Catalytic Hydrogenation of Post-Mature Hydrocarbon Source Rocks Under Deep-Derived Fluids: An Example of Early Cambrian Yurtus Formation, Tarim Basin, NW China

As a link between the internal and external basin, the deep derived fluids play a key role during the processes of hydrocarbon (HC) formation and accumulation in the form of organic-inorganic interaction. Two questions remain to be answered: How do deep-derived fluids affect HC generation in source rocks by carrying a large amount of matter and energy, especially in post-mature source rocks with weak HC generation capability? Can hydrogen and catalysts from deep sources significantly increase the HC generation potential of the source rock? In this study, we selected the post-mature kerogen samples of the early Cambrian Yurtus Formation in the Tarim Basin of China. Under the catalytic environment of ZnCl₂ and MoS₂, closed system gold tube thermal simulation experiments were conducted to quantitatively verify the contribution of catalytic hydrogenation to "HC promotion" by adding H₂. The catalytic hydrogenation increased the kerogen HC generation capacity by 1.4–2.1 times. The catalytic hydrogenation intensity reaction increased with temperature. The drying coefficient of the generated gas decreased significantly as the increasing yield of heavy HC gas. In the simulation experiment, alkane δ¹³C becomes lighter after the catalytic hydrogenation experiment, while δ¹³C_CO2 becomes heavier. In the process of catalytic hydrogenation, the number of gaseous products catalyzed by ZnCl₂ is higher than that catalyzed by MoS₂ under the same conditions, indicating that ZnCl₂ is a better catalyst for the generation of gaseous yield. Meanwhile, Fischer-Tropsch synthesis (FFT) reaction was happened in the catalytic hydrogenation process. The simulation experiment demonstrates that hydrogen-rich components and metal elements in deep-derived fluids have significant catalytic hydrogenation effects on organic-rich matter, which improved the HC generation efficiency of post-mature source rocks.

Introduction

The deep-derived fluids refer to the mantle source volatile fluid below the basement of the sedimentary basin, which is originated from the dehydrating fluid generated in the process of plate subduction or deep metamorphism, and the deep circulating fluid driven by the heat from the mantle or the deep shell source (Jin et al., 2002; Jin et al., 2004; Jin et al., 2007; Liu et al., 2018). A large amount of gas components carried by the deep-derived fluids contain volatile materials, such as H₂, CH₄, H₂S, CO₂, N₂, and He. As one of the main volatile fluids, the distribution of geologic H₂ (molecular hydrogen) is widespread in ocean ridges, active tectonic belts, and the Precambrian or Cenozoic strata distributed on land (Jeffrey and Kaplan, 1998; Allen and Seyfried, 2004; Newell et al., 2007; Tivey, 2007; Bradley and Summons, 2010; Sherwood-Lollar et al., 2014; Meng et al., 2015; Etiope, 2017; Guélard et al., 2017; Etiope and Whiticar, 2019; Bougault et al., 2019; Klein et al., 2019; Klein et al., 2020). H₂ has a variety of genetic types, including the release of deep magma degassing, the serpentinization of ultrabasic-basic rocks (Sherwood-Lollar et al., 2014; Etiope, 2017; Klein et al., 2019; Klein et al., 2020), radiolysis of water due to the presence of radioactive minerals (Sherwood-Lollar et al., 2014; Milesi et al., 2016), mechanical cracking of Si-bearing rock (Hirose et al., 2011; Telling et al., 2015), and the reduction of organic matter by microorganisms (Shuai, et al., 2010). The deep-derived fluids not only carry energy and hydrogen-rich matter, but a large amount of metal catalysts and other elements such as Mg, Fe, Mn, Ni, Zn, Mo, and Cu (Coveney, 1987; Pinto et al., 1999; Tivey, 2007; Proskurowski et al., 2008; Resing et al., 2015). Meanwhile, in sedimentary basins the enrichment of organic matter is normally accompanied by abundant metal elements (e.g., Fe, Mn, Zn, and Mo) (Lv et al., 2018).

The geological kerogen catalytic hydrogenation reaction is a reduction reaction that generates new products and is accompanied by energy conversion. To study the contribution of material conditions in organic-inorganic interactions to hydrocarbon (HC) generation from organic matter, previous studies examined different external hydrogen sources and catalysts (Hawkes, 1972; Jin et al., 2002; Jin et al, 2004). As the energy of the H-H bond is theoretically 436 kJ mol⁻¹, which is less than the H-OH bond energy of 497 kJ mol⁻¹ (Luo, 2004), H₂ is easier to hydrogenate organic matter than H₂O under simulated experimental conditions (Jin et al., 2004). Lewan et al. (1979), Lewan, (1997)and Seewald (2003) confirmed that the contribution of exogenous H plays a key role in HC generation and distribution of underground HC components. Mango et al. (1996, 1997) showed in an open system that the reaction of exogenous hydrogen with organic matter promotes the rate of HC generation under the action of transition metals. Meng et al. (2015) confirmed that deep hydrogen-rich fluids can activate and increase the HC regeneration of ancient source rocks through experiments. Liu et al. (2016) showed that there is Fischer-Tropsch-type (FTT) interaction between H₂ and CO₂ produced by the thermal alteration of source rocks in the geological environment to synthesize HCs.

