Constrain on Oil Recovery Stage during Oil Shale Subcritical Water Extraction Process Based on Carbon Isotope Fractionation Character

Abstract

In this work, Huadian oil shale was extracted by subcritical water at 365 °C with a time series (2–100 h) to better investigate the carbon isotope fractionation characteristics and how to use its fractionation characteristics to constrain the oil recovery stage during oil shale in situ exploitation. The results revealed that the maximum generation of oil is 70–100 h, and the secondary cracking is limited. The carbon isotopes of the hydrocarbon gases show a normal sequence, with no “rollover” and “reversals” phenomena, and the existence of alkene gases and the CH₄-CO₂-CO diagram implied that neither chemical nor carbon isotopes achieve equilibrium in the C-H-O system. The carbon isotope (C₁–C₃) fractionation before oil generation is mainly related to kinetics of organic matter decomposition, and the thermodynamic equilibrium process is limited; when entering the oil generation area, the effect of the carbon isotope thermodynamic equilibrium process (CH₄ + 2H₂O ⇄ CO₂ + 4H₂) becomes more important than kinetics, and when it exceeds the maximum oil generation stage, the carbon isotope kinetics process becomes more important again. The δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ is the result of the competition between kinetics and thermodynamic fractionation during the oil shale pyrolysis process. After oil begins to generate, δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ goes from increasing to decreasing (first “turning”); in contrast, when exceeding the maximum oil generation area, it goes from decreasing to increasing (second “turning”). Thus, the second “turning” point can be used to indicate the maximum oil generation area, and it also can be used to help determine when to stop the heating process during oil shale exploitation and lower the production costs.

1. Introduction

Oil shale has become an important backup energy of conventional fossil energy in the world, and its pyrolysis exploitation mode has changed from aboveground (ex situ) to underground (in situ) [1,2,3,4,5,6,7,8], as the former technology encounters significant challenges, such as environmental pollution and inefficiencies [2]. The in situ technology underground could be further divided into [3] (a) in situ artificial crushing and retorting and (b) real underground in situ retorting. Subcritical water extraction of oil shale is one of the in situ retorting technologies, and the oil generation process can be optimized through altering the heating time, type, and amount of catalyst and maximum temperature [4,5,6,7,8,9]. The quality of oil shale pyrolysis products mainly relates to the heating temperature and time [4], overheating or insufficient heating temperature will lead to a high mining cost with a low resource utilization rate; thus, precise heat transfer and adequate heating conditions are important to maximize oil yield [10]. In this sense, the early detection of the oil recovery stage during oil shale pyrolysis can lead to a quick corrective heating process being implemented, lowering the production costs. However, there are three main traditional methods for forecasting the oil shale pyrolysis process (organic matter decomposition): (1) direct measurement of vitrinite reflectance (%R_o) [11], hydrocarbon generation (T_max), and element ratios (H:C) of rock samples [12]; (2) indirect measurement through analyzing the structure and composition of rocks by wave-substance interaction using terahertz time-domain spectroscopy [13], Raman spectroscopy [14], and infrared spectroscopy et al. [15], or through analyzing the product and its isotope compositions according to empirical formula, using a mass spectrometer and isotope analyzer [16]; (3) numerical modeling method, which establishes kinetics data acquired from pyrolysis experiment (e.g., thermogravimetric analysis) or actual mining data [17,18]. These methods are either time consuming and expensive for the routine control of complicated sample preparations and analytical techniques, or it depends on an amount of experimental data that is hard to achieve in the early mining stage. Hence, a simple and quick method for indication of the oil shale pyrolysis process is still needed.

