Abstract:

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 associated with gas hydrate, mineral species and occurrence modes of carbonates are determined in the Qilian Mountain 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 related to gas hydrate decomposition. The variation of C and O isotopic compositions in carbonates with depth suggests that there probably exist hydrocarbon seepage activities within a certain depth, namely decomposition of gas hydrate, resulting in carbonate formation.

Key words: associated carbonate, gas hydrate, C and O isotopes；Qilian Mountain permafrost