祁连山冻土区天然气水合物伴生碳酸盐C、O 同位素特征及其地质意义

摘要

It is a universal phenomenon for gas hydrate associated carbonates in seafloor sediments,but it is rarely reported in the permafrost.Based on microscopic observations and mineral analyses on carbonates associ-ated with gas hydrate,mineral species and occurrence modes of carbonates are determined in the Qilian Moun-tain permafrost.The results show that occurrence modes of carbonates can be divided into four types according to mineral compositions and geological features.Characteristics of C and O isotopes in different occurrence modes of carbonates indicate that the smoky gray rhombic calcite crystals or microcrystalline calcites may be re-lated to gas hydrate decomposition.The variation of C and O isotopic compositions in carbonates with depth sug-gests that there probably exist hydrocarbon seepage activities within a certain depth,namely decomposition of gas hydrate,resulting in carbonate formation.%伴生碳酸盐矿物在海底与天然气水合物伴生是一种普遍现象，但在陆上冻土区中报道较少。以近两年在祁连山冻土区发现天然气水合物伴生的碳酸盐矿物为研究对象，根据对含碳酸盐样品的显微镜观察及矿物分析，确定了伴生碳酸盐的矿物种属及赋存状态。按碳酸盐矿物组成及地质产状的不同，其赋存状态分成4种类型，即白色薄层状、烟灰色菱形晶簇状、深灰色薄壳状、微细浸染状。根据不同赋存形态碳酸盐的 C、O 同位素特征，认为烟灰色菱形晶簇状方解石或呈（云烟状）微晶方解石可能与天然气水合物分解有关。碳酸盐 C、O 同位素随深度变化特征表明在一定深度处可能存在着烃类物质的活动，即天然气水合物分解，导致碳酸盐的矿物生成。