The influence of extraction of various solvents on chemical properties on Chang 7 shale, Ordos Basin, China

To explore the effect of various solvents extraction on the chemical property of shale, several lacustrine shale samples from the Chang 7 member of the Upper Triassic Yanchang Formation, Ordos Basin, with maturities, from marginally mature (T_max=439^∘) to late mature (T_max=456^∘), were extracted by using acetone, tetrahydrofuran (THF), carbon disulfide (CS₂), and benzene, respectively. Fourier transform infrared spectroscopy (FTIR) was employed to examine the functional groups of the samples before and after extraction with different solvents. The results showed that the extraction yield from shale with THF is significantly higher than that of other solvents, which may be related to the properties of the THF, including the aromatic structure, high boiling point, excellent Hansen solubility parameters, and strong polarity. The total organic carbon (TOC)-normalized yield of the mature sample J1 is significantly higher than that of other samples, which may be related to the fact that the J1 sample is at the peak of hydrocarbon generation; thus, a large number of kerogens were cracked into oil and bitumen. The aromaticity of organic matter (OM) increases with the increase in the maturity. The length of the aliphatic chains of the OM first increased with the increase in the maturity before late maturity stage and then decreased in late maturity stage. The extraction of shale samples with solvents hardly changes the length of aliphatic chains. Higher aromaticity is observed in shale residues after THF extractions than for other solvents (i.e., acetone, CS₂, and benzene).

1 Introduction

Due to the increasingly serious energy shortage problems associated with the exhaustion of fuel energy resources, potential unconventional fossil sources are highly desired (Yang et al., 2019; Li et al., 2022). The quest to produce unconventional petroleum-based fuels and achieve energy independence is being aggressively pursued in China and the United States (Overland, 2015). In the past few years, shale oil has been regarded as an important resource that can be exploited commercially due to the technical innovations of three-dimensional development in horizontal wells and fracture-controlled fracking (Li et al., 2020; S. Zhou et al., 2020; Lei et al., 2021; H. Yang et al., 2021). For China, the government has been encouraging shale oil developments to address the shortage of domestic oil resources and the rising energy demand and to safeguard the country's long-term energy security (Yang et al., 2019; Hu et al., 2020; L. Zhou et al., 2020; Lu et al., 2022).

Figure 1 Locations of sampling wells and simplified structural framework across the Ordos Basin (modified from Cao et al., 2019).

Extraction has become a common shale pretreatment method in the laboratory to study the pore structures related to organic matter (OM; Qi et al., 2019; Wang et al., 2020; S. Yang et al., 2021). However, different solvents could affect shale extraction efficiency and methane adsorption capacity (Cao et al., 2019, 2020). For the shale samples, the extraction by using acetone, CS₂, and benzene can be used to reveal the micropores, mesopores, and macropores, respectively (Cao et al., 2019). In addition, during the development of shale oil, the chemical properties of shale after being extracted by using different organic solvents are also different (Cao et al., 2020). Wei et al. (2014) found that the hydrocarbons in toluene extract exhibit remarkable aromaticity. Furmann et al. (2013) showed that the organic aromaticity of extractions from dichloromethane is greater than that from methanol.

A large number of studies focus on either the changes in shale pore structure before and after extraction (Gorynski et al., 2019; Qi et al., 2019; H. and S. Yang et al., 2021) or the functional groups of OM extracted from underground fossil energy (Chen et al., 2012; Mastalerz et al., 2012; Furmann et al., 2013; Wei et al., 2014). In fact, underground shale is often washed out of OM by organic solvent extraction in the laboratory to explore the relationship between OM and an inorganic mineral skeleton in shale. It is worth mentioning that this study is a continuation of our earlier published study, which details the findings about influence of extraction on shale pore structure with different maturity using various solvents (Cao et al., 2019, 2020). However, the study of the changes in the chemical properties in the lacustrine organic-rich shale of different maturity before and after extraction with different solvents is rarely reported. Our results of the study will help in the selection of organic solvents for oil-washing experiments in shale samples of different maturity.

