四川盆地东部天然气地球化学特征与TSR强度对异常碳、氢同位素影响

TSR Impact on Abnormal Carbon and Hydrogen Stable Isotopes in Sour Natural Gas from Marine Carbonate Gas Fields in the Eastern Sichuan Basin

对四川盆地东部50个天然气样品组分和碳、氢同位素组成分析结果显示,天然气以烃类气体为主,干燥系数高（C1/C1＋=0.975-1.0）,H2S含量变化较大（H2S=0.00%-16.89%）。利用烷烃气碳、氢同位素组成和判识油型气热演化程度图版,确定四川盆地东部天然气主要为原油裂解气,且热演化程度已处于油气裂解阶段。在四川盆地东部,烷烃气碳、氢同位素组成普遍存在局部倒转现象,即δ^13C1〉δ^13C2〈δ^13C3和δD1〉δD2,这主要与研究区域不同硫酸盐热化学还原作用（TSR）强度有关,因为在该反应过程中不仅会产生大量的CH4,其碳同位素较重,同时,水参与了硫酸盐与烃类的化学还原反应使得水中的H＋与烃类中H^＋发生同位素交换,从而引起TSR生成CH4的氢同位素分馏大于干酪根直接生烃过程造成的氢同位素分馏。异常δ^13CCO2值与TSR反应过程中部分碳同位素较轻的CO2与硫酸盐中金属离子（Mg^2＋、Fe^2＋、Ca^2＋等）以碳酸盐的形式沉淀后,导致气藏中残余重碳同位素组成的CO2与酸性气体腐蚀碳酸盐岩储集层形成的CO2相混合有关。

According to gas chemical and carbon and hydrogen isotopic compositions for 50 gas samples from the eastern Sichuan Basin,the natural gas mainly consists of hydrocarbon gas with high dryness coefficient（ C1/ C1＋ = 0. 975 - 1. 0）and various H2 S contents（ H2 S = 0. 00% - 16. 89%）,and the hydrocarbon gas was mainly derived from secondary cracking of oil and gas. In the eastern Sichuan Basin,the phenomenon of partial reversal for stable carbon and hydrogen isotopes of gaseous alkane is widespread,i.e.,δ^13C1δ^13C2δ^13C3and δD1δD2. This abnormality of carbon and hydrogen isotopic composition is mainly caused by different extent of thermochemical sulfate reduction（ TSR） alteration. The TSR process produces a large amount of methane,which enriches heavy carbon isotope. The H^＋isotopic exchange between H2O and hydrocarbons during the TSR process resulted in that hydrogen isotope fractionation in methane produced from TSR is higher than directly from hydrocarbon generation. The CO2 with abnormal δ^13C and CO2 generated from calcite dissolution during TSR process,which has a relatively light carbon isotope composition,precipitated with metal ions in sulfate,such as Mg^2＋,Fe^2＋and Ca^2＋,to form carbonates.