These deep-derived fluids can hydrogenate post-mature kerogen, thereby activating kerogen’s ability to regenerate HCs, by activating the original inert carbon in kerogen and generating HCs (Meng et al., 2015). The post-mature kerogen is rich in polycyclic aromatic HC compounds and polymer groups and has a low H/C ratio. H is a hydrocarbon-generating restrictive reactant for high-maturity kerogen. The hydrocracking reaction can hydro-isomerize and crack (including ring-opening) aromatic compounds and macromolecular groups, i.e., break long-chain alkanes into short-chain alkanes, so that aromatic or naphthenic HCs can be opened to form long-chain alkanes. The hydrocracking reaction can further break the chain (Han, 2001) to promote the reactivation of kerogen to generate HCs (Liu et al., 2018; Liu et al., 2019). In order to simulate the geological regularities of catalytic hydrogenation more accurately in a more consistent environment with deep-derived fluids, on the basis of previous studies, we further optimized the hydrogen quantification in the simulation experiment. The main objectives of this work are to examine the potential of HC generation in high-maturity marine I-type kerogen in the quantitative hydrogenation process, which involves different catalysts under closed system conditions. Meanwhile, it provided to understand the characteristics of HC isotope fractionation of alkane gas under the influence of deep-derived fluids; and to provide theoretical support for the evaluation of organic HC generation.

Experimental Samples and Methods

Experimental Samples

In this study, kerogen was selected from the argillaceous source rocks of the early Cambrian Yurtus Formation in the Dongergou section of the Tarim Basin. Kerogen (Ker) was type I with the vitrinite reflectance (Ro) of 1.91%. The experimental samples were approximately 200 meshes. Total organic carbon (TOC) content of kerogen is 64.0% with purification, atomic ratio H/C = 0.87, T_max = 530°C, pyrolysis parameter S₁ = 1.43 mg HC/g ker, S₂ = 10.95 mg HC/g ker, S₃ = 16.17 mg CO₂/g ker, hydrocarbon index (HI) = 18, oxygen index (OI) = 28. The catalysts used in this study, ZnCl₂ and MoS₂, had a particle size of 200 mesh and met the experimental analytical purity requirements. Deionized water was prepared in the laboratory, and H₂ was high-purity hydrogen gas containing 5% internal standard helium (He).

Experimental Method

The simulation experiment was conducted in the State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences in a closed system composed of gold tubes. During the sample loading process, the loading method of solid-liquid samples described in detail by Liu and Tang (1998) was used. In short, we added the weighed kerogen powder, deionized water, and catalyst powder (10:10:1, wt: wt: wt) into a gold tube (6 mm inner diameter and 70 mm length), which was welded and sealed for cleaning, placed the tube in an argon atmosphere for 25 min to replace the air in the tube, pinched the wall of the gold tube with manual pliers, and sealed the top port with argon arc welding to complete the loading steps. For the hydrogenation sample loading, we made sure precise quantitative volume H₂ injected into a larger gold tube. The tube (10 mm inner diameter, 120 mm long) was used. The pre-solid and liquid reactant injection method was the same as above: when quantitatively injecting H₂ (containing 5% He as the internal standard gas), temperature control was carried out under room temperature (25°C) with two high-sensitivity pressure gauges connected in series to ensure the amount of H₂ injection using the principle of hydrogen gas replacement to effectively eliminate air interference. The mass of the reactants added to the tube was 40 mg kerogen powder, 40 mg deionized water, 4 mg catalyst powder, and 0.65 mg H₂. Then we pressed the top port of the gold tube with a pressure clamp to seal the hydrogen component inside the gold tube and flattened the port with the pressure clamp after the wall of the gold tube was welded by argon arc welding. Next, the gold tube was placed in hot water (>60°C) without bead-shaped bubbles overflowing to ensure gas tightness in the tube.

After the sample loading of gold tubes was completed, the tubes were placed in the high-temperature reaction kettle set for heating. The temperature was first raised in 2 h to reach each predetermined temperature point, and then constant heating was performed for 72 h. There were five groups of experimental samples with each set at six temperature points, 350°C, 375°C, 400°C, 450°C, 500°C, and 550°C. Group I is kerogen + deionized water, group II is kerogen + deionized water + ZnCl₂, group III is kerogen + deionized water + MoS₂, group IV is kerogen + deionized water + ZnCl₂+H₂, group V is kerogen + deionized water + MoS₂+H₂. The simulated experimental pressure was constant at 500 bar, and the abnormal pressure change in the kettle did not exceed 1 bar.

