Crude oil analyses

such as group compositions

carbon isotopes and gas chromatograms of saturated hydrocarbon

as well as characteristics of terpane

sterane and other biomarkers

indicate that crude oil in Yanchang-Yan'an Formations in Ansai-Fuxian area have similar geochemical behaviors and are of low-mature-mature oil.The content of saturated hydrocarbon is over 70%

and the carbon isotope value is relatively light with δ

13

C commonly lower than-31‰.The distribution of normal alkane is complete and appears as single-peak type

with the main peak carbons to be C

17

-C

23

.The biomarkers of the oil are characterized by the followings:the content of pentacyclic terpane is larger than that of tricyclic terpane

with C

30

hopane and C

29

norhopane as the dominant components

and gammacerane as the minor component;sterane is dominated by C

27

cholestane and C

28

ergostane and C

29

sitostane;the homosteranes appear to be in asymmetrical"V"distribution

with relatively low C

28

sterane content and about the same C

27

and C

29

sterane contents.Analyses of geochemical beha-(viors) and correlation of oil and source rocks show that the semideep-deep lake facies sapropelic-mixed type mudstones that occur between Chang 8 and Chang 4+5 reservoirs are the main source rocks.The characteristics of biomarkers reveal that the oil has a certain migration effects.