Fourier transform infrared spectroscopy (FTIR) is an advanced method for the determination of chemical structures (e.g., functional groups) of OM (Wei et al., 2014). In this study, several lacustrine shale samples with different maturities from Chang 7 member of the Upper Triassic Yanchang Formation, Ordos Basin, central China, were collected. The present study is a continuation of our previous research (Cao et al., 2019, 2020); Soxhlet extractions with four different solvents (tetrahydrofuran, carbon disulfide, benzene, and acetone) were performed on the selected shale samples. Interestingly, the Fourier transform infrared spectroscopy (FTIR) was performed on the samples before and after the extraction. After that, the influence of various solvent extraction on chemical properties in lacustrine shale of different maturity was summarized.

Figure 2 The strata and sedimentary characteristics of the Yanchang Formation of the Upper Triassic (modified from Cao et al., 2020).

2 Geological settings and samples

According to the basement and faults, the Ordos Basin is divided into six secondary structural units, namely the Weibei uplift, the Yishan slope, the Tianhuan depression, the Yimeng uplift, the western margin thrust belt, and Jinxi flexural fold belt (Fig. 1; Guo et al., 2014). The Yishan slope is inclined to the west, with a slope of about 1^∘ (Guo et al., 2014). The Chang 7 member was developed during the largest lacustrine flooding in the lake basin (Fig. 1; Guo et al., 2014). Yellowish tuff intervals are interbedded within the Yanchang sediments ranging from 0.2 to 0.45 cm (0.08–0.18 in.; Qiu et al., 2014). The Yanchang Formation of the Upper Triassic can be divided into 10 sub-segments (i.e., Chang 1–10; Fig. 2). The Chang 7 member is mainly made up of silty mudstone and shale samples and has great potential for shale oil or gas (Fig. 2; Duan et al., 2008; Lei et al., 2015). The Chang 7 member shale develops a large area of high-quality source rocks (about 5×10⁴ km²), which is rich in oil-prone OM derived from acritarchs (Leiosphaeridia species) and some Botryococcus (Ji et al., 2010). The cumulative thickness is mostly between 10 and 50 m, and the maximum thickness can reach more than 80 m (Guo et al., 2014). In this paper, several shale samples with different maturities in Chang 7 member of the Upper Triassic Yanchang Formation were obtained from the wells of Huan 317, Jian 1, Luohe 10, and Jinghe 2, respectively (Fig. 1). The samples selected for this study have been analyzed in our previous publications (Cao et al., 2020).

Table 1 Fourier transform infrared (FTIR) bands used in this study (Chen et al., 2012).

Ar is for aromatic; Al is for aliphatic; ν is for stretching vibration; δ is for deformation vibration in plane; γ is for deformation vibration out of plane; as is for asymmetric; and s is for symmetric.

Table 2 Geochemical features of the original shale samples and the corresponding residues.

3 Experiments

3.1 Geochemical analysis

The samples were crushed to 180–200 mesh prior to geochemical analysis. The total organic carbon (TOC) content was tested by a LECO CS-344 carbon and sulfur analyzer. The type, maturity, and hydrocarbon generation potential of OM were determined. The temperature at which the most S₂ is generated is regarded as the maximum pyrolysis yield temperature (T_max; ^∘C), and the hydrogen index (HI; mg HC/g TOC) was obtained from the ratio of S₂ to TOC (Peters, 1986).

3.2 X-ray diffraction

The samples were tested by a D/Max 2500 diffractometer, and the tests follow two separate processes of the CPSC procedure (CPSC, 2010). The actual mineral content of the samples was obtained by quantitative X-ray diffraction (XRD) analysis of samples smaller than 200 mesh (<0.075 mm).

3.3 Soxhlet extraction

The bulk shale samples were uniformly crushed into 60 mesh with a laboratory knife mill and then evenly divided into five parts. The sample was extracted using solvents of different aromaticity, polarity, and permeability, namely acetone, tetrahydrofuran, carbon disulfide, and benzene; the remaining original sample was used as a control. About 1000 g solvent and 50 g shale sample were sealed in a Soxhlet extractor to prevent solvent loss. The extraction time lasted 150 h until the solvent became colorless. Samples were named according to their corresponding abbreviated well names (JH2, J1, LH10, and H317; Table 2), and the original shale samples were marked by the shorthand subscript, following the sample name (JH2_orig, J1_orig, LH10_orig, and H317_orig). The residue (e.g., JH2) from tetrahydrofuran (THF), acetone, CS₂, and benzene extraction was clearly characterized by JH2_T, JH2_A, JH2_C, and JH2_B, respectively.