Experimental Analysis

Following the research method of Pan et al. (2006) and Jia et al. (2014), the gold tube was removed and carefully placed into a custom-made vacuum glass tube. The vacuum tube was connected to an Agilent 6890 N full-component gas chromatograph (GC), which was modified by Wasson ECE Instrumentation for determination of gas molecular composition. The GC contains three detection channels, one flame ionization detector (FID), and two thermal conductivity detectors (TCD). N₂ was the carrier gases for the organic detection channel (FID) and inorganic channel (TCD1), and He was the carrier gas for hydrogen and helium detection channel (TCD2). The detection column was a Paraplot Q-type capillary column. The vacuum environment of the system was maintained by connecting a vacuum pump in the glass tube and piercing the gold tube inside to release the gas component. There are two valves connected in series to the vacuum tube and the chromatograph, which are used to measure the pressure change of the gas component and release part of the gas into the chromatograph to measure its molecular composition. We determined the type and composition of the gas products through different detection response values and corresponding time points. The method used the peak areas of different detection values of FID and TCD to complete the quantification of different gaseous products generated by the peak area ratio of the known quantitative standard gas (Liu and Tang, 1998).

After the GC detection analysis, a gas-tight syringe was used to extract a small amount of gas was extracted from the vacuum tube using a compact gas sampling needle for the carbon and hydrogen isotope analysis. The ratios of carbon and hydrogen isotopes refer to the Vienna Peedee belemnite standard (VPDB). The carbon isotope ratio detection and analysis were performed using a GV Isoprime IRMS interfaced with an Agilent 6890 N-Isoprime 100 instrument. The detection value of each sample was measured two to three times. If there was no obvious difference between the previous two detection values, the average value was taken as the same position value of the sample. If the difference between the first two test values was large, the third test was required, and the average value of the two adjacent values in three tests was taken as the final sample isotope composition. The carbon isotope analysis accuracy of this equipment was within ±0.4‰. Hydrogen isotope ratio detection was performed using the Thermo Trace GC 1310-Delta V Advantage instrument. The detection method was consistent with the carbon isotope analysis, and the hydrogen isotope accuracy was within ±3‰. During the detection process, the pre-determined δ¹³C value calibration gases were periodically analyzed to check the accuracy of the measured value.

Results

Total Gas Yield

The total gas yield of different simulated experimental samples showed an upward trend with increasing temperature (Table 1). The total gas yield of group I was measured at the initial temperature of 350°C at 56.6 m³/t Ker, and the yield increased substantially after 400°C. At the highest temperature of 550°C, the yield reached a maximum value of 241.8 m³/t Ker. After the catalysts ZnCl₂ and MoS₂ were added to groups II and III, the total gas yield increased from the lowest values of 70.0 m³/t Ker (350°C) and 70.5 m³/t Ker (350°C), respectively, and began to increase significantly at 400°C. The maximum yields were 513.4 m³/t Ker and 347.5 m³/t Ker, which were 2.1 to 1.4 times the maximum value in group I, respectively. The addition of ZnCl₂ and MoS₂ significantly promoted the reaction, and the yield increased more by the addition of ZnCl₂ than MoS₂, and the gas increase was mainly from CH₄ and CO₂. In groups IV and V, the total gas yields (without accounting for H₂ yield) were 55.7 m³/t Ker (350°C) and 29.2 m³/t Ker (350°C), respectively, and then the yield increased greatly at 375°C. The maximum yields were 506.5 m³/t Ker and 368.4 m³/t Ker at 550°C, respectively. Compared with groups II and III, the groups IV and V showed little change in the total yield. With the addition of H₂ reaction resulted in the alkane yield increased, but the CO₂ yield decreased, resulting in an insignificant change in the total gas yield overall.

TABLE 1

TABLE 1. Types and concentrations (m³/t Ker) of gas generated during the catalytic pyrolysis experiments at 350°C/72 h, 375°C/72 h, 400°C/72 h, 450°C/72 h, 500°C/72 h, and 550°C/72 h.

Product Components

In terms of the total yield distribution of gaseous HCs, the ∑C_1-5 yield of group I increased continuously with temperature, and the maximum yield was 99.49 m³/t Ker. After the catalysts ZnCl₂ and MoS₂ were added to groups II and III, the maximum yields of ∑C_1-5 were 208.22 m³/t Ker and 137.77 m³/t Ker, which were increased 2.1 and 1.4 times as much as group I, respectively. It proved that the addition of ZnCl₂ and MoS₂ significantly promoted the generation of the gas components. In groups IV and V, the maximum yields of ∑C_1-5 were 375.34 m³/t Ker and 292.32 m³/t Ker, representing 1.8 and 2.1 times increase compared to groups II and III. The catalytic hydrogenation.

