1 Introduction

Bicyclic sesquiterpanes, a type of saturated hydrocarbon, are widely distributed in crude oil and sediments. The presence of bicyclic sesquiterpanes containing the drimane skeleton has been reported in many previous studies since Anders and Robinson (1971) first reported a series of bicyclic sesquiterpanes in their study of the cycloalkane constituents of the Green River shale. They suggested that the bicyclic alkanes may have been derived from degradation of steroid or triterpenoid precursors. Philp et al. (1981) concluded that the distribution of C14, C15, and C16 bicyclanes was not dependent upon the degree of biodegradation of the oils. They also suggested that the tricyclic diterpenoid compounds might possibly be precursors of the bicyclanes or intermediates involved in their formation. Bendoraitis (1973) proposed that the high concentrations of bicyclic alkanes in crude oils from South Texas may result from the degradation of more complex precursors, in particular by the thermal degradation of higher plant triterpenes, whereas Alexander et al. (1984) proposed that bacteriohopanoids were extensively degraded in petroleum source rocks during diagenesis, resulting in functionalized bicyclic compounds of the drimane type. These compounds could then undergo rearrangement involving carbonium ion intermediates or be reduced directly to hydrocarbon. Noble et al. (1987) concluded that bicyclic alkanes were released from the insoluble organic matrix (kerogen) and that drimanes and homodrimanes occurred in sedimentary organic matter in both the free and the bound states.

The evaluation of thermal maturity of organic matter in the previous studies usually was carried out using vitrinite reflectance (% Ro) (Zou et al. 2000; Wang et al. 2008; Hartkopf-Fröder et al. 2015; Fu et al. 2018), graptolite reflectance (Wang et al. 1993), pyrolysis Tmax (Gao et al. 2018), production index (PI) values (Mashhadi et al. 2015), and some biomarkers parameters such as Ts/(Ts + Tm) (Bao et al. 2018), moretane/hopane, oleanane/hopane, C29 20S/(20S + 20R) (Fu et al. 2017). The bicyclic sesquiterpanes may also be very useful as direct maturity indicators during the immature–low-maturity stage due to their relatively high sensitivity to subtle changes in organic matter. The aim of this study was to identify bicyclic sesquiterpanes and analyze their evolution and organic geochemical significance by conducting simulated pyrolysis experiments on an immature organic-rich Lower Cretaceous mudstone collected from the Hesigewula Sag, a relatively new exploration area.

2 Samples and analytical methods

2.1 Sample

The Hesigewula Sag is a Mesozoic sedimentary basin located in the northeastern Erlian Basin, China. The sag belongs to the Daxinganling stratigraphic subregion according to the formation. There are many strata in the periphery of this sag, including Jurassic, Cretaceous, and Upper Paleozoic. The sample for this experiment was collected from the Lower Cretaceous in Well HD2. This well is located at the Hesigewula Sag near the western edge of Daxinganling Prefecture. The source rock sample is brown gray mudstone, and the geochemical parameters of this sample are shown in Table 1.

Table 1 Geochemical parameters of the source rock sample
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2.2 Simulated pyrolysis experiments

In the experimental preparation phase, the source rock sample was crushed to 2 mm–4 mm and then pressed into a cylindrical shape with a pressure of 50 MPa using a custom-made stamper. The weight, height, and diameter of the sample column were 145.22 g, 76.92 mm, and 4.00 cm, respectively.

The simulated pyrolysis experiment was carried out in an experimental system developed in-house, as shown in Fig. 1, and the experimental conditions are listed in Table 2. This simulation experiment mainly includes five steps:

Fig. 1
figure 1

System of simulated pyrolysis experiments

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Table 2 Parameters of the simulated pyrolysis experiments
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  1. (1)

    Clean the experimental system with dichloromethane.

  2. (2)

    Put the sample column into the sealed pyrolysis vessel, evacuate the system using a vacuum pump, and then, fill the system with plenty of helium.

  3. (3)

    Set the initial temperature and pressure of the experiment (Table 2), heat the reactor kettle with a thermowell, and keep the temperature and pressure unchanged for 24 h.

  4. (4)

    Let the experimental temperature drop rapidly to 150 °C and then collect the liquid and gaseous hydrocarbons.

  5. (5)

    Set the next experimental temperature and pressure and repeat experimental steps 3 and 4.

After flowing through a Geiser burette filled with silica gel and alumina (3:1), the liquid hydrocarbons and extractable organic matter (EOM) extracted from the original rock sample were separated into three gross compositions, saturated hydrocarbons, aromatic hydrocarbons, and non-hydrocarbons. Then, the saturated hydrocarbons were tested and analyzed with a gas chromatography–mass spectrometry (GC–MS).

