Geochemistry and genesis on saturated hydrocarbon of crude oils in Gas Hure Oilfield in Qaidam Basin

Gas Hure Oilfield is the biggest one in the Qaidam Basin.Through intensive sampling and GC-MS analysis,the authors analyzed the geochemical characteristics of the crude oils in the Gas Hure Oilfield.The results show that the crude oils have even-odd carbon predominance for n-alkanes.In shallow reservoir,part of crude oils suffered biogenic degradation.As for normal oils,they have low Pr/Ph ratios with abundant gammacerane and C35 homohopane,which indicate that the crude oils are formed in strong reducing salt water environment.Compared deep(E13) reservoir with shallow(N1-N12) reservoir,the ancient salinity of sedimentary environment of N1--N12 crude oils is slightly higher than that of E13 crude oils;the C-21/C+22 ratios of crude oils are 1.02-1.71 with a higher abundance of C27 sterane,which shows that their mainly origin matrix are bacteria and algae;the C29 sterane 20S/(20S+20R)is generally low,the ratio in deep E13 reservoir distributes between 0.31 and 0.40,but in shallow reservoir is relatively low with 0.29-0.36.At the same time,C29 sterane αββ/(αββ+ααα)and Ts/Tm both are extremely lower.All the above show that the crude oils in Gas Hure are mainly low mature oils,although the maturity of deep reservoir crude oils is slightly higher than shallow one.