The methane (C₁) yield increased with temperature and reached the maximum at 550°C. The maximum C₁ yield in group I was 98.55 m³/t Ker. When the catalysts ZnCl₂ and MoS₂ were added to groups II and III, the maximum yields of C₁ were 207.56 m³/t Ker and 136.89 m³/t Ker, which were 2.0 and 1.4 times as much as the yield of group I, respectively. In groups IV and V, the maximum yields of C₁ were 369.82 m³/t Ker and 283.63 m³/t Ker, respectively, which were 1.8 and 2.1 times the yields of groups II and III (Figure 1).

FIGURE 1

FIGURE 1. Characteristics of C₁ yield simulated under different catalytic hydrogenation control conditions of the closed system.

The change in the ethane (C₂) yield of group I with temperature was very small, and the maximum C₂ yield was 0.8 m³/t Ker (500°C). After the catalysts ZnCl₂ and MoS₂ were added to groups II and III, the maximum C₂ yields were 1.16 m³/t Ker and 0.85 m³/t Ker, respectively. The addition of ZnCl₂ and MoS₂ did not increase the C₂ yield significantly. In groups IV and V, the maximum yields of C₂ were 64.76 m³/t Ker and 42.59 m³/t Ker (both 500°C), which were 80 and 50 times those of groups II and III without H₂, respectively. Exogenous H₂ reacts more easily than H₂O to promote the generation of C₂ (Figure 2).

FIGURE 2

FIGURE 2. Change in C₂ yield simulated under different catalytic hydrogenation control conditions of the closed system.

The change in the propane (C₃) yield of group I with temperature was close to 0.0 m³/t Ker. After adding the catalysts ZnCl₂ and MoS₂ to groups II and III, the change in the C₃ yield was still 0.0 m³/t Ker, indicating that the addition of metal salts did not significantly increase the C₃ yield. In groups IV and V, the maximum yields of C₃ were 34.4 m³/t Ker and 21.8 m³/t Ker (450°C), respectively; the addition of external H₂ and catalysts exceedingly improved the C₃ yield (Figure 3). The comparison of the results of C₁, C₂, and C₃ shows that the addition of exogenous H₂ increases the HC generation yield, and the peak temperature of HC generation decreases sequentially with the increasing carbon number.

FIGURE 3

FIGURE 3. Change in C₃ yield simulated under different catalytic hydrogenation control conditions of the closed system.

The catalytic addition of the H cracking reaction had a significant effect on the drying coefficient of natural gas (C₁/∑C_1-5) (Table 1). The C₁/∑C_1-5 ratio of group I (kerogen + deionized water) increased with temperature, from a minimum of 0.90 (350°C) to 0.99 (550°C). When the catalysts ZnCl₂ and MoS₂ were added to groups II and III, the C₁/∑C_1-5 ratio became larger with increasing temperature at the ratios of 0.89–1.00 and 0.87–0.99, respectively. The addition of ZnCl₂ and MoS₂ promoted the formation of C₂₊ HCs at relatively low temperatures (≤400°C). After adding H₂ and catalysts ZnCl₂ and MoS₂, the addition of exogenous H₂ greatly increased the heavy HC (C₂₊) components, and the C₁/∑C_1-5 ratios at 350°C were 0.55 and 0.78, respectively. The C₁/∑C_1-5 ratio gradually decreased with the increasing temperature and dropped to the lowest point at 0.48 and 0.50, respectively, at 400°C. After 400°C, C₁ produced by the hydrocracking of kerogen continued to increase, as well as C₂₊ HCs in the product. The conversion to C₁ significantly increased the C₁ yield, and the natural gas drying coefficient reached the maximum values of 0.99 and 0.97 at 550°C.

The CO₂ yield in each group of experiments increased with increasing temperature and reached the maximum value at 550°C. The CO₂ yield of group I gradually increased with increasing temperature, and the maximum CO₂ yield was 142.25 m³/t Ker. In groups II and III, with the addition of catalysts ZnCl₂ and MoS₂, the maximum yields of CO₂ were 280.91 m³/t Ker and 192.35 m³/t Ker, respectively, which are 2.0 and 1.4 times the maximum yield without the addition of the catalysts. The increase in CO₂ and HC components was similar, indicating that the metal ions mainly catalyze the lysis of kerogen itself rather than decomposing the bond between H and O. The CO₂ yields in groups IV and V were 0.39–0.40 times those of groups II and III, respectively (Figure 4). The significant decrease in the yield of CO₂ resulted from the reaction that occurred with the addition of H₂.

FIGURE 4

FIGURE 4. Change in CO₂ yield simulated under different catalytic hydrogenation control conditions of the closed system.

