Abstract: Oils from different blocks and horizons in Tahe oilfield belong to the same oil group. The difference of stable carbon isotope is not resulted from the parent source.The oils have less distinct differentiation with alkane δ¹³C ranging from -31.8‰ to-33.2‰, while the 813C valne distinction of whole oil, aromat町， nonhydrocarbon and asphaltene fractions of diverse reservoirs become bigger and bigger, es-pecially the Ordovician oils of 4, 6 and 7 districts, which has a lower asphaltene 813C value than the aroma, nonhydrocarbon, and further-more the whole oil of itself. This upside-down phenomenon is due to biodegradation by detailed geochemical analysis. Study shows that there exist two stages of recharging events, with low maturity of oils at early reservoir accumulation and lower carbon isotope value; follow-ing the biodegradation, nonhydrocarbon- and especially asphaltene-dominated group fraction are remained and mixed with late stage of recharging oil, indicating a complex feature of the stable carbon isotope distribution.

Key words: Tahe oilfield, carbon isotope, biodegradation, hydrocarbon accumulation