Carbon isotope geochemistry is one of the major tools for deciphering geologic processes that are both organic and inorganic [19]. It has been widely used to investigate the gas generation mechanism, organic matter formation environment, mixing process [20,21], and so on. Specially, with the maturity of carbon isotope detection technology, the δ¹³C of simple carbon-bearing molecules (CO, CO₂, CH₄, and light hydrocarbons) can be quickly recognized in the field, outside the laboratory, for example, for predicting natural gas recovery with shale gas exploitation [22] and the distribution of combustible ice [19]. A lot of research has been done on the effect of carbon isotope (mainly CO₂ and C₁-C₃) characteristics on the oil shale pyrolysis process, and it can be used to determine the origin and migration pathways of natural gas [23], the thermal maturity stage of organic matter [24], natural gas types, and even to identify the prolific zones for gas recovery [25]. These applications are mainly based on an empirical equation established for actual field data and/or laboratory tests, and a good application effect has been achieved. Specifically, the “rollover” point (e.g., the δ¹³${\mathrm{C}}_{{\mathrm{C}}_2{\mathrm{H}}_6}$ increasing with maturity and then decreasing) and “reversals” point (e.g., δ¹³${\mathrm{C}}_{{CH}_4}$ > δ¹³${\mathrm{C}}_{{\mathrm{C}}_2{\mathrm{H}}_6}$ > δ¹³${\mathrm{C}}_{{\mathrm{C}}_3{\mathrm{H}}_8}$) receive a lot of attention during oil shale pyrolysis process [20]. Nevertheless, those points are out of the maximum oil generation stage during the oil shale artificial mature process, in which the %R_o is generally less than 1 [24]. Thus, this study is mainly focusing on filling this gap and indicating the oil generation area for the in situ exploitation of oil shale based on carbon isotope fractionation character research at the laboratory scale.

Carbon isotopes have a relatively slow exchange rate for many carbon-bearing minerals and are in relatively large fractionations even at high temperatures; those merits made it become an excellent recorder of geological processes [26,27]. Carbon isotope fractionation is a result of mass-dependent ¹³C/¹²C-fractionation during the gas, fluid, or melts generation process [28]. The carbon isotope characteristics in those C-H-O systems are largely considered to be determined by kinetics or/and a thermodynamic equilibrium mechanism [29]. For the organic matter decomposition process, the carbon isotope of gases is mainly related to the former [30], and researchers have established many models to describe the carbon isotope evolution characteristics during natural gas generation process, e.g., the Rayleigh model [31], Cramer model [32], Tang model [33], etc. The effect caused by the thermodynamic equilibrium process has been rarely considered to date; there has been much debate in the literature about the relative importance of kinetics and thermodynamic equilibrium in controlling the isotopic distribution among C-H-O gases, especially in aqueous conditions [28,29]. The existence of H₂O can promote the organic matter decomposition process and alter the gas composition [34], and then, the “frozen” carbon isotope characteristics determined by kinetics will be altered, especially for the carbon isotopes of CH₄ and CO₂ [34]. The carbon isotope thermodynamic equilibrium reversible process [29] introduced some limitations into the existing kinetics models when used. In addition, different organic matter composition will also have a great influence on the established kinetics modes [35]. Thus, it is important to explore new ways of using carbon isotopes. Additionally, a series of experiments have been conducted on the equilibrium partitioning of isotopic exchange reactions between CO₂ and CH₄ from 200 to 1200 °C [28,36], but to our knowledge, the only study on carbon isotope equilibrium characteristics in an organic matter decomposition process is without water [28], and in this condition, there is no obvious carbon isotope exchange between CO₂ and CH₄ without catalytic even at 400 °C with 614 h [28,29].

In this study, we heated an immature oil shale sample with a time series under controlled temperature and pressure in the presence of water at autoclaves. Our goals were as follows: (1) investigate the carbon isotope evolution characteristics below the max oil generation stage; (2) illuminate the possible mechanisms responsible for isotope fractionation; (3) evaluate if the carbon isotope fractionation characteristics based on kinetics and thermodynamic equilibrium can be used to indicate the oil generation characteristics during oil shale pyrolysis within water. This research can fill the gap in the literature on the indicators of the oil generation area, and it also could help optimize the heating process on future in situ oil shale exploitation.