The experimental comparison of groups I, II, and III showed that the H₂ yield did not increase significantly with temperature (the highest is 0.02 m³/t Ker at 550°C in group I), but the maximum yields after adding ZnCl₂ and MoS₂ were 24.27 m³/t Ker and 17.43 m³/t Ker at 550°C. The yield increased by one to two orders of magnitude, indicating that the H₂ production was significantly promoted by the addition of catalysts, combined with the C₂ yield. The dominant reason of the H increase is the cracking of organic matter.

Product Component Carbon Isotope Changes

The results show that δ¹³C becomes heavier with increasing temperature. Since kerogen of the Yurtus Formation is at a high maturity stage, only C₁, C₂, and CO₂ isotope values can be effectively detected in product components. For the δ¹³C value of gaseous HCs under different control conditions at the same temperature point, the δ¹³C value of each gaseous HCs increased with the intensity of reaction between catalysis and hydrogen addition (Table 2). The δ¹³C₁ value of group I increased continuously with the increase in temperature from −39.8 to −34.9‰. When ZnCl₂ and MoS₂ were added to groups II and III, the δ¹³C₁ values changed from −44.7 to −36.7‰ and from −43.6 to −36.2‰. In groups IV and V, the δ¹³C₁ values changed from −45.9 to −40.2‰ and from −52.4 to −43.4‰, showing an increase before the 400°C stage, then a decrease in the 400–450°C interval, and finally becoming heavier with increasing temperature. For the C₁ yield after 400°C, catalytic hydrocracking plays a major role in promoting the disproportionation of small low-carbon molecules. Since the bond energy of ¹²C–¹²C is less than the bond energy of ¹²C–¹³C, the δ¹³C₁ fractionation follows; as the intensity of catalytic hydrogenation increases, δ¹³C₁ becomes lighter (Figure 5).

TABLE 2

TABLE 2. δ¹³C values of gas generated during the catalytic pyrolysis experiments at 350°C/72 h, 375°C/72 h, 400°C/72 h, 450°C/72 h, 500°C/72 h, and 550°C/72 h.

FIGURE 5

FIGURE 5. Change in δ¹³C₁ simulated under different catalytic hydrogenation control conditions of the closed system.

The δ¹³C₂ value increased sequentially with the increasing temperature. In group I, the value of δ¹³C₂ changed from −28.2 to −24.4‰ at 350–400°C. After 400°C, the δ¹³C₂ value only changed from −24.4 to −23.6‰. In groups II and III with catalysts ZnCl₂ and MoS₂, the δ¹³C₂ value was generally stable, similar to group I, changing from −29.9 to −24.0‰ and from −29.1 to −23.7‰, respectively. The addition of ZnCl₂ and MoS₂ promoted the reaction and the exchange of C elements to a lesser extent. The δ¹³C₂ value changed in groups IV and V from −38.4 to −14.1‰ and from −37.6 to −12.1‰, indicating that catalytic hydrogenation significantly increases the formation of C₂ and the exchange of C elements. At 550°C, the δ¹³C₂ value was the largest because the addition of H₂ promoted the catalytic cracking reaction. When the temperature exceeded the gas generation window (<500°C), a large amount of C₂ cracked to form C₁, resulting in an abnormally large δ¹³C₂ value (Figure 6).

FIGURE 6

FIGURE 6. Change in δ¹³C₂ simulated under different catalytic hydrogenation control conditions of the closed system.

As one of the main reaction products, the CO₂ isotope fractionation value is mainly controlled by its own molecular quality. The comparison of the experimental results between the hydrogenated and unhydrogenated groups showed that the δ¹³C_CO2 value in the hydrogenated group was larger than the unhydrogenated group, which is negatively correlated with the change in the fractional distillation of the δ¹³C value of alkanes. The δ¹³C_CO2 value of CO₂ generated by hydrocracking was relatively stable. The δ¹³C_CO2 values of groups I, II, and III were distributed between −35‰ and −33‰. The δ¹³C_CO2 values in groups IV and V showed a decreasing trend first and then an increasing trend. The participation of ZnCl₂ catalyst increased the δ¹³C_CO2 value of the product. The joint addition of H₂ and the catalyst further increased the fractionation of the δ¹³C_CO2 value. The effect of MoS₂ on the δ¹³C_CO2 value was larger at the early stage of the reaction (Figure 7). At the same time, relative to the changes in δ¹³C values for C₁ and C₂, the range of carbon isotope fractionation of CO₂ was smaller than that of alkanes because of the oxidative decarboxylation of its controlled long-chain HCs, rather than the bond energy difference controlled by the C-C bond leading to preferential cracking (Lu et al., 2010).

FIGURE 7

FIGURE 7. Change in δ¹³CO₂ simulated under different catalytic hydrogenation control conditions of the closed system.

