A Comparison of the Geochemical and Stable Carbon Isotopic Characteristics of Extracts Obtained from Source Rocks Using Different Solvents

Abstract

The choice of the solvent to use in the Soxhlet extraction process dramatically affects the extraction yield. In this work, ten hydrocarbon source rocks were extracted using different solvents, and the chemical compositions of their products were analyzed to assess the extraction efficiency and the differences between fractions. The results indicated that using a mixed dichloromethane (DCM) and methanol (MeOH) reagent instead of the traditional chloroform (TCM) reagent can improve extraction efficiency for all rock types except for coal. The improvement in extraction efficiency was attributed to the contributions of non-hydrocarbon compounds (NOSs). A comparative study of the biomarkers of the fractions extracted using different reagents showed no significant differences in geochemical parameters, such as ∑C₂₂₋/∑C₂₃₊, Pr/Ph, Pr/nC₁₇, Ph/nC₁₈, OEP₁, OEP₂, CPI, and hopane distribution. Additionally, the carbon isotopic compositions of the fractions varied by less than 1‰, indicating that the TCM and DCM: MeOH regents did not significantly affect the results of the oil–source correlation.

1. Introduction

Extraction is a sample preparation step in analytical chemistry [1]. Solvent extraction from solid samples, commonly known as solid–liquid extraction, is one of the oldest reliable sample preparation techniques. Soxhlet extraction is a reference extraction technique for some authors and the most widely used process in petroleum geochemistry [2].

Research conducted in the field of organic geochemistry frequently deals with organic matter extracted from sedimentary rocks using single or mixed organic solvents, such as chloroform (TCM) [3], methylene chloride (DCM) [4], methanol (MeOH), DCM-MeOH [5], and methanol–acetone–chloroform (MAC, 23:30:47 by weight) [6]. The data extracted include chloroform bitumen “A” content, total hydrocarbon content, and group compositions, which are critical parameters used to qualitatively and quantitatively analyze source rocks and essential for reservoir evaluation and oil–source correlation studies [7,8,9,10,11]. High-efficiency and high-precision extract yields are required for results to be properly interpreted, and rapid sample extraction is desirable whenever organic geochemical methods are used during petroleum exploration [12]. As the choice of solvent and the methodology used for the extraction process are fundamental and critical factors in geochemical studies, much research has been conducted to examine the effects of sequential extraction and solvent type on biomarker distributions [7,13,14]. Chen et al. (2006) studied the extraction of coal and coal-measures mudstone. They showed significant differences in the group composition and biomarker compound distribution between chloroform extracts of coal and oil from coal measures, which made it very difficult to confirm the oil source of oil-measures crude oil in coal-bearing sedimentary basins [13]. Guo et al. (2000) used CS₂:N-methyl-2-pyrrolidinone (NMP), chloroform, and ternary reagents (MAC) as super-polar mixed solvents to extract low-maturity source rocks, finding that the amount of CS₂:NMP extracted was two to nine times that of the chloroform and ternary solvents [15]. These previous research results all show that the polarity of the solvent has a decisive effect on the extraction yield and product distribution in the extraction of source rocks. Most researchers perform Soxhlet extraction using conventional solvents, such as TCM, at temperatures near their boiling points [16]. However, TCM has been replaced in many recent studies by different reagents (e.g., DCM:MeOH, v/v = 9:1 [6]; CS₂: NMP, v/v = 1:1) [15,16,17,18] due to the restrictions on the use of chloroform reagents. Researchers have had no choice but to gradually start using a mixture of DCM and MeOH to replace chloroform. This solvent change has affected the extracted products’ yields and characteristics. The change in extraction reagents has also introduced many uncertainties into the geochemical analyses of source rocks. Specifically, these uncertainties concern whether the extraction yield will be improved, whether the biomarker characteristics of the extract will change, and whether the stable carbon isotope of the extract will be affected. Unfortunately, few investigations have been conducted into these issues.

In this work, we report the results of experiments designed to evaluate how extraction yields, biomarkers, and stable carbon isotope signatures vary after extraction using different solvents. The saturated hydrocarbon fraction was identified using gas chromatography–mass spectrography (GC-MS), and elemental analyzer–isotope ratio mass spectrometry (EA-IRMS) was used to measure the bulk carbon isotopes of group components.

