Extended hopanes and 8,14-secohopanes up to C40 in unbiodegraded marine oils from the Tarim Basin, NW China

Extended 17α(H), 21β(H)-hopanes and three series of 8,14-secohopanes up to C40, including 8α(H), 14α(H), 17α(H), 21β(H)-, 8α(H), 14α(H), 17β(H), 21α(H)- and 8α(H), 14β(H), 17β(H), 21α(H)-, were detected by GC-MS-MS method in the branched/cyclic hydrocarbon fractions of some unbiodegraded marine oils from the Tazhong uplift in the Tarim Basin, NW China. The coexistence of extended hopanes and 8,14-secohopanes up to C40 in unbiodegraded oils suggests that they are primary and independent on biodegradation. The similarity of distribution and composition for extended hopanes and 8,14-secohopanes up to C40 in unbiodegraded oils proposes that they could be derived from a similar biological precursor. However, an abrupt decrease up to 3–5 times in the relative abundance from C35 to C36 in C31-40 extended hopanes and extended 8,14-secohopanes suggests that C31-35 and C36-40 extended hopanes and extended 8,14-secohopanes should have their own biological precursor. The known C35 bacteriohopanetetrol should be biological precursor of C31-35 extended hopanes and 8,14-secohopanes, but an unknown C40 functionalized hopanoid could be biological precursor of C36-40 extended hopanes and 8,14-secohopanes. More attention should be paid to their potential roles in oil-source correlation for severely biodegraded oils based on their widespread occurrence in various source rocks, unbiodegraded and severely biodegraded oils.