Discussion

Catalytic Hydrogenation Mechanism

Catalytic hydrogenation showed that the yield and isotopic value of the gaseous product components changed significantly and were also accompanied by FTT synthesis reaction products. The source supply angle mainly depends on the control influence of H₂ and different metal elements in the reaction environment.

Product Component Comparison

The post-mature kerogen of the Yurtus Formation has very weak pyrolysis and HC generation capacity. With the increase in temperature and the catalysis of hydrogenation by different catalysts, the kerogen HC generation capacity significantly improved. As the most stable HC molecule, C₁ had an increased yield. In addition to the catalytic hydrogenation reaction of kerogen, there were other gaseous HCs produced by catalytic hydrocracking. The yield of different catalysts changed significantly. Under the same hydrogenation conditions, the yield of C₁ under the action of ZnCl₂ was greater than that of MoS₂, with a yield ratio of 1.0–1.4 and average of 1.3. The results of the control experiment between the experimental group with added H₂ and catalyst and the experimental group with catalyst only showed that the addition of H₂ delayed the decomposition of heavy HCs such as C₂ and C₃, leading to a higher temperature of HC generation peaks. Meanwhile, the heavy HC group bond energy was smaller than the methyl group energy, which led to the preferential participation of heavy HC groups in hydrocracking and thus to a significant increase in the yield of C₂₊ HCs and a significant reduction in the drying coefficient of the product. The addition of exogenous H₂ greatly changed the kerogen's own cracking mode, and heavy HC component C₂₊ increased significantly. It was well consistent with the results that the drying coefficient of the product gases with the increased intensity of catalytic hydrogenation is significantly lower than that of the ordinary geological environment (Lewan et al., 1979; Mango et al, 1994; Ma et al., 2018).

Change in the i/n-Alkane Ratio

Among gaseous HC components, normal paraffins are formed by free radical reactions, and isoparaffins are derived from free radical cracking on kerogen and asphalt branches under acidic cationic ion reaction (Eisma and Jurg, 1969; Almon and Johns, 1977; Kissin, 1987; Pan et al., 2006). The Yurtus Formation kerogen is high maturity, and the branched chains of its molecular structure is not developed; thus, the main source of isoparaffin is the acidification cation reaction. Thompson and Creath, (1966) showed that in North American industrial HC reservoirs mainly generated by free radical HC generation, the iC₄/nC₄ ratio is around 0.5, generally not greater than 1.0; the iC₅/nC₅ ratio is around 1.0 and generally less than 2.0. In this study, the comparison of the same component at the same temperature showed that the value of iC_n/nC_n under the action of ZnCl₂ was larger than MoS₂. For the products of butane (C₄), from the initial temperature of 350°C–400°C, the iC₄/nC₄ value of the products produced by the two catalysts was relatively stable, ranging from 0.241 to 0.309 and 0.184 to 0.217, respectively. The maximum values were 1.045 and 1.042, respectively, at 450°C. For the products of pentane (C₅), the temperature changed greatly from the initial temperature of 350°C–500°C; the range of the iC₅/nC₅ ratio under the action of ZnCl₂ was 2.200–6.252 and under the action of MoS₂ was 0.096–1.409 (Table 1, Figure 8). The results showed that under the action of ZnCl₂, more HCs were generated by the cationic reaction, which promoted more formation of the isoparaffins. The higher the temperature is, the more significant changes in the cationic reaction are. The results further reveal that the catalytic effect of ZnCl₂ on promoting HC generation by the cation reaction is stronger than that of MoS₂.

FIGURE 8

FIGURE 8. Comparison of the iC₅/nC₅ alkane ratio between butane and pentane under the condition of ZnCl₂/MoS₂ of the closed system.

Reaction Mechanism

HC generation of catalytic hydrogenation is mainly carried out in the form of H radicals and H ions. There are two modes of catalysis: “Lewis acid” catalysis and “Brønsted acid” catalytic processes (Figure 9). When performing Lewis acid site catalytic processes, the main chemical reactions are decarboxylation and C-C bond breaking (Li et al., 2002; He et al., 2011; Ma et al., 2018). Since the Lewis acid site composed of Zn²⁺ or Mo²⁺ is an empty orbital position with high affinity for electrons, when the organic matter is decarboxylated, Zn²⁺ or Mo²⁺ at this position gets an electron from the adsorbed organic molecule, and carboxylic acid loses CO₂. Free radicals further undergo a rearrangement reaction with H radicals, leading to the breakage of the C-C bond and the formation of free HCs with shorter bond lengths. The product of the decarboxylation reaction is mainly CO₂. The breakage of the C-C bond mainly promotes the generation of free HCs. When performing Brønsted acid catalytic processes, through contact with Zn²⁺ and Mo²⁺, H₂ or water molecule lose an electron to generate H ions, promoting the kerogen unsaturated cycloalkane addition reaction. As the hydrogenation reaction continues, low-molecular-weight saturated chain HCs are eventually formed (Wu et al., 2012; Ma et al., 2018).

