Gas Source of Carbon Dioxide and Its Degassing Model in Songliao Basin
松辽盆地二氧化碳的气源及其脱气模式

Normal hydrocarbon gas and CO2 coexist widely and a number of gas reservoirs with large volumes of carbon dioxide have been discovered in Songliao basin. Carbon dioxide can be sourced from inorganic, organic and mixed origins, in which the inorganic origin is dominated. Based on comprehensive analysis of carbon dioxide natural gas and regional geology, gas source of carbon dioxide and its degassing model are discussed. Study reveals that carbon dioxide in Songliao basin can be from mantle-derived magma, volcanic rocks, carbonate rocks and organic matters. Mantle-derived magma in crystalline basement, especially Cenozoic alkali basaltic magma, is the most significant gas source. Volcanic rocks in sedimentary cover are another significant gas source, and medium-acidic volcanic rocks of Yingcheng and Denglouku Formations could be principal gas source in east Changde gas pool. Carbonic-Permian carbonate rocks in metamorphic basement need more attention to study as gas source in future exploration. There are abundant organic matters in sedimentary cover and metamorphic basement, but its contribution to form large volumes of carbon dioxide accumulations is little. Degassing models varies with different gas sources. There are four types of degassing models for mantle-derived carbon dioxide to migrate into basins, including direct degassing through lithospheric faults from the upper mantle, degassing from mantle diapir bodies, degassing through low\|velocity and high\|conduction zones as intermediate repeater and basement faults, and after-releasing of absorbed gases by volcanic rocks. Degassing of carbonate rocks in contact and dynamic metamorphisms is favorable for carbon dioxide accumulations.