Systematic geochemical analyses show that the reservoir bitumen in Feixianguan and Changxing Formations in Puguang gas pool are residuals of the original oil in fossil oil pools after they have been thermally cracked into gas.The organic elementary compositions of the bitumen are characterized by poor hydrogen and rich oxygen and sulfur

suggesting that it is of pyrobitumen as a result of high maturity and TSR.The bitumen has δ

13

C values ranging from-25.60‰ to-27.78‰ and fits isotopically with kerogen in the Upper Permian source rocks(-26.5‰ to-29.2‰)

indicating a genetic relationship between them.The n-alkanes with a complete range of carbon number are observed in the saturated hydrocarbons of the bitumen and no evidences of severe biodegradation occur.They exhibit a phytane predominance with Pr/Ph ratios less than 1.0; predominance of C

27

compounds occur in the steranes with detectable C

30

and C

26

steranes;and the terpanes are characterized by abundant tricyclic terpanes and gammacerane.All these indicate that aquatic organism-dominated marine source rocks have been deposited in a reducing environment.As a result of aromatization of organic matter during the thermal cracking of original bitumen into gas

the aromatic hydrocarbons in the bitumen are characterized by abnormally abundant tetra-and penta-cyclic compounds

and predominated by parent aromatic compounds without alkyl chain

such as fluoranthene

pyrene

chrysene

benzofluoranthene etc.The distribution pattern and MPI index of aromatic hydrocarbons reveal a high thermal evolution for the bitumen with a corresponding R

o

value of over 2.0%.