2. Materials and Methods

2.1. Chemicals and Samples

n-Hexane (analysis grade), methanol (LC grade), dichloromethane (analysis grade), and chloroform (analysis grade) were all purchased from Sigma-Aldrich (St. Louis, MO, USA). A total of ten samples, including coal and source rocks, were selected from different basins in China. Two source rocks were taken from wells He-6 (H-6) and Ji-101 (J-101) in the depression of the Turpan–Hami basin, and two samples were taken at different depths from well Fuye-1 (FY1-4 and FY1-25) in the Ordos basin. The remaining two samples were collected in the Sichuan basin from well Baoye-2 (BY2) and the outcrop (Cam-shale). Samples ZDS-9 and ZDS-16 were collected from the Qiangtang basin, and samples G11-8 and G11-18 were collected from the Qaidam basin. Before extraction, the source rock samples were washed, dried, and ground to <100 mesh. All the samples were analyzed using a Rock-Eval instrument with Rock-Eval parameters. S₁, S₂, S₃, and T_max values were determined, and then the hydrogen index (HI) and oxygen index (OI) were calculated. The Rock-Eval pyrolysis results for the samples are shown in

Table 1. Total organic carbon (TOC) and pyrolysis data for source rock samples.

Samples	TOC (%)	TS (%)	Tmax (°C)	S1 (mg/g)	S2 (mg/g)	S3 (mg/g)	HI	OI	PI	MINC (%)
ZSD-09	1.96	1.69	443	0.46	13.10	0.13	670	7	0.03	2.54
ZSD-16	1.20	1.47	443	0.08	7.08	0.24	589	20	0.01	4.53
FY1-25	1.09	0.20	457	0.86	1.68	0.19	155	18	0.34	1.61
FY1-10	7.38	1.20	446	5.20	14.16	0.30	192	4	0.27	0.82
BY-2	6.86	3.78	385	0.05	0.06	0.15	1	2	0.46	1.44
Cam-shale	3.39	2.18	444	0.38	0.53	0.29	16	9	0.42	3.12
J-101 coal	35.47	0.50	429	1.38	90.18	2.02	254	6	0.02	0.58
He-6 coal	80.57	0.44	453	10.49	141.94	3.41	176	4	0.07	1.08
G11-8	0.51	1.51	426	0.50	2.64	0.19	522	38	0.16	1.86
G11-18	1.62	1.90	430	0.96	7.78	0.35	479	22	0.11	0.93

TOC = total organic carbon; TS = total sulfur; T_max = maximum peak temperature; S₁ = free hydrocarbon content; S₂ = pyrolyzed hydrocarbon content; S₃ = pyrolyzed inorganic carbon content; HI = hydrocarbon index; OI = oxygen index; PI = production index; MINC = degree of mineral carbonization.

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2.2. Soxhlet Extraction (SE) Procedure

The Soxhlet extraction (SE) method used here was adopted from Parera et al. [19] and further optimized. A thimble containing 50 g of source rock sample was placed in a Soxhlet chamber fitted with a condenser and connected to a round-bottom flask containing 100 mL of mixed dichloromethane: methanol (9:1 v/v) solvent. The solvent flask was refluxed for 72 h at 40 °C. After extraction, the extract was concentrated at low pressure until it was dry.

2.3. Fractionation of the Extracts

The asphaltene fraction produced using different extraction methods was precipitated with n-hexane. The maltene fraction (~20–50 mg) in ~0.3 mL of solvent was absorbed into 0.2 g of silica gel, after which the solvent was removed. The absorbed sample was then added to the top of a glass column 40 cm in height and 1 cm in diameter. It was packed with n-hexane slurry consisting of 3 g of activated silica gel in the lower part of the column and 2 g of activated neutral alumina in the upper part. This mixture was sequentially eluted with the following solvents: (1) 100 mL of n-hexane to elute the saturate fraction, (2) 50 mL of CH₂Cl₂ to elute the aromatic fraction, and (3) 20 mL of methanol to elute the non-hydrocarbon fraction.

