Simulation Experiments Study Of Abiogenic Hydrocarbons Generation In Subsurface Hydrothermal Conditions

It was found that there were abiogenic methane and ethane in hydrothermal fluids of Mid-Ocean Ridge,Precambrian shields,igneous rocks and even ultra-deep basins.The study of these abiogenic hydrocarbons is helpful for understanding the origin of life,global carbon cycles and petroleum exploration.But the generation mechanisms and identification methods of these abiogenic hydrocarbons are not clear for people now.The theme of this paper are abiogenic hydrocarbons generation underground.In this paper,the possibility of abiotic hydrocarbon generation and the composition of hydrocarbon products under hydrothermal conditions by different carbon sources and catalysts were simulated by a closed gold tube system,furthermore the isotope fractionations and generation mechanisms of hydrocarbons generation,the simple identification criteria of the abiogenic hydrocarbons were studied.There are a variety of inorganic carbons such as（bi）-carbonates,CO,CO₂,and organic carbons carbons such as formic acid/formate underground.While hydrogen-bearing material is relatively simple,the main material is water.In this dissertation,simulation experiments of abiogenic hydrocarbons were carried out with a variety of carbonaceous materials underground as carbon sources and water as the main hydrogen source.The temperatures of all simulation experiments were between300-400 ^oC,which were close to the temperature of mid-ocean ridge fluids in Rainbow area.Pressure was 50 MPa,which was similar to the pressure of sea water in5 km depth.This dissertation is a study on Fischer-Tropsch-type synthesis reaction in hydrothermal conditions underground essentially.From the compositions and yields of hydrocarbon products,abiogenic hydrocarbons were produced in each series,indicating that the Fischer-Tropsch synthesis reactions in subsurface hydrothermal conditions were successfully realized in the gold tube system.The main experimental results are as follows:1)When Na BH₄as hydrogen source,the conversion rate of hydrocarbons generation of each carbonate/bicarbonate series was high,generally above 40%,especially when Al₂O₃（Ni）was used as catalyst,the conversion rate of methane can reach up to more than 80%.The possibility of abiogenic methane under hydrogen-rich conditions and the universality of abiogenic methane underground were confirmed by our experiments.2)Hydrocarbons with the highest carbon number of 30 were formed in the CO experiments,which realized the synthesis of long chain compounds such as biofilms and provided experimental supports for a study of the origin of life,as well as guidance for the direction of abiogenic petroleum explorations under ground.3)In the simulation experiments with CO₂as carbon source（represented by siderite）,only simple iron and ferrous oxide as catalyst,the long chain gaseous hydrocarbons and polycyclic aromatic hydrocarbons were generated,suggesting that ordinary catalytic process can generate butane and pentane,compositions of polycyclic aromatic hydrocarbons in the experiments were similar to those in carbonaceous chondrite,this provided experimental supports for the generation of PAHs in meteorites,siderite was probably an important precursor of PAHs in meteorites.Montmorillonite and cobalt composite catalysis experiment is easier to realize the H₂generation by water cracking,this research is of great significance to hydrolysis to produce hydrogen.This series produced more hydrocarbons than iron mineral catalytic series,and long chain hydrocarbons were generated in this series,which was the first time reports for liquid hydrocarbons generation with the tetravalent carbon as carbon source under hydrothermal conditions,the abiogenic liquid hydrocarbon generation in this series has more practical significance under ground.The stable carbon and hydrogen isotopic compositions and distributions of hydrocarbon products can reflect the formation process and path to a certain extent,so this was one of the focus of this dissertation.1)It was found that the carbon isotope distributions of gaseous hydrocarbons was present in partially reversed order when CO was used as carbon source(δ¹³C₁<δ¹³C₂>δ¹³C₃>δ¹³C₄>δ¹³C₅).In other simulation experiments,the carbon isotope distributions were in positive order(δ¹³C₁<δ¹³C₂<δ¹³C₃<δ¹³C₄<δ¹³C₅),which was different from the previous understanding that the carbon isotope distributions of abiogenic hydrocarbons were in reversed order.2)The mechanism of hydrocarbon generation determines the fractionation directions of isotopes.According to the composition and distribution of carbon and hydrogen isotopes of hydrocarbons,it was considered that the process of hydrocarbons generation was likely to follow the alkyl mechanism when CO was the carbon source,resulting in the inverse distribution of carbon isotopes of C₂-C₁₀.When CO₂was used as carbon source,carbonyl insertion mechanism may be more dominant for chain growth,leading to positive order of carbon isotopes of gaseous hydrocarbons.This indicates that carbon isotope compositions and distributions of hydrocarbons are not reliable indicators of their origins.3)The hydrogen isotope distributions of the gaseous hydrocarbon products were in positive order,which was consistent with the actual underground samples,this may be related to the faster reaction rate of M-CD₂than M-CH₂.Due to the uncertainty of underground carbon sources,the diversity of carbon isotopes and the complexity of hydrocarbons generation,the compositions and distributions of stable carbon/hydrogen isotopes of abiogenic hydrocarbons underground change vary greatly.In this study,the abiogenic hydrocarbons in the Precambrian shield(-60‰<δ¹³C_C1<-20‰,δD_C1<-320‰,5<C₁/C₂+C₃<200),MOR(-20‰<δ¹³C_C1<-8‰,-100‰<δD_C1<-200‰,C₁/C₂+C₃>200)and thermogenic gases(-60‰<δ¹³C_C1<-20‰,-300‰<δD_C1<-180‰,C₁/C₂+C₃<200)can be distinguished simply by combining the plots ofδ¹³C_C1–C₁/C₂+C₃,δ¹³C_C1–δD_C1,δD_C1–C₁/C₂+C₃and isotope distribution characteristics of the simulation experimental products.