2. Material and Methods

2.1. Sample Preparation

The oil shale used in this study was sampled from the fourth layer of the Dachengzi mine area, Huadian, Jilin Province, China, with 0.35% R_o. Before being separated into 100 g samples for later experiments, the oil shale samples were crushed and sieved into a diameter around 0.60−0.90 mm, and to reduce the experimental errors caused by sample heterogeneity, the samples were uniformly mixed [4]. To know the basic properties of HD oil shale, the proximate analysis, ultimate analysis, and Fischer assay analysis were conducted according to [6] (

Table 1. Properties of HD oil shale samples.

Proximate Analysis (wt %, ad)		Ultimate Analysis (wt %, ad)		Fischer Assay Analysis (wt %)
Moisture	3.36	C	40.40	Shale oil	16.08
Volatiles	36.52	H	5.97	Gas	4.80
Fixed carbon	3.42	N	0.75	Water	5.77
Ash	56.70	S	0.83	Residue	73.35

).

2.2. Pyrolysis Experiments

First, the 100 g oil shale samples were put in 0.5 L stainless-steel autoclaves manufactured by Hai’an Oil Research Co., Ltd. of Jiangsu, China, with 200 mL of distilled water. Second, to remove the air in the top space of the autoclaves, high-pressure purity N₂ (99.999%) was used at 4 MPa, and then, the N₂ was released at ordinary pressure. This process was repeated three times. Third, the autoclaves were heated to 365 °C by 1.5 h; when they reached the set temperature, they were kept for 2, 4, 6, 8, 10, 20, 30, 50, 70 and 100 h. The temperatures of the autoclaves were monitored by thermocouples, and the standard deviation was around ±0.1 °C. During the heating, the pressure was monitored by piezometers. The pressure was approximately 25 MPa during all experiments at 365 °C, and this pressure was almost provided by the vapor pressure of water. At the end of each experiment, the autoclave was cooled down to room temperature at ice water; then, the gas and oil products were subsequently collected for further analyses (Figure 1). Additionally, here, we use oil recovery per unit rock sample mass (ORR) to represent the oil recovery stage during the oil shale pyrolysis process, which is equal to the ratio of oil product weight collected in the autoclaves to the oil shale sample weight. This term is not equivalent to the oil gas recovery factor in oil gas geology, which is the ratio of final oil production from a reservoir to petroleum geological reserves.

2.3. Gas Compositional and Isotopic Analysis

The gas products were collected at 2, 4, 6, 8, 10, 20, 30, 50, 70, and 100 h and analyzed by an Agilent 7890B gas chromatograph (GC) manufactured by Agilent, California, USA, with one injection at Jilin University. The GC was equipped with two thermal conductivity detectors (TCD) and a flame ionization detector (FID). C₁–C₆ were detected by FID using HP-AL/S columns (25 m × 0.32 mm × 8 μm). TCD1 was used to detect CO₂, O₂, N₂, and CO using HayeSep A 80−100 mesh columns, and TCD2 was used to detect H₂ using Molsieve 5 A 60/80 Mesh columns. All detection processes used He as the carrier gas. The content of the gas component was quantified based on a calibration gas sample (item number: A0908685), which was manufactured by the Scott company. The software used in calculation was Agilent OpenLab manufactured by Agilent, California, USA. A stable carbon isotope of gas (CO₂ and C₁–C₃) products is selectively tested at 2, 6, 10, 20, 30, 50, 70, and 100 h. The analysis was performed using Thermo Delta V Advantage isotope ratio mass spectrometry (IRMS) manufactured by Thermo Fisher Scientific, Waltham, USA, at the Daqing Petroleum Exploration and Development Research Institute, China. Carbon isotope ratios are reported in ‰ with respect to the VPDB standard. The precision in the measurements is ±0.5‰ for δ¹³C.

3. Results and Discussion

3.1. Products Character

The ORR has a faster increasing trend before 8 h from 0 to 11.36%, and then, it is almost unchanged until 20 h; between 20 and 70, it gradually increased (11.50–16.52%) and then, it declined at 100 h (15.74%). The maximum value of ORR may be around 17% (

Table 2. The yields and stable carbon isotopic compositions of gaseous products.

