Discovery and Significance of High CH4 Primary Fluid Inclusions in Reservoir Volcanic Rocks of the Songliao Basin, NE China

摘要

Comparing compositions of the fluid inclusions in volcanic rocks to the contents and isotopes of the gases in corresponding volcanic reservoirs using microthermometry, Raman microspectroscopy and mass spectrum analysis, we found that: (1) up to 82 mole% methane exists in the primary inclusions hosted in the reservoir volcanic rocks; (2) high CH4 inclusions recognized in the volcanic rocks correspond to CH4-bearing CO2 reservoirs that are rich in helium and with a high 3He/4He ratio and which show reversed order of δ 13C in alkane; (3) in gas reservoirs of such abiotic methane (>80%) and a mix of CH4 and CO2, the enclosed content of CH4 in the volcanic inclusions is usually below 42 mole%, and the reversed order of δ 13C in alkane is sometimes irregular in the corresponding gas pools; (4) a glassy inclusion with a homogeneous temperature over 900°C also contains a small portion of CH4 although predominantly CO2. This affinity between gas pool and content of inclusion in the same volcanic reservoirs demonstrates that magma-originated gases, both CH4 and CO2, have contributed significantly to the corresponding gas pools and that the assumed hydrocarbon budget of the bulk earth might be much larger than conventionally supposed.