Abstract: The origin of natural gas of Xujiaweizi fault depression in Songliao basin is heavily debatable on that account whether it is biogenetic or abiogenetic. There are different interpretations on the origins of natural gas in the nature. The carbon isotopic fractionation can be taken place in Fischer-Tropsch synthesis just like in plants' photosynthesis possibly without indication of reverse distribution of carbon isotope. Fischer-Tropsch reaction under the hydrothermal conditions is very common in the nature, but the kinetic fractionation mechanism of carbon and hydrogen isotopes is not clear. Carbon isotope used for the trace distinguishing of oil-gas sources is still of great uncertainty. The deep crust structures of Songliao basin show that there exists a low velocity-high conductive layer in the mid-crust, in which the mantle fluids enriched with CO₂ , H₂ could take part in reactions of Fischer-Tropsch synthesis and generate hydrocarbon compounds. Based on this thought, much more natural gases could be explored and discovered in deep Songliao basin (including volcanic rocks, granites and other basement rocks). In addition, giant oil-gas fields could be found out in other domestic basins by such a thought.

Key words: Songliao basin, natural gas, inorganic origin, Fischer-Tropsch synthesis, carbon isotope, mid-crust, low velocity-high conductive layer, mantle fluid