Five biodegraded oil samples from the north slope of Biyang Depression were analyzed by integrating the two-dimensional gas chromatography/time-of-flight mass spectrometry(GC×GC/TOFMS)

gas chromatography(GC)and gas chromatography-mass spectrometry(GC-MS)to reveal the composition and genesis of ‘baseline hump’.All the oil samples analyzed are of similar original compositions and thermal maturities

but different degradation levels at 2

3

5

6 and 8

respectively.With the increasing of oil degradation level

n-alkanes

iso-alkanes

aromatic hydrocarbons with low molecular weight

steranes

terpanes and polycyclic aromatic hydrocarbons are degraded successively

resulting in the significant decrease of the number of compound types.The cyclic alkanes and isomers with similar polarities and strong resistance to biodegradation are enriched relatively in the biodegraded oil

and their co-elution in GC chromatograms results in ‘baseline hump’ or so-called ‘unresolved complex mixtures (UCMs)’.Total 2D characteristic mass spectrums and standard compound tests show that cyclic alkanes and their homologues are the major components of the UCMs

with a basic framework of cyclohexane.Additionally

the UCMs can be divided into six types(typeⅠto typeⅥ).The typeⅠandⅡare composed of C

16

-C

26

long chain alkyl-cyclohexanes

C

11

-C

16

alkyl-decalins and their isomers.These cyclic alkanes with one or two rings might be the new compounds from the microbial transformation of normal oils.Type Ⅲ-Ⅵ are composed of sesquiterpenoids

tricyclic terpanes

low molecular weight steranes

diasteranes and norhopanes

originated from the enrichment of hydrocarbons having relatively strong resistantance to degradation in the normal oils.