FIGURE 9

FIGURE 9. Catalytic mechanism of transition metal based on free radical cracking and H ion reaction (modified according to Johns, 1979; Mango et al, 1994; Mango, 1996; He et al., 2011; Wu et al., 2012; Ma et al., 2018).

The yield results show that Zn²⁺ has a better catalytic effect than Mo²⁺ because Zn²⁺ has a more stable and long-lasting catalytic activity than Mo²⁺, and it is easier to fully contact the reactants. The chemical properties of Cl⁻ are extremely stable. Previous studies suggest that Cl⁻ has a certain inhibitory effect on the catalyzed cracking of kerogen (Li et al., 2002). In the catalytic hydrogenation experiment of H₂, ZnCl₂ shows a strong catalytic effect on hydrogen generation. S²⁻reacts with H₂ to generate part of H₂S, which has a catalytic effect on HC generation (He et al., 2011), but the product test results show that the H₂S content is relatively low, and the catalytic contribution is not significant.

Stable Carbon and Hydrogen Isotope Change Mechanism

δ¹³C₁/δ¹³C₂

For the HC isotope composition of oil and natural gas, relative to its parent material, kerogen is lean in ¹³C and ²H. This is because the kerogen ¹²C–¹²C bond and the C-C bond in ¹H-CC-¹H are more easily broken than the ¹²C–¹³C bond and the ²H-CC-¹H CC bond, thereby allowing lighter isotopes to enter HC (Lu et al., 2010).

The change in δ¹³C₁-δ¹³C₂ values can reflect the catalytic hydrogenation effects effectively. The data of groups I, II and III showed that δ¹³C₁-δ¹³C₂ has a strong linear relationship (Figure 10). After adding the catalysts, the change in the δ¹³C₁-δ¹³C₂ value was decreased significantly and the range changed widely, but C₂ yield changed little (Figure 2). The results showed that C₂ mainly promotes the elemental exchange between carbon isotopes after the catalyst were added and has little effect on the yield reaction equilibrium. The comparison of the experimental results showed that the catalyst addition leads to lighter δ¹³C₁ and δ¹³C₂. Meanwhile, the metal ions are involved in enhancing the reaction process and promoting the isotope fractionation level during the reaction. The catalytic hydrogenation extremely promotes the activation of the carbon source in post-mature kerogen. Within a certain temperature range (≤450°C), the distribution of δ¹³C₁-δ¹³C₂ is clustered and distributed in a stable lighter interval. At 500°C, the δ¹³C₁/δ¹³C₂ ratio is close to the range with no H₂ added. The C₂ component passed the peak of HC generation because the C₂ component with a smaller δ¹³C value starts to crack into C₁, which led to the leaving C₂ with heavier δ¹³C. At the same time, C₃ cracked completely due to the δ¹³C kinetic fractionation to promote the formation of lighter δ¹³C₁ and heavier δ¹³C₂. Restricted by high temperature and the amount of carbon material, the heavier carbon content retained in generated C₂ increases at 550°C, resulting in larger change of linear variations. It also implies the δ¹³C₁-δ¹³C₂ parameters can provide supplementary verification to evaluate whether the HC generation process of source rocks was affected by catalytic hydrogenation in sedimentary basins.

FIGURE 10

FIGURE 10. Change in the δ¹³C₁-δ¹³C₂ contrast simulated under different catalytic hydrogenation control conditions of the closed system.

Relationship Between CO₂ and Alkane δ¹³C

CO₂ is one of the main by-products of the reaction process. Its δ¹³C fractionation change is opposite to that of alkane δ¹³C. δ¹³C_CO2 increases with the enhancing intensity in the reaction (Figures 11, 12). When the catalytic hydrogenation stage is weak, the kerogen HC generation is mainly carried out by self-cracking; thus, the changes in kerogen δ¹³C in the low temperature stage are greatly affected by temperature and the external H₂ participation in the reaction. The δ¹³C_CO2 value became larger accompanied by an increasing intensity of the catalytic reaction. In group I, the δ¹³C_CO2 change from the large to the small was at the 400 °C inflection point. In group II and III, the δ¹³C_CO2 change from large to small was at the 450°C inflection point and the HC generation mode of kerogen was affected distinctly by the catalytic elements during pyrolysis reaction. In group IV and V, the addition of H₂ changed the characteristics of the original kerogen evolution process and promoted isotope exchange in the HC generation. The change in δ¹³C_CO2 from lighter to heavier was mainly controlled by the dynamic fractionation of carbon isotope of CO₂. The maximum δ¹³C_CO2 temperature under the action of ZnCl₂ is delayed compared to that under the action of MoS₂. The δ¹³C₂ value under the action of ZnCl₂ is less than that under the action of MoS₂, which demonstrates that the catalytic effect of ZnCl₂ under the same conditions is stronger than that of MoS₂.

