Abnormal elevations of C34 2α-methylhopane and C34 2α-methylbenzohopane in a Lower Triassic mudstone sample, NW Sichuan Basin

Highly abundant 2α-methylhopane and 2α-methylbenzohopane series were observed in a Lower Triassic mudstone sample, NW Sichuan Basin. Unlike the linear decrease of homohopanes (C31–C35) with increasing carbon numbers, the 2α-methylhomohopanes (C32–C36) show a different distribution that is characterized by an abnormal elevation of C34 2α-methylhopane. Correspondingly the C34 2α-methylbenzohopane is also elevated in comparison with C33 benzohopane. The calculated C34 2α-methylhopane index and C34 2α-methylbenzohopane index is up to 42% and 31%, respectively. Pyrolysis data show that the abnormal elevation of C34 2α-methylhopane can be preserved to the thermal stage of Ro ≈ 0.97%. These results indicate that there might be unknown and independent precursors of 2-methylbacteriohopanepolyols (2-MBHPs) with side chains that may have a more fragile functional group than hydroxyl group on C-34, and it is the easier bond cleavage between C-33 and C-34 that accounts for the abnormal elevations of both C34 2α-methylhopane and C34 2α-methylbenzohopane in reducing depositional environments.