Time	C1H4	C2H6	C2H4	C3H8	C3H6	CO2	CO	H2	Wetness Index a	ORR b	CH4	C2H6	C3H8	CO2
h	Volume (%)									Weight (%)	δ13C (‰, VPDB)
2	3.84	0.87	0.18	0.28	0.12	54.95	1.03	3.90	28.36	6.13	−36.8	−31.96	−29.79	−8.19
4	4.89	1.74	0.19	0.77	0.28	63.18	0.53	4.73	42.20	7.18	-	-	-	-
6	5.78	1.98	0.18	0.78	0.25	64.38	0.45	3.49	38.05	9.02	−36.96	−29.58	−27.94	−5.95
8	6.08	2.19	0.17	0.96	0.29	62.84	0.30	5.84	41.26	11.36	-	-	-	-
10	6.74	2.45	0.17	1.08	0.30	64.11	0.29	6.23	41.14	11.39	−37.54	−29.68	−27.11	−5.69
20	8.30	3.14	0.14	1.42	0.34	64.02	0.24	7.81	41.67	11.50	−39.55	−30.69	−29.39	−7.11
30	10.88	3.95	0.09	1.81	0.32	61.93	0.11	8.14	39.52	15.31	−38.79	−30.16	−28.89	−6.65
50	12.4	4.60	0.09	2.25	0.38	56.8	0.14	9.27	40.95	15.73	−38.59	−29.85	−28.49	−6.83
70	13.45	5.12	0.06	2.66	0.34	61.94	0.05	6.46	41.95	16.52	−38.55	−30.8	−29.11	−7.56
100	14.69	5.54	0.05	2.85	0.30	59.25	0.03	6.16	41.03	15.74	−38.95	−30.98	−29.37	−7.54

Note: “-“ means no detection; “^a” is calculated by ∑C_2–5/∑C_1–5; “^b” is calculated by the oil product weight/oil shale sample weight. In addition, since the C₄₊ gas components are not shown, their total volume is not equal to 100%.

and Figure 2a), and this point corresponds to the secondary cracking speed of oil equal to its formation [37,38]. The evolution character of wetness (∑C_2–5/∑C_1–5) after 8 h changed little with the experiment time (Figure 2b). The CH₄ shows the largest yields among the C_nH_m gases generated (Figure 2c). The paths of C₂H₆ and C₃H₈ are similar to the continuously increasing yields of CH₄ at all times; this character made it difficult for the alkane to indicate the ORR evolution characteristics during the oil shale exploitation process. Small amounts alkene gases are generated, and in contrast to C₂H₆, the C₂H₄ continued declining as the time increased (Table 2). The existence of unstable alkene gases indicated that the C-H-O gases have not achieved equilibrium in the system, and then, a second cracking process happened, which will lower the generated oil weight [39]. The plot of ln(C₁/C₂) versus ln(C₂/C₃) is useful for distinguishing the secondary cracking process [40] in which the secondary cracking of forming heavy hydrocarbon gases and oil will increase the ln(C₂/C₃) value and decrease the ln(C₁/C₂) value. If the decomposition rates exceed the generation rates, the ln(C₂/C₃) will increase sharply as ln(C₁/C₂) increases. The ln(C₁/C₂) value of pyrolysis gas varies from 0.29 to 0.67, whereas the ln(C₂/C₃) value varies from 0.16 to 0.58 with a linear relationship in this study (Figure 2d), and the slope around 1 of Figure 2d may imply that the decomposition and generation rate of oil is closed to balanced and the ORR has reached its maximum.

There are also considerable amounts CO₂ and H₂ generated, but the formation of CO is limited, and its formation rate continues declining as the time increases (Figure 3a). The formation of CO₂ can occur from the breaking of C-O, C=O bonds during organic matter thermal decay and the decomposition of carbonate minerals [41], and the hydration and/or oxidation process (CH₄ → CO → CO₂) can also form it [29,35]. The reaction process is shown below:

CH₄ + 2H₂O ⇄ CO₂ + 4H₂

(1)

CH₄ + 3CO₂ ⇄ 4CO + 2H₂O

(2)

2CO ⇄ C + CO₂

(3)

The obviously high concentration of H₂ and CO at all experiment times implied the existence of hydration (1) and oxidation reactions (2)–(3) throughout. Those reactions may be the main processes for CO₂ generation, as it has a very high content (54.95–64.3%; Table 2), and this character has been verified by comparison experiments between the oil shale pyrolysis process with water and without water [42]. Additionally, the CO is an intermediate product, and its decreasing formation rate over time implied that the C-H-O system is developing toward equilibrium. This evolution character can be seen in the CH₄-CO₂-CO diagram (Figure 3b), and this conclusion is also similar to the analysis of C_nH_m gases outlined before.