FIGURE 11

FIGURE 11. Change in the δ¹³C values of different alkanes and CO₂ under the simulated ZnCl₂ catalyzed hydrogenation control condition of the closed system.

FIGURE 12

FIGURE 12. Change in the δ¹³C values of different alkanes and CO₂ under the simulated MoS₂ catalyzed hydrogenation control condition of the closed system.

Accompanying Secondary Reaction—Fischer-Tropsch-Type Synthesis

As one of the main modes of inorganic HC generation, FTT synthesis is well verified in geological discoveries and experiments (Fu et al., 2007; Hosgörmez, 2007; McCollom, 2013; Suda et al., 2014; McCollom, 2016; Etiope, 2017; Liu et al., 2018) and greatly affected by the redox conditions of the environment. The catalytic hydrogenation experiment provides a good reduction environment and source supply conditions. As shown in Figure 13, the relationship between CO₂ and CH₄ in the catalytic hydrogenation state of ZnCl₂ was y = 1.1359x-17.687, R² = 0.9375. Under the catalytic hydrogenation state of MoS₂, the relationship between CO₂ and CH₄ changed to y = 2.1496x-97.873, R² = 0.9533. Under the stable reaction at >400°C, the relationship between CO₂ and CH₄ changed to y = 1.9199x-73.472, R² = 0.9921, indicating that the decrease in the yield of CO₂ in the hydrogenation reaction showed a strong linear relationship with the increase in C₁. The CO₂ yield decreased rapidly at 375 °C owing to the solubility of water at high temperatures. In the high-temperature stage (≥400°C) under the catalytic state, the activation reaction of hydrogen radicals and carbon molecules was accelerated. In the catalytic hydrogenation reaction of CO₂, a large quantity of HCs and water mainly composed of C₁ are produced in the FTT synthesis reaction accompanying H₂ and CO₂. The change in the slope of the equation shows that the catalytic effect of ZnCl₂ on kerogen pyrolysis and HC generation is stronger than that of MoS₂.

FIGURE 13

FIGURE 13. Changes in the characteristics of C₁ and CO₂ under different catalytic hydrogenation control conditions simulated of the closed system.

With H₂ added to the HC generation reaction, the CO₂ yield decreased significantly. However, the CO₂ yield remained stable after falling to a certain value. The yield was stable under the influence of different temperature points. According to the δ¹³C_CO2 isotope change, a heavier trend in both H₂ added groups implies that ¹²C is more likely to participate in the catalytic hydrogenation response than ¹³C. Combined with the experimental results described in Table 1, Figures 4, 13, the addition of H₂ inhibited the CO₂ generation. The FTT synthesis decreased the generated CO₂ and H₂ yield. The results of the control experiment showed that the final CO₂ conversion rate was a constant under a different reaction environment. The amount of activated C with the H₂ addition reaction indicated that activated C reactants would not react in the geological environment.

The catalysis reaction by the addition of H₂ was carried out in a neutral reduction system, and the FTT synthesis accompanied by CO₂ generated HCs, which had a significant effect on the total HC yield. In geological processes, the FTT synthesis reaction plays an important role as a link between the deep hydrogen-rich fluid and the hydrogen generation of post-mature organic matter.

Conclusion

For the ancient source rocks of the Yurtus Formation in the Tarim Basin, the catalytic substances and exogenous hydrogen carried under deep-derived fluids can “secondarily generate HCs” for the activation of source rocks. ZnCl₂ and MoS₂ have a strong catalytic effect on the reaction of post-mature kerogen and H₂. Catalytic addition of H₂ promotes the HC generation capacity of kerogen and the increase in HCs by 0.4–1.1 times, significantly reduces the natural gas drying coefficient, and promotes FTT synthesis of secondary reactions. Isotope fractionation mainly follows the laws of kinetics and shows good identification characteristics. By comparing the composition of gaseous products and isotope characteristics, the catalytic effect of ZnCl₂ is stronger than MoS₂ under the same conditions. Meanwhile, it implies deep-derived hydrogen fluids containing Zn, Mo and other metal elements can make a remarkable contribution to reactivate the post-mature source rocks for HC generation.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author Contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

Funding

This work was supported by National Key R&D Program of China (Grant No. 2017YFC0603102), National Natural Science Foundation of China (Project No.: 41625009 and 41673066), Strategic Priority Research Program of Chinese Academy of Sciences (Project No.: XDA14010404) and National Oil and Gas Major Project (No. 2017ZX05008-002-050).

Conflict of Interest

Author JZ, LQ, MQ, ZD, and LJ were employed by the company SINOPEC.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We are grateful to Yunpeng Wang, Hong Lu, and Qiang Wang for their experimental supports. The authors also thank Xiaoqi Wu for his critical comments that significantly improved the quality of manuscript.

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