2.2 DAI Jinxing

Discrimination of abiogenic and biogenic alkane gases

Jinxing Dai, Yunyan Ni

Research Institute of Petroleum Exploration and Development, PetroChina, Beijing 100083, China

In nature hydrocarbons are formed largely by the digestion of organic compounds by microorganisms (microbial origin) and the thermal decomposition of organic matters (thermal origin), which belong to biogenic origin (referring to the thermogenic and bacterial or microbial). A number of recent studies demonstrated the existence of abiogenic alkane gases, which might result from the mantle-degassing or chemical processes such as Fischer-Tropsch synthesis. Since natural gases are mainly dominated by methane, plus small amount of heavy hydrocarbons (C2+) and nonhydrocarbons (CO2, N2, H2S, etc.), the available geochemical information is limited and studies of gas origin and gas source correlation are largely dependent on their chemical composition and carbon, hydrogen and helium isotopes. However, due to insufficient knowledge of abiogenic gases, the discrimination of biogenic and abiogenic gases is a hot debate. In order to further investigate the discrimination criteria of the biogenic and abiogenic gases, we made a comprehensive study on the geochemical data of gases from China, USA, Russia, Germany, Australia and other countries. It is concluded that abiogenic gases are characterized by the following features: 1. δ13CCH4>－30‰ in general; 2. carbon isotope decreases with increasing carbon number among methane and its homologues (δ13C1> δ13C2> δ13C3> δ13C4); 3. R/Ra >0.5 and δ13C1−δ 13C2>0. The discrimination of abiogenic alkane gases will relieve the world oil-gas crisis and open up a new exploration field, which has great economic and scientific impacts.