S configuration and the ααα configuration transformed into αββ configuration, the ratios of 20S/(20S + 20R) and αββ/(αββ + ααα) gradually show an apparent increasing (Seifert and Moldowan 1986; Moldowan 1989). Additionally,the ratios of C2920S/(20S + 20R) and C29αββ/(αββ +ααα) have been widely used in petroleum geochemistry as indicators of thermal maturity.

Figure 7c-e reveal that the relationship between regular sterane parameters and Roin lacustrine source rocks of Songliao Basin. Clearly, the ratios of C27/C29regular steranes, C2920S/(20S + 20R) and C29αββ/(αββ + ααα)show significant differences with increasing Ro. The C27/C29regular steranes show an initial decrease with Ro, until the Rois about 1.0 then it increases markedly. This phenomenon may be related to the difference of sterane side chains of C27and C29. From the perspective of molecular mechanics, C29steranes have one more ethyl group at C-24 than C27steranes, which makes the standard enthalpy of formation of C29sterane is 61.97 kJ/mol less than that of C27sterane (Wang et al. 1996), and the heat required for the formation of C29sterane is lower than that of C27sterane (-C2H5= C-(C)(H)3+ C-(C)2(H)2, the standard enthalpy of formation of C-(C)(H)3and C-(C)2(H)2are- 39.09 and - 22.88 kJ/mol, respectively). Therefore, it is easier to produce C29regular steranes by kerogen degradation than C27regular steranes in the mature stage of crude oil (Ro: 0.65-1.20), resulting in lower C27/C29steranes. On the contrary, The ratios of C27/C29regular steranes increase in the high-over maturity stage, which is consistent with Lewan et al. (1986) and Sun et al. (2011).It’s generally believed that the alkanes containing branched chains are prone to breaking and C29steranes are one more ethyl group than C27steranes. In high-over maturity stage,the C29regular steranes will be demethylated to form C27regular steranes, resulting in an increase in C27/C29regular steranes.

As presented in Fig. 7d, e, the ratios of C2920S/(20S + 20R), C29αββ/(αββ + ααα) and C27/C29regular steranes in lacustrine source rocks display a distinct variation with increasing Ro. The C29regular sterane parameters increased at the maturity stage within the range of Ro0.65-1.20% (in the oil window), suggesting it’s a good maturity parameter. In high maturity stage (Ro-= 1.20-1.61%), oil-cracking gradually takes a dominant position. The isomers with the 20S structure show a total faster cracking rate compared to the isomers with the 20R structure and αββ configuration faster than ααα configuration, resulting the values of C29-20S/(20S + 20R) and αββ/(αββ + ααα) persistently decrease (Chen et al.1997, 2013; Sun et al. 2011). In short, the variation of C27/C29regular steranes is mainly caused by the difference of standard enthalpy of formation and demethylation, while the variation of isomerization parameters of C29steranes is controlled by thermal cracking rate. Therefore, the regular sterane parameters have different organic geochemistry significance in different evolutionary stages, the influence of maturity should be carefully considered when using these parameters.

3.3.3 Rearranged steranes

Diasterane is a kind of biomarker widely used in organic geochemistry. It is a sterol formed in the early stage of diagenesis by carbon skeleton rearrangement under the acidic catalysis of clay mineral (Leeuw et al. 1989).Rearranged steranes generally increase with the increase of maturity, it appears that the diasteranes/regular steranes change in source rocks as a function of maturity (Philp 1985; Liu et al. 2007). It’s generally believed that rearranged steranes are affected by total organic carbon (TOC),clay minerals and maturity. Kong (2018) found that the content of clay minerals in Songliao Basin is 28.5-42.3%,rich in illite and illite/mengmontmorillonite interlayer, and lacking kaolinite. Meanwhile, the TOC values of lacustrine mudstones are in the range of 0.34-2.09 wt%, (Table 1),showing little change. The above research shows that the acid catalyzing effect of clay minerals and TOC content in this area are not the main reasons for the change of rearranging sterane. As shown in Fig. 7f, complete C27and C29rearranged steranes can be detected in lacustrine source rocks of Songliao Basin, and the ratios of rearranged steranes/regular steranes increase with maturity. With increasing thermal evolution, the absolute concentration of rearranged steranes increases because of methyl rearrangement, it’s obvious that the ratios of rearranged steranes/regular steranes increased. Additionally, the ratio of diasteranes/regular steranes persistently increases, suggesting that the thermal stability of rearranged steranes is better than that of regular steranes.

4 Conclusions

Thermal action is one of the important factors affecting the formation and distribution of steranes in lacustrine source rocks, steranes exhibit different evolution regularities in different evolutionary stages. The composition and distribution characteristics of steranes revealed that thermal action contributed to the formation of steranes, thermal cracking is the main factor for the decrease of sterane content. Secondary pregnane in the thermal evolution process was not formed by the side chain breaking of high carbon number regular steranes, but are associated with the degradation of kerogen organic matter.

The variation of steranes parameters has apparently distinct characteristics at different maturity stages. In the maturity stage, the sterane parameters decrease except for the C29sterane isomerization parameters. In the high-maturity stage, the ratios of C27/C29regular steranes increase,while the C2920S/(20S + 20R) and αββ/(αββ + ααα)decrease, which are associated with the thermostability of steranes. It is also verified that the different distribution characteristics of steranes in lacustrine source rocks of Songliao Basin are affected by thermal action. Moreover,the values of rearranged steranes/regular steranes in source rocks have a good correlation with maturity, indicating that it can be used as an effective maturity parameter at maturity to high- maturity stage.

Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Grant No. 41772124).