High sour gas with >10% H2S (hydrogen sulfide) has been reported in reservoirs from the Upper Permian Changxing Formation and Lower Triassic Feixianguan Formation in the NE Sichuan Basin  - . The elevated H2S (up to 62%) is attributed to thermochemical sulfate reduction (TSR) in deep buried Triassic and Permian reservoirs with vertical depths over 4.8 km . TSR is an organic-inorganic interaction between sulfates and hydrocarbons in high temperature diagenetic settings, in which sulfates are reduced to H2S, organic sulfides and elemental sulfur (S0) while hydrocarbons are oxidized to carbon dioxide (CO2) and water . TSR-derived H2S not only critically affects the economic proportion of hydrocarbon gas, but is highly toxic to human beings and corrosive for petroleum processing facilities. H2S gas risk has become one of the major concerns for exploration in deeply buried carbonates   . Some laboratory experiments with gaseous saturated hydrocarbons were carried out to investigate the formation mechanism of elevated H2S in sour gas from TSR zones  - . However, up to now, few TSR simulation studies utilized alkenes as reducing agents for sulfates . Although alkenes are absent or only trace components in most natural gas and petroleum, they are produced in appreciable amounts during thermal degradation of kerogen  as well as in many previous TSR experiments      . Notably, it was reported that alkenes were the most reactive hydrocarbon type during TSR , and the rapid increase in the yield of ethanethiol and propanethiol was closely related to the generation of ethylene and propylene from hexadecane thermal cracking at high temperatures . Nevertheless, the kinetics of TSR by alkenes needs further investigation due to no reports related. To fill this gap, a series of simulation experiments on the reactions between magnesium sulfate (MgSO4) and ethene (C2H4) were conducted under controlled hydrothermal conditions. C2H4 was selected as a representative of intermediate hydrocarbon species during TSR. Anhydrous MgSO4 was utilized as oxidants for hydrocarbons due to its effective oxidant of hydrocarbons without the addition of H2S . The objectives of the present study are to:
1) Investigate the thermodynamic characteristics of TSR by ethene;
2) Determine the kinetic parameters for sulfate reduction by ethene;
3) Discuss the reason that alkenes were not detected in natural TSR zones.
MgSO4 is analytical pure, purchased from Aladdin Industrial Corporation (Shanghai, China). C2H4 (99.999%) was supplied by Wuhan Newradar Special Gas Co., Ltd.. Simulation experiments on the reduction of MgSO4 by C2H4 were operated at elevated temperatures using an autoclave. The autoclave was made of Hastelloy C-276 alloy with a volume of 200 mL. The sample basket is a quartz glass cylinder with a height of 40 mm and diameter of 20 mm. The quartz glass cylinder has a bottom to hold MgSO4 powder and its other side is left open. The basket was placed on the bottom of the reactor. Each time, the basket with about 2.000 g MgSO4 powder and 1.0 mL distilled water was put into the reactor, and then the reactor was vacuumed. Ethene was charged to a given pressure. In the reaction system of C2H4-MgSO4-H2O, the initial pressure was about 2.5 MPa at room temperature and the final pressure of the reaction system ranged from 4.5 MPa - 5.6 MPa. The reactor was first heated to 250˚C directly and then to the final temperatures by a program control. The final temperatures were 280˚C, 300˚C, 320˚C, 340˚C and 360˚C, and the heating times were 240 h, 400 h, 560 h, 720 h and 880 h, respectively. Constant heating rate β is set as 0.125˚C/h. After simulation experiments, the basket with reaction products was taken out of the autoclave, firstly put into an oven to be heated at 120℃ for about 6 h to remove water, and then heated in a muffle at 500˚C. The weight of solid products was measured by an electron balance. The calculation of TSR conversions was based on gravimetry. The products were characterized by gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and X-ray diffractometer (XRD) methods, respectively.
After thermal experiments, headspace gaseous products of the tube reactor were removed through the valve, and characterized using an Agilent 6890 gas chromatograph (Agilent Technologies, Palo Alto, CA, USA). Subsequently, 2 mL of dichloromethane (DCM) was then injected into the reaction vessel through the valve to extract organic products, and then organic sulfur products in the DCM extract were analyzed by gas chromatography-mass spectrometry (GC-MS). The analysis of the extracted organic sulfur products was carried out at Beijing Risun Chemical Technology Institute Co., Ltd., China (courtesy of Shijuan Du). Characteristics of the crystalline structure of the solid products at the termination of each experiment were confirmed by a PANalytical X’Pert PRO X-ray diffractometer (PANalytical Co., Almelo, Netherlands). The analysis of the crystalline structure of the solid products was carried out at Beijing Risun Chemical Technology Institute Co., Ltd., China (courtesy of Xinpo Feng). Detailed operation conditions for GC, GC-MS and XRD are available in Ding et al. (2014, 2016)  .