Before discussion of the carbon isotope fraction characteristics, it is necessary to know the gas species formation mechanism, as the carbon isotope will be “frozen” after the gas is generated; thus, the carbon isotope fractionation is limited during collisions between molecules of gases [29]. There are three possible pathways for organic–inorganic reactions during hydrous pyrolysis [43,44]: (1) free radical mechanisms, where free radicals generated by the homolysis of organic matters were captured by water or water-derived hydrogen [34]; (2) ionic mechanisms, consisting of the reaction between water-derived H⁺ and alkenes and alkanes [28,37]; (3) oxidation or hydration mechanisms, which the direct reaction of alkenes by CO₂ or water [35]. Those reaction mechanisms can all occur in the hydrous pyrolysis, but the former may relate to the kinetic fraction of the carbon isotope, while the other two are related to thermodynamic equilibrium [23,35]. According to the quantum chemistry calculation, the ionic mechanism is more significant during high-temperature water–hydrocarbon reactions, while the free radial mechanism may play a more important role in geological conditions with temperature lower than 250 °C, but their effect on the carbon isotope fractionation result is the same [35]. Even though the oxidation or hydration mechanism has a low reaction rate, as it has to overcome higher energy barriers, the phenomenon in which the amounts of CO₂, CO, and H₂ generated in the presence of water are always significantly higher than without water indicated that the presence of H₂O can foster the oxidation and/or hydration process [42]. Thus, as the isotope fractionation result is similar between the free radial and ionic reaction mechanisms as gases formed through organic matter decomposition [23], the final carbon isotope character in the C-H-O system may be decided by the degree of oxidation and/or hydration process through Reactions (1)–(3) at the end of the time period.

3.2. Carbon Isotopes Character

Carbon isotopes produced during the thermal cracking of organic matter are useful for assigning the degree of gas recovery with water presence [45]. This is mainly based on the detection of the carbon isotope “rollover point” (1.5%R_o), which correlates well with the highest production of hydrocarbon gases [23], or the “reversal point”, which needs an even higher maturity with a %R_o always higher than 2.0 [46]. However, those points are all beyond the highest oil generation area and were not discovered in our low time (0–100 h) oil shale subcritical water recovery process (Figure 4). The time-series carbon isotope evolution of methane, ethane, and propane shows a similar isotope trend δ¹³C₁ < δ¹³C₂ < δ¹³C₃. The δ¹³C ranges for CH₄, C₂H₆, and C₃H₈ were −39.55‰ to −36.80‰, −31.96‰ to −29.58‰, and −29.79‰ to −27.11‰, respectively (Table 2). The hydrocarbon gas shows the origin of organic matter, while the δ¹³${\mathrm{C}}_{{CO}_2}$ is much higher than without the water pyrolysis experiment and moving toward an inorganic carbon isotope. Its increasing characteristics may be related to the continuous isotope exchange between the gas product and carbonates in the rock [34,35].

