Could Heavy Carbon Dioxide be of Organic Origin?

Shuai, Yanhua; Zhang, Shuichang; Peng, Pingan; Zou, Yanrong

Carbon dioxide is one of the most common non-hydrocarbon gases found in sedimentary basins. It is of critical importance in investigating the evolution of volatile fluids and in helping to develop regional strategies for natural gas exploration, but its origins remain enigmatic. Carbon dioxide has several possible origins, of which three kinds of sources for carbon dioxide are significant for sedimentary rocks buried at over 80°C: the mantle, carbonate dissolution, and organic substances.

Light δ13CCO2 values have been suggested as a good indicator for discriminating CO2 of organic origin from that of inorganic origin (such as from the mantle or from carbonate mineral dissolution). However, the fact that carbon dioxide with high δ13C values can be liberated in quantity from deeply buried source rock has been generally ignored. We therefore presents a study of CO2 generation and isotope evolution during an artificial maturing process of coal. The startling results reveal that gases derived from organic matter in the deep crust should be rich in CO2 with heavy isotopic compositions (δ13C ranging from -6‰ to +5‰), which could easily be considered to have a non-organic origin. We will present two cases highlighting organic, isotopically heavy CO2 in natural gases occurring at variable concentrations at different sedimentary backgrounds.

Therefore, some organic CO2 may have been mistaken as having an inorganic origin and hence underestimated in many basins worldwide. By assuming the existence of such carbon dioxide, the origin of certain mysterious CO2 accumulations in many sedimentary basins around the world might be elucidated.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013