东北漠河盆地甲烷碳氢稳定同位素异常及形成机制

东北漠河盆地是我国陆域多年冻土区天然气水合物聚集成藏及勘探的潜在区域。近几年,在漠河盆地实施天然气水合物钻探实验井和气源探井的过程中发现,该区甲烷碳、氢同位素δ^13 CCH4、δDCH4值明显偏低,负向偏移特征明显:碳同位素δ^13 CCH4 值多数小于-60‰,最低达-82.9‰;氢同位素δDCH4 值绝大多数低于-350‰,最低达-450‰。通常,埋深小于1 500m的范围,甲烷碳同位素δ^13 CCH4 值随深度增大,氢同位素δDCH4 值随深度减小;深度大于1 500m,甲烷碳、氢同位素δ^13CCH4 、δDCH4 值随深度同步增大。研究表明:漠河盆地的甲烷碳稳定同位素负向偏移,主要与甲烷的微生物成因有关;甲烷氢稳定同位素负向偏移,则是盆地所处较高纬度独特气候引起的地表水蒸发或大气冷凝降水过程的瑞利蒸馏和乙酸发酵的甲烷生成方式共同作用的结果。

Abnormalities of stable carbon and hydrogen isotopes of methane in the Mohe Basin,Northeast China and mechanisms of isotopic change

The Mohe Basin in northeastern China possesses the potential for gas hydrate accumulation and exploration in the permafrost zone of China. During a recent investigation of wells used for gas hydrate testing and gas source exploration, we discovered significant negative shifts in stable carbon and hydrogen isotopes of methane in the Mohe Basin. The measured δ13CCH4 and δDCH4 values commonly ranged from -60‰ to -82.9‰ and from -350‰ to -450‰, respectively. We also observed that at burial depth of approximately less than 1500 m, δ13CCH4 value generally increased with depth, whereas δDCH4 value decreased; however, both values increased synchronously with burial depth at greater than 1500 m. We believe that the negative shifts of stable carbon isotope are associated with the microbial origin of methane; whilst the negative shifts of stable hydrogen isotope are the results of a combined effect of Rayleigh distillation during the evaporation of surface water or atmospheric condensation process driven by basin‘s unique climate at higher latitude, and methane production through acetate fermentation.