Chagan Sag is a Meso-Cenozoic fault depression.The crude oil in it is of low density

low viscosity

low sulfur content

mid paraffin and lightweight oil characterized by high saturated hydrocarbon contents(80%～90%)and rich C

27

sterane.The second member of Bayingebi Formation(K

1

b

2

)and the first member of Suhongtu Formation(K

1

s1)of the Lower Cretaceous are potential source rocks of the sag.The ratio of ααα(20R)C

27

/C

29

and ααα(20R)C

28

/C

29

of crude oil and the extract from oil bearing sand is 0.39～0.75 and 0.4～0.7 respectively.The ratio of Pr/Ph and Pr/nC 17 is 0.79～0.94 and 0.83～1.34.The carbon isotopic characteristic curve of crude oil is more similar to dolomitic mudstone of K

1

b

2

and differs from the dark grey mudstone of K

1

s1.Synthetical correlation shows that the crude oil is mainly derived from the second member of Bayingebi Formation(K

1

b

2

)dolomitic mudstane.