Experimental Study On The Oxidation Mechanism Of Methane And Other Hydrocarbons By Fe³⁺-bearing Silicate Minerals And Its Implications

The conversion mechanism between CH4 and CO2,two essential volatiles in earth system,is one of the most important scientific problems,which is significantly related to many geological processes,such as carbon cycle and fluid-rock interaction.Previous studies explored the formation mechanism of CO2 through mainly carbon isotope analysis and other geochemical means,however,the generating mechanism of CO2 through the oxidation of CH4 by Fe3+-bearing silicate minerals is still unclear.For example,it is widely accepted that the oxidation of methane by biotite alteration(chloritization,muscovitization)is the most important reason for the zonal distribution of paleo-fluids in the Carboniferous,Permian,and Triassic strata from central Swiss Alps,i.e.,methane zone,methane-water transition zone,and water zone.It is found that the fluid composition is evolved from CH4 system with high salinity to CH4-CO2 system with lower salinity in the Baogutu porphyry Cu-Mo-Au deposit where the chloritization and muscovitization of biotite are significant.Therefore,the oxidation of methane driven by biotite alteration could be one of the most important reasons for the evolution of fluid composition.However,the relevant experimental geochemical evidence is lacking for explaining these field geological phenomenon.In those experimental researches on the oxidation mechanism of CH4,iron or manganese oxides,sulfate and other compounds are mostly used as the oxidants,rather than those rock-forming minerals with more general significance,especially biotite and other silicate minerals,therefore it is difficult to simulate and explain the phenomenon of hydrocarbon oxidation observed in the field.In this study,aiming to reveal the reaction conditions and mechanism of methane oxidation by minerals in geological environment,Fe3+-bearing silicate minerals(biotite,amphibole and pyroxene)were used as oxidants when carrying out the oxidation experiments of CH4 and other hydrocarbons.In this study,Fused Silica Capillary Capsule(FSCC)was used as the reaction chamber for all experimental systems,except for the thick quartz tube in biotite-Al4C3-H2 O system.Mineral oxidants,hydrocarbons and ultra-pure water were sealed into the reaction chamber before reaction.The reaction temperature was controlled by the heating-cooling stage and the cold-sealed autoclave.The gas and liquid products were detected by laser Raman in situ/quenching analysis,and the solid products were detected by scanning electron microscopy(SEM)-energy dispersive spectroscopic analysis.Through the experimental studies on biotite-CH4-N2-H2 O,amphibole-CH4-N2-H2 O and pyroxene CH4-N2-H2 O systems,this study confirmed that biotite,amphibole,pyroxene and other Fe3+-bearing silicate minerals are able to oxidize the CH4 to CO2 at initial reaction temperatures of about 150 ℃,140 ℃ and 210 ℃,respectively,which are lower than the observed geological temperature in the Alps.After the reaction of biotite-Al4C3-H2 O system,the energy dispersive spectroscopic analysis of residual biotite showed that the content of the variable element Fe decreased significantly,indicating that the Fe3+ in biotite is participated in the reaction as an oxidant and migrated.Meanwhile,the comparative experimental results of CH4-N2-H2 O system showed that CH4 will not be oxidized to CO2 if there isn’t Fe3+-bearing oxidizing mineral,which further proves that the oxidation of methane is caused by mineral oxidants.The experimental results of biotite-C3H8-N2-H2 O,biotite-C13H28-H2 O,biotite-C2H5OH-H2 O,amphibole-C13H28-H2 O and pyroxene-C13H28-H2 O systems show that the initial reaction temperature of methane is lower and it is oxidized earlier than long-chain hydrocarbon when silicate minerals are used as oxidants,which could result from the fact that methane is the smallest hydrocarbon in molecular radius,thus it is able to enter the mineral structure and reacts with electron acceptors easily.The experimental results suggest that the thermochemical oxidation of CH4 is a common phenomenon,which provides an important experimental and theoretical support for the oxidation of CH4 to CO2 by biotite and other Fe3+(Mn4+、Mn3+)-bearing dark minerals under low temperature environment in geological system.Meanwhile,the transformation of CH4 to CO2 would benefit for understanding the mechanism of hydrocarbon loss and the preservation conditions of oil-gas reservoir,moreover,it would also provide a new mechanism for the genesis of CO2-rich ore-forming fluids.