2.3 Analytical methods of GC–MS

The GC–MS used was an Agilent-6890 N/5975 MSD. The column was an HP-5MS (30 m × 0.25 mm × 0.25 μm), and the carrier gas used in this experiment was helium. The temperature of the injection port was set to 300 °C. The pulse splitless injection technique was chosen as the injection mode, and the flow rate of the sample was kept unchanged at 1.0 ml/min. The process of increasing temperature has two stages. Firstly, the temperature was kept at 50 °C for 1 min and then was increased to 100 °C at a rate of 20 °C per minute. Secondly, the temperature was increased to 315 °C at a rate of 3 °C per minute and then remained unchanged for 16 min. The mass spectrometer was an Agilent 5975 quadrupole mass spectrometer, and the temperature of the quadrupole rod was 150 °C. The ion source of the mass spectrometer was an electron impact ionization source, with an energy of 70 eV and a temperature of 230 °C. The scanning methods were full scan (50 amu–550 amu) and selective ion scan.

3 Results and discussion

3.1 Identification of the bicyclic sesquiterpanes

Selected ion monitoring (SIM) of m/z 123 was used to determine whether any bicyclic sesquiterpanes were present in the saturated hydrocarbons separated from 10 products (Fig. 2). The sesquiterpanes ranging from C14 to C16 usually elute out between n-C13 and n-C16 in the total ion chromatograms (TIC)/SIM chromatogram (Figs. 2 and 3) (Philp et al. 1981; Stout et al. 2005; Wang et al. 2005a; Yang et al. 2009; Yessalina et al. 2006). Peaks A, B, C, and D were identified as C14 (peak A), C15 (peak B), and C16 (peaks C and D) bicyclic sesquiterpanes (Fig. 3). Figure 3 shows that each of these products yields a notable m/z 123 fragment due to the formation of a C9H15 fragment ion (Fig. 4). The partial mass chromatograms of various confirmation fragment ions (m/z 179, 193, 207) for the different sesquiterpanes are also shown in Fig. 3, which aided in the confirmation of the identification of these compounds compared to previously published data (Alexander et al. 1983, 1984; Dimmler et al. 1984; Noble et al. 1987; Huang et al. 2004; Zhang et al. 2005; Wang et al. 2005b; Yang et al. 2009; Xu et al. 2010; Zhang and Zhang, 2012; Hou et al. 2013; Ji et al. 2016; Song et al. 2016; Cesar and Grice 2018), including mass spectral data, GC retention data, and the distribution pattern data of sesquiterpanes at m/z 123.

Fig. 2
figure 2

Mass chromatogram of m/z 123 and total ion chromatogram (TIC) of the saturated hydrocarbon of products (400 °C). A C14 bicyclic sesquiterpanes, B 8β(H)-drimane, C 8β(H)-homodrimane, D 8α(H)-homodrimane

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Fig. 3
figure 3

GC-MS chromatograms of bicyclic sesquiterpanes (m/z 123, 179, 193, and 207) of saturated hydrocarbon (400 °C) eluting in the n-C13 and n-C16 range

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Fig. 4
figure 4

Molecular structure of compounds A, B, C, and D. A C14 bicyclic sesquiterpanes, B 8β(H)-drimane, C 8β(H)-homodrimane, D 8α(H)-homodrimane

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The mass spectra for the C14, C15, and C16 bicyclic sesquiterpanes usually show intense M+-15 fragment ions at m/z 179, 193, and 207, respectively (Philp et al. 1981; Alexander et al. 1983,1984; Stout et al. 2005; Zhang et al. 2005).

In addition, a relatively intense ion at m/z 123 was present in the spectra. These ions were used in the present study as the initial basis for determining whether the C14, C15, and C16 bicyclic sesquiterpanes were detected. Moreover, the molecular ions of these compounds are also present in Fig. 5, i.e., m/z 194, 208, and 222, corresponding to the molecular formulas of C14H26, C15H28, and C16H30, respectively. Peaks A, B, C, and D were identified as C14 bicyclic sesquiterpanes, 8β(H)-drimane, 8β(H)-homodrimane, and 8α (H)-homodrimane, when a comparison was made between the present GC–MS data (Figs. 3 and 5) and the previously published literature data (Alexander et al. 1984; Dimmler et al. 1984; Noble 1986; Noble et al. 1987; Jiang et al. 1988; Weston et al. 1989; Luo et al. 1990; Cheng et al. 1991; Wang 1993; Xia and Luo, 1994; Stout et al. 2005; Wang et al. 2005a; Zhang et al. 2005; Lu and Zhang 2008; Nytoft et al. 2009; Yang et al. 2012; Ji et al. 2015; Zhu et al. 2015).