Table 3 Mineral compositions of the studied samples.

Unitless indicates that the content of individual mineral was not determined.

3.4 Potassium bromide (KBr) and FTIR analysis

Both powdered original shale samples and their corresponding extracted shale residues (∼2 mg) were blended with potassium bromide (KBr; ∼200 mg) and pressed into pellets. Through the use of a Nicolet 6700 FT-IR spectrometer collecting 200 scans per sample at a resolution of 4 cm⁻¹, the vibration frequencies of specific atomic bonds were analyzed. Thus, the chemical functional groups could be assessed using a standard semi-quantitative technique. For the KBr–FTIR characterization, each spectrum was recorded in the spectral range of 400–3250 cm⁻¹. The bands of absorption were identified by comparing the spectrums with published studies (Drobniak and Mastalerz, 2006; Furmann et al., 2013).

According to a manual mode, the band area was determined using Origin software. These integrated band areas were used to calculate ratios for the (1) aromaticity, including AR1 = [(δArC = C$/\mathit{\nu }$AlC-H) + δArC = C], AR2 = (νArC-H$/\mathit{\nu }$AlC-H), and AR3 = (γArC-H$/\mathit{\nu }$AlC-H); (2) length of aliphatic chains, namely LAC = (CH${}_{\mathrm{2}}/$CH₃) in νAlC-H; and (3) degree of substitution (DOS) of aromatic sites by alkyl groups, namely DOS = (γArC-H(1 adjacent H)$/\mathit{\gamma }$ArC-H (4 adjacent H); Wei et al., 2014). The detailed definitions of the parameters can be found in Tables 1 and 4.

The representative spectrums of the original shale samples and corresponding residues are presented in Figs. 5 and 6, respectively. In terms of the length of aliphatic chains (LAC), longer n-alkyl chains are determined by CH₂ bands, whereas shorter n-alkyl and branched aliphatic chains are indicated by CH₃ bands (Table 4). The DOS of aromatic rings represents the fraction of aromatic sites where hydrogen atoms are substituted by alkyl groups (Table 4).

4 Results

4.1 Geochemical characteristics

The geochemical features of shale samples are listed in Table 2. Four selected samples span from marginally mature (JH2; 439 ^∘C) to late mature (H317; 460 ^∘C), based on the Chinese continental hydrocarbon source rock geochemical evaluation method (SY/T 5735-1995, 1995). The kerogen type of selected samples is type I or II₁ (Fig. 3; Tong et al., 2011). TOC content is generally high, ranging from 3.79 wt % to 22.19 wt % (Table 2). The hydrocarbon generation potential (S₁+S₂) ranges from 8.11 to 127.42 mg g⁻¹. Table 2 shows that the hydrogen index (HI) of the samples is in the range of 155–528 mg g⁻¹.

Figure 3 Plot of HI versus T_max for the original shale samples (based on Espitalie et al., 1985).

4.2 Mineral compositions

The samples contain a large number of clay minerals (42.8 wt %–54.2 wt %, with an average of 48.15 %). An illite–smectite mixed layer or illite are the most dominant clay types (Table 3). In general, the content of the felsic component is between 19.5 wt % and 52.2 wt %, with the average of 40.63 %. In addition, the pyrite content of the studied samples varies greatly, ranging from 0.9 wt % to 25.5 wt % (Table 3).

Figure 4 Extraction data of the studied samples with solvents of acetone, THF, benzene, and CS₂. (a) Original extraction yield and (b) TOC-normalized yield of extracts.

4.3 Extraction yields

The extraction data of the studied samples with different solvents are shown in Fig. 4. Among all the solvents, THF has the highest extraction efficiency (2.44 wt %–4.43 wt %; Fig. 4a). According to the TOC-normalized yield of extracts, the mature samples J1 (T_max+449^∘) had significantly higher extractable samples (13.48 wt %–56.91 wt %) than other samples (≤5.28 wt %; Fig. 4b).

