我国生物气藏碳、氢同位素特征、形成途径及意义

Carbon and Hydrogen Isotopic Compositions:generation pathway of bacterial gas in China

生物气碳、氢同位素组成是探讨其形成途径和成藏特征的基本手段,研究基于我国10个生物气气藏31个气样的碳、氢同位素组成资料探讨了这些气藏的形成途径和成藏特征。研究表明:这些气藏的氢同位素组成可以分为三个区间,即δDCH4>-200‰;δDCH4值在-250‰～-200‰之间和δDCH4<-250‰。前人认为在陆相淡水条件下生物气的形成途径主要是乙酸发酵作用,我国10个气藏31个气样碳、氢同位素研究表明,海相及盐湖相条件下生物气形成途径为典型的CO2还原途径,具有重的氢同位素组成,其δDCH4>-200‰,而陆相条件下成藏的生物气也主要为CO2还原途径,但氢同位素组成较典型海相成因生物气轻,其δDCH4值在-250‰～-200‰之间。其值可能与古湖泊水介质的咸化程度有关。从柴达木的资料来看,随水介质咸度增大,生物甲烷氢同位素组成也具有相应增大趋势。陆相条件下有处于CO2还原和乙酸发酵两种作用过渡区形成的生物气气藏,其形成可能与古水介质无咸化过程和地温梯度较高有关,如保山盆地。该区形成的生物气具有轻的氢同位素组成,δDCH4<-250‰,碳同位素组成则相对较重,其碳、氢同位素组成之间具有较好的负相关。生物气碳、氢同位素组成的成气机理及途径有可能成为判识自然界采集的生物气气样是否具有工业意义,一般而言,乙酸发酵途径形成的生物气不利于成藏。

The carbon and hydrogen isotopic composition of biogenic gas could indicate its forming mechanism and pool-forming characteristics. In this paper,the forming ways and pool-forming characteristics of the biogenic gas are researched according to the carbon and hydrogen isotopic composition of thirty-one samples from ten biogenic gas reservoirs. The studies show that the hydrogen isotopic composition of these biogenic gas reservoirs can be divided into three zones,i. e. δDCH4 -200‰,-200‰ δDCH4 250‰,and δDCH4 -250‰. The research results of forerunner showed that the main way of biogenic gas forming under the condition of continental fresh water facies is acetate fermentation. Our research results show that the typical way of biogenic gas forming under marine and saline lake facies is CO2reduction,and the biogenic gas have heavy hydrogen isotopic composition,such as the values of δDCH4 are greater than -200‰; however,the main way of biogenic gas forming under the condition of land facies is the same way too,but they have light hydrogen isotopic composition which may relate to salty degree of ancient lakes as water medium,such as the values of δDCH4 are between -200‰ and -250‰. As compared with the relational data of Qaidam Basin,it is showed that the hydrogen isotope composition of the biogenic methane has the same trend as salty degree of water medium increases. The biogenic gas reservoir formed in continental transition region,in which CO2 reduction generated CH4 in company with fermentation together,may relate to constant salinity of ancient water medium and relatively high geothermal gradient,such as Baoshan basin. It has light hydrogen isotope,such as the value of δDCH4 less than-250‰,and heavy carbon isotope,which result in a negative correlation between its carbon and hydrogen isotope. The carbon and hydrogen isotope composition of biogenic gas controlled by the formation mechanism and ways can be used to identify whether it has industrial utilization value. In general,the biogenic gas from acetate fermentation is not favorable for the formation of gas reservoir.