3. Results and Discussion
3.1. TSR Products
According to the GC analysis, the composition of gaseous products from the reaction between C2H4 and MgSO4 was shown in Table 1. It was found that gaseous products in the temperature range of 280˚C - 360˚C were composed of CH4, C2H6, C3H8, C3H6, n-C4H10, i-C4H10, H2S, CO2, H2, CH3SH, C2H5SH, CH3SCH3 (Table 1). Thereinto, C1~C4 hydrocarbons and H2 gas were derived from complex polymerization-decomposition of C2H4 and/or alteration of hydrocarbons by TSR at elevated temperatures. H2S and CO2 were typical TSR products as had been documented in previous geological observations    as well as laboratory simulation experiments        . The contents of sour gases increased with increasing temperatures at 280˚C - 360˚C (Figure 1), which indicated elevated temperatures were favorable for TSR occurring. The reaction pathway for sour gas formation could be written as:
Labile organosulfur compounds (LSC), such as CH3SH, C2H5SH, CH3SCH3 (Table 1), were generated from H2S reacting with hydrocarbons  . Figure 2 indicated the contents of ethene and organic sulfides vs. temperature. Figure 2 indicated the contents of ethene and organic sulfides vs. temperature. With the decrease of C2H4, contents of LSC vs. temperature exhibited a positive relationship in Figure 2. The diminishing C2H4 resulted from TSR effect as well as thermal chemical alteration (Figure 1 and Figure 2).
Table 1. Composition of gaseous products at elevated temperatures (%).
Figure 1. Contents of ethene and sour gases vs. temperature.
The qualitative and quantitative results of sulfur compounds in the DCM extract were listed in Table 2. Besides H2S and LSC, thiophene derivatives and benzothiophene were also detected in the DCM extract (Table 2). Thiophene derivatives and benzothiophenes are stable organic sulfur compounds (SOSCs) relative to LSC. It can be inferred from Ding et al. (2016)  that the thermal stability order for mercaptans, sulfides, thiophene derivatives and benzothiophene is benzothiophene > thiophene derivatives > mercaptans or sulfides. Therefore, the formation of the SOSCs in Table 2 should be attributed to thermal alteration of LSC . The positive correlation between contents of SOSCs and temperature in Figure 3 indicated that the conversion of inorganic sulfur of MgSO4 to thiophene sulfur in TSR was enhanced by elevated temperatures. Tentative formation pathways for LSC and SOSCs in this study were given in Equations (2) and (3):
Figure 2. Contents of ethene and organic sulfides vs. temperature.
Some black material was observed on the surface of solid products at 360˚C. After calcinations at 500˚C, the solid products turned white. Therefore, the black deposit corresponded to be coke . X-ray powder diffraction patterns of solid reaction products at 360˚C were shown in Figure 4. From the X-ray patterns, two distinct crystallographic phases, MgO and MgSO4 were found to coexist in the products. These results suggested that MgO was produced during TSR by C2H4. Based on the above analysis, it was concluded that H2S, CO2 and MgO were the main TSR products. Various organic sulfur compounds such as mercaptans, sulfides, thiophenes derivatives and benzothiophene were mainly derived from H2S reacting with hydrocarbons as well as thermal chemical alteration of LSC at elevated temperatures.
Figure 3. Contents of thiophene derivatives and benzothiophene vs. temperature.
Figure 4. X-ray patterns of the solid products after calcinations.
3.2. Thermodynamic Analysis
Our present experimental results indicated that TSR by ethene could be written as the general Equation (4). Previous study of Ding et al. (2011)  showed that thermochemical reduction of MgSO4 by ethane (C2H6) conformed to Equation (5). Based on Equations (4) and (5), the standard Gibbs free energy of molar reaction for TSR by ethene or ethane at various temperatures was calculated by use of thermochemical modeling program HSC 6.0 (Outokumpu Research Oy, Pori, Finland) and shown in Figure 5. It is evident that the reactions (4) and (5) proceed spontaneously at 0 - 600˚C according to the negative values of Gibbs free energy (Figure 5). The absolute value of Gibbs free energy of the two reactions increased with increasing temperature, which implied that elevated temperature is favored. Comparing the absolute value of Gibbs free energy of the two reactions (4) and (5) at the same temperature, it can be concluded that ethene thermodynamically more easily participates in TSR than ethane.