2.4. Gas Chromatography-Mass Spectrometry (GC-MS)

The saturated hydrocarbons extracted from source rocks were analyzed with gas chromatography–mass spectrometry (GC-MS) to determine the samples’ molecular compositions. All GC analyses were carried out using an Agilent 5890A gas chromatograph (Agilent Technologies Inc., California, PA, USA) equipped with a fused silica column (HP-5MS, 30 m × 0.32 mm × 0.25 μm) and coupled with a mass spectrometer (MSD, Agilent 5975C) for saturated hydrocarbon analysis. The carrier gas was helium, with a 1.0 mL/min flow rate. Injections were performed in splitless mode at 280 °C, and the injection volume was 1.0 μL. The GC oven was initially set to a temperature of 80 °C for 3 min, then heated to 280 °C at a rate of 2 °C/min, and then held at that temperature for 30 min.

2.5. Stable Carbon Isotope Analysis

The stable carbon isotope compositions of source rock extracts and fractions were measured using an elemental analyzer (Flash 2000, Thermo-Fisher, Bremen, Germany) coupled with an isotope ratio mass spectrometer (MAT253, Thermo-Fisher, Bremen, Germany) via a combustion interface (GC Combustion III). Helium was used as the carrier gas with a flow rate of 2 mL/min. The isotopic composition is reported here using the δ notation relative to Vienna Pee Dee Belemnite (V-PDB) for carbon isotope ratio traceability. The δ¹³C values were pre-calibrated against an international standard material (International Atomic Energy Agency, caffeine, IAEA-600, C₈H₁₀N₄O₂, −27.771 ± 0.043‰, V-PDB). In this study, all samples’ stable isotopic compositions were better than ±0.2‰ [20].

3. Results and Discussion

3.1. Extraction Yield and Bulk Composition Comparison

To examine variations in the extracts obtained using different solvents, we fractionated each extract into saturated hydrocarbon, aromatic hydrocarbon, hetero component (NOSs), and asphaltene fractions after quantifying the products. The results for the extraction yield and extract fractionation are shown in Figure 1 and Figure 2.

Figure 1 shows that the extraction yields for each sample when using different solvents were significantly different. The TCM reagent resulted in more products from coal samples, whereas the mixed solvent obtained higher yields from the source rock. Our data show that the properties of a solvent have a significant influence on the yield of the extract; for example, the TCM and mixed DCM: MeOH reagents produced extraction yields of 1.06 and 2.37, respectively, and the mixed DCM:MeOH reagent obtained higher extraction yields for other samples.

After extraction was performed on the source rocks using both reagents, we observed that the group compositional characteristics of the extracts were significantly different in each case. Figure 2 shows that more saturated hydrocarbon fractions (Sat.) were obtained when TCM was used as the extraction reagent and the amounts of heteratomic compounds (NOSs) and asphaltenes (Asph.) were reduced accordingly. There were no significant differences in the extracted aromatic hydrocarbon content with different solvents. We interpreted the differences in extracted group components as being related to differences in each solvent’s polarity. The results of this study are consistent with previous reports [19,21].

3.2. Comparison of Biomarkers

Much geochemical research involves analyzing the origins of various oil sources, which is achieved by correlating the main-peak carbon number, peak type, odd–even dominance, ∑C₂₂₋/∑C₂₃₊, and other parameters. GC–MS was used to analyze all saturated fractions in the individual samples and reveal the distribution characteristics of the biomarker compounds. Due to differences in the maturity of the source rocks, m/z 217 ions were not detected in the saturated hydrocarbon fractions of the extracts from some of the samples. Therefore, only representative samples were selected for discussion in this study.

Figure 3 shows that the peak type and compound distribution characteristics of the m/z 85 and m/z 191 mass fragments were very similar. When TCM was used as the extraction solvent for sample J101, the carbon number range was C₁₃–C₃₉ and the main peak was nC₂₁ (

Table 2. Biomarker parameters for saturated hydrocarbons from TCM and DCM: MeOH extraction.

