硫酸盐热化学还原反应对烃类的蚀变作用

硫酸盐热化学还原反应(TSR)是指硫酸盐与烃类作用,将硫酸盐矿物还原生成H2S等酸性气体的过程,是高含H₂S天然气形成的重要机制。由于硫酸盐热化学还原反应是热动力驱动下烃类和硫酸盐之间的化学反应,因此伴随着烃类的氧化蚀变,烃类气体的组分和碳同位素则会发生相应的变化。对四川盆地东北部下三叠统飞仙关组和渤海湾盆地古生界高含硫化氢气藏中烃类气体组分和碳同位素的研究发现,在硫酸盐热化学还原反应消耗烃类的过程中,重烃类优先参与了该反应,从而导致天然气的干燥系数增大。同时,在硫酸盐热化学还原反应过程中,¹²C—¹²C键优先破裂,¹²C更多地参与了该反应,而¹³C则更多地保留在残留的烃类中,使反应后残留的烃类中相对富集¹³C,烃类气体碳同位素值增重2.0‰~4.0‰,且重烃类碳同位素的增重幅度大于甲烷。

Alteration of thermochemical sulfate reduction to hydrocarbons

The thermochemical sulfate reduction (TSR) refers to the process of sulfate reaction with hydrocarbons，which is the important mechanism for generating natural gas with high content of H₂S and CO₂. Because TSR is the reaction between hydrocarbons and sulfate under the driving of thermal force，the composition of hydrocarbons gas and the carbon isotope maybe vary correspondingly along with the oxidation and alteration of hydrocarbons.The composition of hydrocarbons gas and the carbon isotope of gas reservoirs with high content of H₂S in the Feixianguan Formation of the lower Triassic (T₁f) in the northeastern area of Sichuan Basin and the formation of Paleozoic Erathem in Bohai Bay Basin were studied. It is proved that the heavy hydrocarbons preferentially participated in the reactions of TSR and led to the drying coefficient of natural gas to increase during the process of hydrocarbon depleted by TSR. In the process of TSR，the bond of ¹²C-¹²C ruptured preferentially and more ¹²C participated in the reactions of TSR，while more ¹³C remained in the residual hydrocarbons，which causes the abundant ¹³C distributed in the residual hydrocarbons.Therefore，the drying coefficients of hydrocarbons gas increased after the alteration of TSR. Meanwhile，the increased values of carbon isotope of hydrocarbons gas are from 2.0‰ to 4.0‰. Also，the increasing extent of carbon isotope for heavy hydrocarbons is greater than that of methane.