Fig. 5
figure 5

Mass spectra of compounds corresponding to peaks A, B, C, and D. A C14 bicyclic sesquiterpanes, B 8β(H)-drimane, C 8β(H)-homodrimane, D 8α(H)-homodrimane

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Figure 6 shows the m/z 123 mass chromatograms of the extracts from the original source rock and 9 products at different experimental temperatures. Except for the extracts from the source rock, only a few species of bicyclic sesquiterpanes can be detected in other liquid hydrocarbons in the simulated pyrolysis experiment (Fig. 6). This study will investigate the evolution characteristics and geochemical significance of 8β(H)-drimane (peak B), 8β(H)-homodrimane (peak C), and 8α(H)-homodrimane (peak D) considering their relatively high abundance and obvious change characteristics during thermal evolution.

Fig. 6
figure 6

m/z 123 mass chromatograms of the extracts from the original source rock and nine products. A C14 bicyclic sesquiterpanes, B 8β(H)-drimane, C 8β(H)-homodrimane, D 8α(H)-homodrimane

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3.2 Evolution of bicyclic sesquiterpanes with increasing temperatures

As shown in Fig. 6, there are many species of C14, C15, and C16 bicyclic sesquiterpanes present in the EOM extracted from the source rock. However, only four types of predominant bicyclic sesquiterpanes (peaks A, B, C, and D) could be detected in the pyrolysis simulation products, and a small amount of C14 bicyclic sesquiterpanes could still be detected after 360 °C. Thus, the bicyclic sesquiterpanes may be derived from more complex drimane-related precursors by a rearrangement process involving carbonium ion intermediates during diagenesis and then turned into other hydrocarbons at lower experimental temperatures by thermal degradation because of their relatively poor thermal stability, except compounds A, B, C, and D. At the high experimental temperature (≥ 360 °C) stage, the low concentration of C14 bicyclic sesquiterpanes (peak A) shown in Fig. 6 may be derived from compounds B, C, and D by some secondary reactions, such as thermal degradation, methylation shift, and demethylation. In contrast, compounds B, C, and D have relatively good thermal stability, and their material sources are mainly derived from the pyrolysis products of kerogen, and only a small part of their material sources is derived from soluble organic matter (Xia and Luo 1994).

From Figs. 6 and 7, it is obvious that the relative abundance of bicyclic sesquiterpanes B, C, and D in liquid hydrocarbons in the experiment has a regular pattern of change. The relative abundance of 8β(H)-drimane shows an upward trend at first and then decreases with increasing experimental temperature, and the relative abundance of 8α(H)-homodrimane presents a completely opposite trend. In addition, the relative abundance of 8β(H)-homodrimane first increases sharply and then remains unchanged after 360 °C.

Fig. 7
figure 7

Comparison of the relative abundance of the bicyclic sesquiterpanes at various temperatures

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3.3 Discussion of maturity indicators for bicyclic sesquiterpanes

To find suitable maturity indicators using compounds B, C, and D, ratios 8β(H)-drimane/8β(H)-homodrimane, 8β(H)-homodrimane/8α(H)-homodrimane, and 8β(H)-drimane/8α(H)-homodrimane were calculated and discussed (Fig. 8).

Fig. 8
figure 8

Maturation index of bicyclic sesquiterpanes. B 8β(H)-drimane, C 8β(H)-homodrimane, D 8α(H)-homodrimane

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Figure 8 clearly shows that all three selected indicators can reflect the evolution trend of organic matter, and all three parameters have the same change characteristics—a rapid increase at first and then a sharp decrease with increasing temperature. There is a conversion point between the two stages corresponding to a temperature of approximately 360 °C–380 °C. Among the ratios, the tendency of the ratio 8β(H)-drimane/8β(H)-homodrimane, which is widely used in geochemistry studies, is consistent with that in the study by Luo et al. (1990). Luo et al. (1990) found that the conversion point of the ratio 8β(H)-drimane/8β(H)-homodrimane corresponds to Ro = 1.25%. Thus, it can be inferred that the Ro corresponding to the transformation point of the other two parameters is close to 1.25%. In particular, the variation level of ratio 8β(H)-drimane/8β(H)-homodrimane with increasing temperature is smaller than the other two indexes in this experiment, indicating that 8β-(H)-homodrimane/8α(H)-homodrimane and 8β(H)-drimane/8α(H)-homodrimane may be more suitable than the former ratio as a maturity parameter at the immature and low-maturity stages.

