Four series of rearranged hopanes in the Mesoproterozoic sediments

Abstract

Analysis of the Mesoproterozoic molecular markers by gas chromatography–mass spectrometry (GC–MS) and GC–MS metastable reaction monitoring (MRM) revealed four series of rearranged hopanes in the solvent extractable hydrocarbons of the 1.45 billion-year-old Hongshuizhuang and 1.32 billion-year-old Xiamaling formations, the North China Craton. It includes 17α(H)-diahopane (C27D and C29-C35D), 18α(H)-neohopane (C27Ts and C29Ts), early eluting rearranged hopane (C27E, C29-C35E), and 28-nor-spergulane (C29Nsp). The strong positive correlations between different parameters (e.g. C29D/C29H, C30D/C30H, C30E/C30H, C29Nsp/C29H and C29Ts/C29H) indicate an identical genetic biological source and similar formation mechanism. The rearranged hopanes may not originate from widespread bacteria but from the bacterial communities living in a specific environment, which are probably restrained by high salinity water conditions. The variable concentration of rearranged hopanes in the Mesoproterozoic sediments, particularly the extremely low abundance in the sediment of the Gaoyuzhuang Formation (1.56 Ga), is mainly controlled by biological source and sedimentary environment rather than clay minerals. Moreover, the four series of rearranged hopanes could serve as the effective molecular fossils for recognizing the Proterozoic oils in the North China Craton.