An abnormal assemblage of molecular markers and its geochemical significance in the Da'anzhai Member of the central Sichuan Basin

An abnormal assemblage of molecular markers was found in the source rock extracts from the Da'anzhai Member in the central Sichuan Basin, including extremely rich in rearranged hopanes, rearranged drimanes, rearranged steranes and C₂₆₊ long-chain tricyclic terpanes.Integrated with the organic matter source, depositional environment and source rock maturity, the significance of the above-mentioned special steranes and terpenes assemblages is discussed.The results show that rearranged compounds and C₂₆₊ long-chain tricyclic terpanes have the similar enrichment pathways, among which the enrichment pathway of rearranged hopanes is more than that of rearranged drimanes, rearranged steranes and C₂₆₊ long-chain tricyclic terpanes.The special assemblages of steranes and terpenes may indicate that the organic matter is mainly contributed by lagoonal autochthonous sources(algae and bacteria, etc.) and formed in a restricted weakly reduction environment.The high abundance of rearranged compounds indicate that the organic matter has been thermally mature.In addition, it may indicate specific acidic clay minerals and calcareous sediments catalysis and special biotic community in the depositional environment.In summary, this study has an important demonstration significance for the geochemical application of abnormal assemblages of molecular markers.