The attainment of chemical equilibrium is a prerequisite for the attainment of isotopic equilibrium because isotopic exchange is most likely driven by the same elementary chemical reactions among C-H-O gases [29]. The existence of the alkene gases and CH₄-CO₂-CO diagram all implied that the chemical has not achieved equilibrium; this character also can be seen from the difference of the fractionation coefficient (δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$) between the actual measurement and equilibrium condition at 365 °C (Figure 5a). The δ¹³C₂-δ¹³C₃ versus ln(C₂/C₃) shows the gases that emerged mainly from primary cracking before 70 h (Figure 5b) featured larger ratios of ethane to propane and smaller carbon isotopic differences between ethane and propane with the differences below −3‰ [47]. This implied that the carbon isotope fraction of CH₄ was only slightly influenced by the secondary cracking of C₂ and C₃ to C₁. However, while the continuous primary cracking process will lead to the δ¹³${\mathrm{C}}_{{CH}_4}$ increasing with time, the newly formed CH₄ will have a low δ¹³C value based on secondary cracking, as the ¹²C will enter gases more than ¹³C [45]. thus, the decreasing of δ¹³${\mathrm{C}}_{{CH}_4}$ after 70 h is mainly related to the mixing process between primary and secondary cracking. As studied by others, the isotope exchange between CO₂ and CH₄ below 500 °C at 614 h cannot be seen [29] at a low temperature, and it only takes place with a catalyst, e.g., Ni [28].

In general, the carbon isotopes of gases formed through the kinetics process will continue increasing, as ¹²C will enter gases first [48]. However, the evolution of the carbon isotopes took on a wavy shape, especially in the initial stage of pyrolysis (before 20 h; see Figure 4). This phenomenon has been observed in numerous studies, which state that it is mainly caused by the heterogeneity of the carbon isotopic composition of the organic structures. For example, [20,33] assumed that the initial trend of decreasing δ¹³C values is due to mixing with the methane that is generated from lighter isotopic and heavy methyl groups. As mentioned before, the oxidation and/or hydration process is throughout our time-series experiments. Thus, the carbon isotope fraction formed by kinetics has been altered by thermodynamic fractionation to varying degrees. The speed of the thermodynamic fractionation process is mainly controlled by the chemical reaction rate, which is mainly influenced by the gas concentration, temperature, and pressure [29,35]. In our experiment, the temperature is constant, and the pressure has little changes. Therefore, the thermodynamic fractionation speed is mainly controlled by the gas concentration, especially the concentration of CH₄. This process, in contrast to kinetics, will make the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ decrease until reaching equilibrium. Hence, the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ is the composition results from the kinetics and thermodynamic fractionation. To check the real CH₄ generation characteristics, we recovered it based on chemical Reactions (1)–(3). During calculation, the CO₂ was regarded as formed all from Reaction (1), as the release of CO₂ from organic matter decomposition during the immature stage is limited. The recovered result can be seen in

Table 3. The recovered gas characteristics based on Reactions (1)–(3) at 365 °C.

Time	C1H4	CO2	Recovered Dry Coefficient a
h	Volume (%)
2	5.07	55.72	97.52
4	6.21	63.58	95.05
6	6.77	64.72	95.21
8	7.62	63.07	94.19
10	8.37	64.33	93.79
20	10.31	64.20	92.44
30	12.94	62.01	91.12
50	14.75	56.91	88.97
70	15.08	61.98	88.58
100	16.24	59.27	87.86

Note: “^a” is calculated by C₁/∑C_1–5.

, in which the concentration of CH₄ increased with time, leading the carbon isotope altered by thermodynamic fractionation to also increase. However, the generated rate of CH₄ is not constant (Figure 6a), and this change is reflected in the changing δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ values as time increases (Figure 5a): it has a great increase before 20 h, and after this point, the change decreases before increasing again (70–100 h). This change feature is similar to the evolution of ORR (Figure 2a).

3.3. The Indicative Significance of the Carbon Isotope to ORR

As the δ¹³C_CO2-CH4 shows phase change characteristics, we further checked the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ vs. ORR relation. Figure 7a shows three obvious stages within the experiment time. Before 20 h, the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ increases faster with the ORR (stage 1); then, it decreased until the ORR reaches its maximum at 70 h (stage 2). In the ORR reduction area, the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ increases again (stage 3). According to the pyrolysis products characteristics, stage 1 corresponds to the gas generation stage with a faster generation rate (Figure 6a), but the ORR increases fast before 8 h and then changes little between 8 and 20 h (Figure 2a). Stage 2 is the main oil generation stage; the ORR shows an obvious increase, but the increased CH₄ generation rate slows down (Figure 6a). Stage 3 is related to the rapid oil cracking stage, in which the C₂/C₃ decreases rapidly (Figure 7b) and the CH₄ generated rate quickly increases again (Figure 6a). Additionally, there is a large variation of pyrolysis products before 8 h, and after this point, the variation of ORR is little; this area may be matching the immature stage of organic matter. As shown in the calculation results (Table 3; Figure 6b), the dry coefficient is higher than 95% before 8 h; according to the empirical results, the dry coefficient higher than 95% is related to immature stage of organic matter, and under 95%, the organic matter enters the mature stage [49]. Thus, the large variation of pyrolysis products during the immature stage may be contributed to its retained oil and gas before with little newly formed oil [50].