3.4 Discussion of the possible formation mechanism of bicyclic sesquiterpanes

The different thermal stabilities of these compounds lead to the abovementioned evolutionary characteristic (Figs. 7 and 8). The thermal stability of a compound depends on its molecular chemical structure. The molecular structures in Fig. 4 show that compounds C[8β(H)-homodrimane] and D[8α(H)-homodrimane] have the same substituents and that the substitution positions of methyl and ethyl are exactly same. However, compound D[8α(H)-homodrimane] contains axial methyl groups at C-8, which results in unfavorable 1,3-diaxial interactions with their respective C-10 methyl groups (Fig. 4). In contrast, the C-8 methyl groups of the 8β(H)-epimers (compounds C) are oriented in the less-crowded equatorial positions, resulting in greater stability of the compounds C[8β(H)-homodrimane] (Wang et al. 2005a). Thus, compounds with the 8α(H)- configuration (compounds D) will be easily converted into stereoisomers with an 8β(H)- configuration (compounds C) by isomerization reactions via an ionic mechanism owing to the greater stability of the latter (Noble et al. 1987). In addition, it is generally believed that compound B is formed by demethylation of compound C under thermodynamic activity (Xia and Luo, 1994), which can indicate that compound B is more thermostable than compound C. In summary, the thermal stability of these three compounds can be ranked as follows: B > C > D, and this conclusion has been confirmed in the study by Xia and Luo, 1994. (Wang et al. 2005a). Therefore, during this experiment, with the increasing experimental temperature, a large amount of compound D converted to compound C by isomerization reactions and rearrangements at the beginning, and compound C simultaneously degraded into compound B by some secondary reactions, such as thermal degradation, methylation shift, and demethylation at the same time.

However, due to the low temperature (below 360 °C), the demethylation configuration conversion of compound D to compound C may be easier than the conversion of compound C to compound B. Therefore, with the relative abundance of compound D decreasing sharply, the relative abundance of compound C increases significantly, and the relative abundance of compound B increases relatively gently (Fig. 6). The change in the relative abundance of all three compounds is not obvious from 360 °C to 400 °C, which indicates that a large amount of hydrocarbon generation from kerogen makes a dynamic balance of this conversion process in this temperature range. At temperatures above 400 °C, the relative abundance of compound D has an obvious upward trend. This phenomenon may be attributed to the release of these bicyclic alkanes from the insoluble organic matrix (kerogen) (Noble et al. 1987; Zhao et al. 2018) at this stage, and the rate of hydrocarbon expulsion is higher than the rate of conversion from compound D to compound C. In addition, the relative abundance of compound C does not change significantly, indicating that the process of the formation of compounds C by isomerization reactions (D → C) and the process of conversion C → B may reach a dynamic balance. However, the relative abundance of compound B shows a certain downward trend. There may be a massive conversion of compound B into low-carbon hydrocarbons in this temperature range, and this process likely led to a phenomenon in which a small number of species of C14 bicyclic sesquiterpanes can be detected at this stage.

4 Conclusions

The results of the simulated pyrolysis experiment show that the ratios 8β(H)-drimane/8β(H)-homodrimane, 8β(H)-homodrimane/8α(H)-homodrimane, and 8β(H)-drimane/8α(H)-homodrimane show a regular upward trend at first and then decrease with increasing experimental temperature. This pattern is consistent with the thermostability of these three compounds, namely, 8β(H)-drimane > 8β(H)-homodrimane > 8α(H)-homodrimane. At the first stage before the conversion point, a large amount of compound D converted into compound C, and compound C simultaneously demethylated to form compound B at the same time as the experimental temperature increased. Thus, the three selected parameters show the same upward trend. However, after the turning point, all three parameters show a downward trend, which may be due to the following two reasons; (1) In addition to the conversion of D → C→B during the high temperature phase, compound B will simultaneously convert to hydrocarbons of lower carbon numbers. (2) The hydrocarbons in the kerogen structure will be released in large quantities once this temperature is exceeded, leading to a substantial increase in the relative content of compound D.

The ratios 8β(H)-drimane/8β(H)-homodrimane, 8β(H)-homodrimane/8α(H)-homodrimane, and 8β(H)-drimane/8α(H)-homodrimane all show a clear upward trend with increasing temperature before the temperature turning point. Therefore, all three parameters can be used as the maturity indicators of source rocks in the immature–low-maturity stage. The last two ratios may be more suitable than the first ratio as valid parameters for measuring the extent of thermal evolution of organic matter in the immature–low-maturity stage because their amplitude of change with increasing temperature is more obvious.