Sample	Solvent	Carbon Number Range	Main Peak	∑C22−/ ∑C23+	Pr/Ph	Pr/nC17	Ph/nC18	OEP1	OEP2	CPI
ZSD-09	A 1	C13–C39	nC21	0.95	1.21	0.73	0.62	1.12	1.15	1.16
	B	C14–C35	nC20	1.18	1.25	0.79	0.65	1.14	1.21	1.16
ZSD-16	A	C12–C39	nC25	0.83	2.67	0.55	0.22	1.09	1.27	1.24
	B	C13–C36	nC23	0.86	2.46	0.55	0.22	1.08	1.33	1.29
FY1-25	A	C13–C32	nC16	2.57	1.51	0.15	0.10	1.00	1.03	1.07
	B	C12–C35	nC16	3.91	1.65	0.16	0.11	0.99	1.05	1.10
FY1-10	A	C12–C32	nC16	2.86	1.44	0.16	0.11	0.99	1.01	1.07
	B	C12–C33	nC16	3.04	1.47	0.17	0.12	1.00	1.06	1.08
BY-2	A	C12–C21	nC14	n.d.	1.27	0.45	0.74	0.90	n.d.	n.d.
	B	C14–C21	nC16	n.d.	1.02	0.33	0.74	1.00	n.d.	n.d.
Cam-shale	A	C12–C34	nC27	3.96	1.08	0.12	0.12	1.01	1.03	1.06
	B	C15–C29	nC27	3.82	1.05	0.44	0.26	0.69	1.86	1.94
J-101 coal	A	C12–C33	nC23	1.32	7.69	2.13	0.25	1.00	1.48	1.69
	B	C13–C33	nC23	1.11	6.86	2.20	0.28	0.97	1.52	1.66
He-6 coal	A	C12–C30	nC16	10.27	2.34	0.04	0.02	1.01	1.22	1.42
	B	C13–C29	nC17	7.12	2.45	0.04	0.02	0.97	1.09	1.37
G11-8	A	C12–C36	nC17	2.30	0.45	0.31	0.70	1.03	0.94	1.00
	B	C13–C33	nC17	2.35	0.57	0.41	0.78	1.04	1.01	1.05
G11-18	A	C13–C37	nC27	0.19	0.29	1.12	2.98	0.98	1.08	1.09
	B	C13–C34	nC25	0.43	0.31	1.09	2.73	0.98	1.05	1.12

¹ A, TCM extraction; B, DCM: MeOH (9:1, v:v) extraction. ${\mathrm{OEP}}_1=\frac{({\mathrm{C}}_{15}+6{\mathrm{C}}_{17}+{\mathrm{C}}_{19})}{4({\mathrm{C}}_{16}+{\mathrm{C}}_{18})}$, ${\mathrm{OEP}}_2=\frac{({\mathrm{C}}_{25}+6{\mathrm{C}}_{27}+{\mathrm{C}}_{29})}{4({\mathrm{C}}_{26}+{\mathrm{C}}_{28})}$, $\mathrm{CPI}=\frac{1}{2}\left(\frac{{\mathrm{C}}_{25}+{\mathrm{C}}_{27}+{\mathrm{C}}_{29}+{\mathrm{C}}_{31}+{\mathrm{C}}_{33}}{{\mathrm{C}}_{26}+{\mathrm{C}}_{28}+{\mathrm{C}}_{30}+{\mathrm{C}}_{32}+{\mathrm{C}}_{34}}+\frac{{\mathrm{C}}_{25}+{\mathrm{C}}_{27}+{\mathrm{C}}_{29}+{\mathrm{C}}_{31}+{\mathrm{C}}_{33}}{{\mathrm{C}}_{24}+{\mathrm{C}}_{26}+{\mathrm{C}}_{28}+{\mathrm{C}}_{30}+{\mathrm{C}}_{32}}\right)$.

); however, when the extraction solvent used for sample J101 was DCM: MeOH, the carbon number range was C₁₄–C₃₅ and the main peak was nC₂₀.

