Improved Understanding of the Alteration of Molecular Compositions by Severe to Extreme Biodegradation: A Case Study from the Carboniferous Oils in the Eastern Chepaizi Uplift, Junggar Basin, Northwest China

Abstract

Biodegraded oils have been widely discovered throughout the world, whereas the alteration of the molecular composition of oils at extreme levels (>PM8) has been insufficiently documented. A suite of crude oils from Carboniferous volcanic reservoirs in the eastern Chepaizi Uplift, Junggar Basin, experienced severe to extreme biodegradation (from PM6+ to PM9+), which provided an ideal case for the present study. This investigation showed that the variations in molecular composition were not strictly consistent with their stepwise fashion in established schemes. The idea that 25-norhopanes are derived from hopanes was confirmed by the sharp decreases in the C29 hopane/gammacerane (C29H/G) and C30 hopane/gammacerane (C30H/G) values at the level of extreme biodegradation, which were associated with the increases in their counterparts of C28 25-norhopane/gammacerane (C28 25-NH/G) and C29 25-norhopane/gammacerane (C29 25-NH/G). 25-Norhopanes were also biodegraded at an extreme level, with C29 25-NH being more susceptible than C28 25-NH. The preferential biodegradation of individual homohopanes by carbon number occurred at an extreme level, whereas C29H featured more bioresistance than C30H and shared a similar susceptibility to biodegradation as 18α-30-norneohopane (C29Ts). The formation of 22R isomers for 25-norhopanes seemed to be favored over that of 22S isomers, although the 22S isomer was degraded faster that the 22R epimer for the C31, C32, and C33 homohopanes. However, the constant values of 22S/(22S + 22R) for the C34 homohopane implied no preferential biodegradation of 22S or 22R isomers for this extended hopane. Lower molecular weight tricyclic terpanes (TTs) were preferentially removed at extreme biodegradation levels, and the late eluting stereoisomers were degraded faster than the early eluting stereoisomers for C26TT, C28TT, and C29TT. C24 tetracyclic terpane (C24Tet) is much more resistant to biodegradation than TTs. Pregnanes have a similar susceptibility to biodegradation as gammacerane, but they are more resistant than C23TT. The biodegradation of regular steranes was characterized by their faster depletion than diasteranes and the preferential depletion of C27 regular sterane to the C29 homologue and the 20R isomers to the 20S isomers. At the extreme level, even C20 and C21 triaromatic steroids (TAS) were distinctively reduced, coexisting with the relatively highly degraded steranes and terpanes, although water washing can also be responsible for the decreases in (C20 + C21)-TAS/C26–28-TAS values.