Songliao Basin has similar favorable geological conditions for biogenic gas accumulation as the Cretaceous basins in North America

including the both dimension and high quality of source rocks.At present

17 gas reservoirs in the Songliao Basin have been discovered to have the features biogenetic gas such as high dryness coefficient(C

1

/ΣC

1+

>

0.95)and light stable carbon isotope(δ

13

C

1

ranging from -60‰ to -50‰).Therefore

much attention has been paid to the biogenic gas potential in this area and study of their occurrence.Ho-wever

comprehensive analysis demonstrates that the discovered biogenic gas reservoirs are dominated by secondary biologic modification gas reservoirs with mixed secondary biogenic gas from biodegradation of thermal oil/gas and residual thermal gas.The following evidences were obtained from detailed geochemical character analysis:significant increase of isoparaffin content and significant decrease of propane due to biodegradation;biodegradation trace of stable carbon isotope ratios of heavy gaseous hydrocarbons;features reflecting biodegradation getting prominent along with the increasing N

2

content;significant biodegradation of liquid hydrocarbons associated with biogenetic gas reservoirs.Biodegradation degree

preservation conditions and the replenishment of thermal gas at later stage are the factors that complicate the characteristics of biodegradation gas.No primary biogenic gas reservoirs have been discovered so far.It is mainly caused by that the Songliao Basin has been in uplifting and cooling since the Late Cretaceous

unfavorable for the sustained generation and preservation of primary biogenic gas.Primary biogenic gas reservoirs possibly occur in lithologic traps in relatively stable blocks.Methods and marks for identification of secondary biogenic gas reservoirs are summarized to guide exploration of shallow biogenetic gases in other areas with similar background.