Pristane (Pr) and phytane (Ph) are the essential acyclic isoprenoids used for evaluating redox conditions during source rock deposition. The Pr/Ph ratios can be used to identify specific paleoenvironmental conditions. Other parameters, such as OEP, CPI, Pr/nC₁₇, and Ph/nC₁₈, can also reflect specific geochemical characteristics. In this study, the responses in these parameters were used to determine the effects of extraction reagents on the extracted products of the source rocks [22,23]. The saturates obtained from extractions with different solvents showed negligible differences for all parameters, including ∑C₂₂₋/∑C₂₃₊, Pr/Ph, Pr/nC₁₇, Ph/nC₁₈, OEP₁, OEP₂, CPI, and hopane distribution. Table 2 shows that only ∑C₂₂₋/∑C₂₃₊ exhibited a slight variation after the same sample was extracted with a different solvent, whereas other biomarker parameters showed no significant changes. For example, pretreatment of sample FY1-25 with the TCM reagent produced values of 1.51, 0.15, 0.10, 1.00, 1.03, and 1.07 for the parameters Pr/Ph, Pr/nC₁₇, Ph/nC₁₈, OEP₁, OEP₂, and CPI, respectively; however, when the extraction reagent was DCM: MeOH, these parameter values were 1.65, 0.16, 0.11, 0.99, 1.05, and 1.10, respectively. These differences were likely related to either the loss of low-mass components during the separation process or differences in the injection volume during the GC–MS analysis.

These experimental results indicate that replacing TCM with DCM: MeOH affects the relative proportions of group components extracted from a source rock but does not affect the distribution characteristics of biomarkers. However, further evaluation of their effects should be undertaken when using solvents with large polarity differences.

3.3. Stable Carbon Isotope of the Extracts and Kerogen

Correlating oil reservoirs with their source rocks is an important objective in hydrocarbon exploration. Isotopic methods that compare the ¹³C/¹²C ratios of kerogen, extract, and crude oil can help to identify such relationships [10].

Here, we present a comparative study of the stable carbon isotopes of fractions obtained using different solvents. Isotopic curves were plotted using measured carbon isotope data for all group components (Sat., Arom., NOS, and Asph.), after which the values for kerogen were measured and added to these plots, and linear fits were produced for all data points (Figure 4).

As shown in Figure 4, except for samples ZDS-9, G11-8, and G11-18, pretreatment with different solvents led to the saturated hydrocarbons being most depleted in δ¹³C and the heavier carbon isotope being most enriched in asphaltenes (Figure 4). The variation trend for the carbon isotopes of group components was consistent with the results of previous studies. However, due to slight changes in the carbon isotopic composition between fractions and the polarity of the two extraction reagents being similar, there were no apparent differences in the carbon isotope values of the group components in some of the samples, which weakly correlated with kerogen. The predicted carbon isotope values of kerogen were in agreement with the measured values from the carbon type-curves of other samples. These results indicate that replacing a chloroform reagent with a mixed dichloromethane and methanol reagent does not significantly affect group components’ carbon isotope test results during pretreatment of source rocks.

Therefore, whether using DCM:MeOH or TCM reagent to extract source rocks, the conclusions reached in the oil–source correlation study using the stable carbon isotopic type-curves method were consistent. The carbon isotopic correlation between fractions and kerogen was not affected.

4. Conclusions

Ten source rocks were pretreated with both DCM:MeOH and TCM reagents in separate experiments, which allowed differences in the geochemical and stable carbon isotopic characteristics of the extracts and fractions to be investigated.

Pretreatment regimes that used different reagents produced significantly different extracts in each case; in particular, the TCM reagent is more suitable for extracting coal samples, while mixed solvents are more suitable for extracting source rock samples. The experimental results also showed that the TCM reagent made it possible to extract more asphaltene, while the DCM: MeOH mixed reagent made it possible to extract more NOSs. A comparative study of the characteristics of the biomarkers and bulk carbon isotope values for the fractions produced by each reagent showed that there were no significant effects on their geochemical parameters, including ΣC₂₂₋/ΣC₂₃₊, Pr/Ph, Pr/nC₁₇, Ph/nC₁₈, OEP₁, OEP₂, CPI, and hopane distribution. Additionally, the difference in the carbon isotopic composition of each component was less than 1‰, indicating that TCM and DCM: MeOH do not significantly affect the results of oil–source correlation.