Figure 5. Effect of temperature on standard Gibbs function of TSR by ethene or ethane.
3.3. Kinetic Study
In the present study, the molar conversion for the reaction between C2H4 and MgSO4 was calculated from the weight change of the solid products and reactants, and the results were given in Figure 6. From Figure 6, it can be seen that TSR conversions increased with increasing temperature. So, temperature is a key factor for TSR by ethene, as had been documented in most previous studies        .
The reaction of C2H4 and MgSO4 is assumed as first order reaction, and its kinetic equation could be written as :
where constant heating rate , x is reaction conversion, A is the apparent
Figure 6. Reaction conversions of MgSO4 vs. temperatures.
frequency factor, E is apparent activation energy, T is absolute temperature and R is the gas constant. According to the study of Ding et al. (2008b) , the approximate integration of Equation (1) will give
According to the slope and intercept of regressed line in Figure 7, the apparent activation energy E and apparent frequency factor A for TSR by ethene were determined as 76.370 kJ/mol and 4.579 s−1, respectively. Notably, Xia et al. (2014)  reported that the activation energy for reduction of MgSO4 by ethane is 264.5 kJ/mol. The lower activation energy for ethene involved in TSR relative
Figure 7. The kinetic regression line for the TSR by ethene.
to ethane suggested that the reactivity of ethene is much higher than that of ethane, in accordance with the above thermodynamic analysis. High reactivity of alkenes is possibly due to their susceptibility to electrophilic attack . The reaction rate constant k and half-live t1/2 for TSR by ethene at geological temperatures was determined using Equation (6). The results were listed in Table 3.
Table 3. The reaction rate constants and half-lives for the reduction of MgSO4 by ethene at geological temperatures.
Rate constants determined experimentally using first-order kinetics extrapolate to MgSO4 half-lives of 1. 948E+05 years −67.329 years at 20˚C - 120˚C, 67.329 years −7.783 years at 120˚C - 160˚C and 7.783 years −0.590 years at 160˚C - 220˚C. These values demonstrated that the reaction rate for TSR by ethene is very fast in high-temperature gas reservoirs (120˚C - 220˚C). The new formed ethene from thermal cracking of natural gas and/or petroleum could not survive after TSR process. Therefore, alkenes were rarely detected in natural TSR reservoirs. Although the present results of the simulation experiments could not yet be directly applied to the oil & gas exploration in TSR zones, they are theoretically significant for further understanding of the formation mechanisms of thiols in the Triassic Wolonghe gas field as well as elevated thiophene concentrations in the Jurassic reservoirs from the Sichuan Basin, China .
Thermal reduction of sulfates by alkenes may have implications for origin of OSCs in deep buried geologic settings. In the present study, TSR by ethene was demonstrated under laboratory experimental conditions. The threshold temperature for reduction of MgSO4 by ethene was determined as low as 280˚C, with H2S, CO2 and a series of OSCs as the main TSR products. The reactions between ethene and MgSO4 can proceed spontaneously at 0 - 600˚C. Ethene thermodynamically more easily participates in TSR than ethane. The lower activation energy of 76.370 kJ/mol for ethene involved in TSR relative to ethane suggested that the reactivity of ethene is much higher than that of ethane. The reaction rate for TSR by ethene is very fast in high-temperature gas reservoirs (120˚C - 180˚C), indicating the newly formed ethene from thermal cracking of natural gas and/or petroleum could not survive after TSR process. Therefore, alkenes were rarely detected in natural TSR reservoirs. The present results of the simulation experiments are theoretically significant for further understanding of the formation mechanisms of thiols in the Triassic Wolonghe gas field as well as elevated thiophene concentrations in the Jurassic reservoirs from the Sichuan Basin, China.
We gratefully acknowledge financial support by the National Natural Science Foundation of China (No. 41472095), the Open Fund of Key Laboratory of Exploration, Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education (No. K2018-05) and the Foundation of Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology (Grant No. DMSM2018041).
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