Abstract:The bitumen from the Kaitang and the Luomian outcrop sections in the Kaili residual reservoir in southern Guizhou depression have been severely biodegraded. Their hydrocarbon compositions were analyzed in detail by aid of gas chromatography- mass spectrometry (GC- MS) and gas chromatography- mass spectrometry- mass spectrometry (GC- MS- MS) to determine their exact biodegradation extent and discern whether there remain some primary biomarkers suitable for the severely biodegraded oil- source correlation. Analytical results show that in the reservoir bitumen from the Kaitang outcrop sections, relatively whole C19- 30 tricyclic terpanes and C27- 35 hopanes could be detected, but secondary biomarkers derived from biodegradation of crude oils such as C19- 29 demethylated tricyclic terpanes and C26- 34 25- norhopanes are abundant and complete, C21- 22 steranes and diasteranes are dominant in steroids, suggesting that the bitumen has been severely biodegraded. Because triaromatic steroid biomarkers are preserved in the bitumen, their biodegradation may have been up to 8- 9 level based on the known criteria to determine the biodegradation extent of crude oils. However, in the reservoir bitumen from the Luomian outcrop section, hopanes have been almost destroyed, the distributions of C19- 30 tricyclic terpanes and their demethylated counterparts and 25- norhopanes are completely different from those in the bitumen from the Kaitang outcrop section, some biomarkers such as C23T, C24T, C22NT, C23NTE and C26- 29 NH are dominant components, triaromatic steroids biomarkers have completely disappeared, demonstrating that the biodegradation extent of the bitumen in this section may be up to 10 level or more. Because the severe biodegradation has changed the distributions and compositions of common steranes and terpanes in the bitumen from two sections, their practical values have been completely lost. However, in the severely biodegraded bitumen from two sections, three series of C27- 35 8,14- secohopanes are detected. It is noted that this kind of biomarkers are present also in the marine end- member oils from the Tazhong uplift in the Tarim Basin, with a very similar distribution, suggesting that they are primary biomarkers and not related to biodegradation of crude oils. Moreover, their existence in the severely biodegraded bitumen show that they have unusual resistance to biodegradation may have potential practical value in the oil- source correlation of severely biodegraded oils.