In stage 1, the increase in the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$, before 8 h, is perhaps related to the diffusion process with the continued desorption of the retained gas, as the CO₂ is heavier than CH₄, and the ¹²C has spill precedence [48]. In addition, due to the existence of H₂, CO₂, and CO, we think it is also influenced by the varying degrees of thermodynamic fractionation. When the organic matter matures (8–20 h; Figure 6a) in stage 1, the organic matter begins to decompose with the lowest δ¹³${\mathrm{C}}_{{CH}_4}$ value (Figure 4), which causes the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ value to suddenly go up, and almost no oil is formed (Figure 7a). In this area (8–20 h), the influence of the kinetics fraction is stronger than the thermodynamic fractionation; thus, the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ also increases faster than before. When entering stage 2, compared to stage 1, the ORR increased slowly over time (20–70 h, Figure 2a); on the contrary, the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ decreases, and there is the first “turning” of δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ from increasing to decreasing (Figure 7a). Even though it still shows a kerogen cracking character with limited oil and gas cracking (Figure 5b), the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ decreased with time, which means that the thermodynamic fractionation becomes more important than the kinetic fraction, which is mainly controlled by hydration (Reaction (1)) and the oxidation mechanism (Reactions (2) and (3)). While entering stage 3, the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ appears to have a second “turning” from decreasing to increasing again, as the oil is cracking faster than its generation, which indicated that the kinetic fractionation plays a dominant role again (Figure 7b). Thus, the first δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ “turning” point indicates that new oil begins to form, and the ORR increased with time, while once the second “turning” point appears, the ORR has reached its maximum and is about to start to decline.

Specially, in this study, we did not consider the effect of pressure, temperature, and rock characteristics on the carbon isotope fraction process. According to other research, the pressure has a suppression effect of gas generation in the early stage from 20 to 90 MPa, with a maximum carbon isotope fractionation effect of ≈3‰, and it increases with temperature from 350 to 373 °C [51]. However, the effect on the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ characteristics may be less, as it is a relative change. The organic matter type with a heterogenous structure in the rock may also have an influence on the carbon isotope fractionation, but the evolution characteristic of δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ may be slight, as the gas product at different decomposition stages is similar to that of different organic matter [30]. Thus, the result of this research can be used to indicate the oil shale pyrolysis process, and it can help determine when to stop heating during oil shale exploitation and lower the production costs.

4. Conclusions

The carbon isotope fractionation characteristics and its constraints on the oil recovery stage during oil shale in situ exploitation are studied based on the subcritical water extraction process of Huadian oil shale at 365 °C. The results revealed the following: (1) the carbon isotope of hydrocarbon gas shows a normal sequence, before ORR decreasing, the secondary crack process has little effect on the carbon isotope composition, which is mainly controlled by the kinetics during the gas formation; (2) after the decomposition process, the formed carbon isotopes are altered by the thermodynamic equilibrium process based on the oxidation and/or hydration mechanism; (3) the two “turning” points of δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$, which is controlled by the competition between the kinetics and thermodynamic fractionation process, can be constrained at the oil generation stage. Specially, the influence of organic matter type, heterogeneity, temperature, pressure, and catalyst on the δ¹³${\mathrm{C}}_{{CO}_2-{CH}_4}$ evolution characteristics needs further research before using. During the in situ pyrolysis process, the mixing and migration process will also have an influence on the carbon isotope fractionation in some way.