4.4 FTIR spectroscopy

The representative spectrums of the original shale samples and corresponding residues are presented in Figs. 5 and 6, respectively. The FTIR results after extraction are summarized in Table 4. The OM in shale is a three-dimensional macromolecule composed of aromatic clusters and aliphatic chains (Tong et al., 2011; Guan et al., 2015; Wang et al., 2016, 2017; Zhao et al., 2018). The aromaticity (i.e., AR2 and AR3) of the samples follows the order H317 (3.327) > LH10 (3.225) > J1 (3.038) > JH2 (2.828; Table 4), which is consistent with maturity. In addition, shale residues from THF extraction possess higher aromaticity than those from other solvents. Unlike aromaticity, the length of aliphatic chains (LAC) does not increase monotonically with the increase in the maturity (Table 4). However, there is no regularity in the DOS (i.e., the fraction of aromatic sites where hydrogen atoms are substituted by alkyl groups) of different maturity shale samples, and the DOS of shale before and after extraction with different solvents demonstrated an irregular change (Table 4).

Figure 5 FTIR spectra of original (a) marginally mature sample JH2_orig (T_max=439 ^∘C), (b) the mature sample J1_orig (T_max=449 ^∘C), (c) late mature sample LH10_orig (T_max=454 ^∘C), and (d) late mature sample H317_orig (T_max=456 ^∘C).

Figure 6 A comparison of FTIR spectra of original samples and residues.

Table 4 FTIR results from original shale and extracted shale residues.

^a The integrated peak areas were used to calculate FTIR ratios. ^b AR1 = 0 for residues because aromatic bands (δArC = C, νArC-H) were not present in the FTIR spectrum.

5 Discussion

5.1 The influence of solvents on extraction yield and composition of the soluble mixture

Previous studies have found that the extraction yields of shale and the composition of the soluble mixture are related to the properties of the solvent, extraction temperature, the geochemical parameters of the shale, and so on (Hu et al., 1999; Olukcu et al., 1999; Shaohui et al., 2000; Abourriche et al., 2013; Cao et al., 2019). As the polarity of the solvent increases, the extraction yield increases accordingly (Masaki et al., 2004). The solvent dissolution capacity has obvious temperature dependence, and the dissolution capacity increases with the increase in the temperature (Hu et al., 1999). The increase in extraction pressure and temperature will promote the activation energy of the sample (Allawzi et al., 2011).

The Hansen solubility parameters of the four solvents are shown in Table 5; the extraction ability of CS₂ is similar to that of benzene (Table 5). Because of the aromatic structure, high boiling point, and strong polarity, THF could have a stronger extraction ability than other solvents (Table 5).

Table 5 The properties of the four solvents (Cao et al., 2020).

Allawzi et al. (2011) found that the OM obtained by solvent extraction is mainly composed of low molecular weight compounds (e.g., saturated hydrocarbons, olefin hydrogen, and aromatic compounds). Our results show that the extraction yield of shale with THF is significantly higher than that of other solvents (Fig. 4a), which could be attributed to the fact that a mass of the heaviest macromolecular compound of asphaltene was screened out (Cao et al., 2020). The significant elimination of OM (i.e., asphaltene) by THF may be related to the properties of the THF, including the aromatic structure (Johnson et al., 1967), high boiling point (Cao et al., 2019), excellent Hansen solubility parameters, and strong polarity (Toolan et al., 2016; Table 5), which could assist to destroy the interact between shale matrix and the asphaltenes (i.e., a complex molecular architecture with a central polycyclic aromatic hydrocarbon (PAH), with peripheral alkane substituents; Schuler et al., 2017) of PAH molecular polymerization (Schuler et al., 2015).

The extraction yields of the studied samples are of the order of LH10, JH2, H317, and J1, regardless of solvents (Fig. 4a), which may be due to the soluble hydrocarbon content (S₁) of the studied sample (Table 2). According to the TOC-normalized yield of extracts, the mature samples J1 (T_max=449^∘) had significantly higher extractable samples than other samples (Fig. 4b), which may be related to the fact that the J1 sample (T_max=449^∘) was at the peak of hydrocarbon generation. Thus, a large number of kerogens were cracked into oil and bitumen (Ertas et al., 2006; Wei et al., 2014). However, a little kerogen was converted to a small amount of oil in the early maturity stage (Cao et al., 2020), so the TOC-normalized extraction yield of the JH2 sample is low. However, the TOC-normalized extraction yield of late mature shale H317 is similar to that of early mature shale JH2, which could be attributed to the fact that some of the oil is cracked to natural gas or transported to other locations with maturation (Cao et al., 2020).

5.2 Effect of solvent extraction on the chemical structure of shale samples

The application of FTIR spectroscopy provides an evaluation of the aromaticity and length of aliphatic chains for original shale samples and residues from extractions. Table 4 showed that the aromaticity (AR2 and AR3) of the original shale OM with different maturity is obviously different and ranges from 2.828 to 3.327. Figure 7 shows there is a positive correlation between aromaticity and maturity in shale (Feng et al., 2013). Thus, the abovementioned facts indicate that the thermal maturation can increase the aromaticity of shale OM.

Figure 7 Relationships between the aromaticity and the maturity in shale OM.

Table 4 showed that the length of aliphatic chains (LAC) from late mature shale (i.e., LH10 and H317) is higher than those of mature shale J1 and marginally mature shale JH2. In addition, the LAC of the early mature shale J1 is also higher than the marginally mature shale JH2 (Table 4). However, the LAC of the higher-maturity H317 shale (T_max=456^∘) is lower than that of mature LH10 shale (T_max=454^∘). Those values indicate that the aliphatic chains lengthened with the increase in the maturity before the late maturity stage (Feng et al., 2013) and then shortened as the maturity further increases. That may be attributed to the fact that kerogen cracking to form a large amount of bitumen at the low-maturity stage leads to longer aliphatic chains with increasing maturity, while at the high-maturity stage, bitumen is largely consumed, and the OM represented by bitumen is further cracked, thus shortening the aliphatic chains with increasing maturity (Feng et al., 2013). However, previous studies have also observed that aliphatic chains lengthened become monotonically shorter and/or more branched with maturity, which indicates that longer aliphatic chains can be preferentially cleaved. The cleavage of the C–C bonds between carbons α and β to a ring could explain this result (Lin and Ritz, 1993). Moreover, the large differences in the sources of OM in different regions may be the cause of the above contradictions.

The DOS of aromatic rings represents the fraction of aromatic sites, where hydrogen atoms are substituted by alkyl groups, and there is no regularity in the DOS (i.e., the fraction of aromatic sites where hydrogen atoms are substituted by alkyl groups) of different maturity shale samples (Table 4). Therefore, the fraction of aromatic sites where hydrogen atoms are substituted by alkyl groups in shale is independent of maturity.

In addition, the FTIR-derived LAC values smaller than 1 show the superiority of CH₃ bands (i.e., asymmetric stretching vibration) in shale samples corresponding to short and branched aliphatic chains (Furmann et al., 2013). Our results demonstrate that aliphatic chains are shorter with higher branching in the original shale samples, which could not be changed during the extractions (Table 4). In addition, the FTIR of shale samples after different solvent extractions is irregular, which may be influenced by the inorganic minerals. Therefore, it seems much more reasonable to conclude that FTIR was unable to detect any significant change in the LAC values due to solvent extraction. Furthermore, the aromaticity (both AR2 and AR3) of residues after THF extractions is significantly higher than those after the extraction of other solvents (Table 4), which may be related to the fact that THF only dissolves relatively fewer aromatic hydrocarbons from shale samples (Cao et al., 2020), and the residual abundant aromatic hydrocarbons in shale benefits OM aromaticity (Monger, 1984).

6 Conclusions

Soxhlet extractions with THF, benzene, CS₂, and acetone, respectively, have been applied to the shale samples with a progressive maturation gradient in order to gain insights into the proportion of soluble hydrocarbon and the chemical characteristics of shale with different maturity. The following conclusions have been obtained.

1. The extraction yield of shale with THF is significantly higher than that of other solvents, which may be related to the properties of the THF, including the aromatic structure, high boiling point, excellent Hansen solubility parameters, and strong polarity. The TOC-normalized yield of the mature sample J1 is significantly higher than that of other samples, which may be related to that the J1 sample is at the peak of hydrocarbon generation; thus, a large number of kerogens were cracked into oil and bitumen.

2. Thermal maturation of shale samples can increase the aromaticity of OM. The aliphatic chains in the shale OM lengthened with the increase in the maturity before the late maturity stage, and then shortened with the further increase in the maturity in the late maturity stage.

3. The extraction of shale samples with solvents hardly changes the length of aliphatic chains. Higher aromaticity is observed in shale residues after THF extractions than other solvents (i.e., acetone, CS₂, and benzene).