Distribution and origin of rearranged hopanes in Yanchang Formation, Wuqi-Gaoqiao area, Ordos Basin

A high abundance of rearranged hopanes has been detected from source rocks of the Triassic Chang 7,8,9 formations in the Wuqi-Gaoqiao area of the central Ordos Basin.We classified 3 types of source rocks and separately analyzed their genesis compared with the characteristics of biomarkers from different formations.The gas chromatography-mass spectrometry (GC-MS) analyses indicated that there were three series of rearranged hopanes in the source rocks,which are 17α(H)-diahopanes,early-eluting rearranged hopanes and 18α(H)-neohopanes.Moreover,early-eluting rearranged hopanes are commonly associated with high abundant 17α(H)-diahopanes and 18α(H)-neohopanes,meanwhile,rearranged drimanes and rearranged steranes are detected in source rocks which also retain high amounts of those three types of rearranged hopanes.This indicates that the genesis and alteration of rearranged compounds are probably affected by similar processes.Many geochemistry parameters showed that, the formation of rearranged hopanes could be closely related with higher plant source materials.Sub-oxic,freshwater and clay-rich depositional environment are beneficial for forming a significant proportion of rearranged hopanes. Higher thermal evolution favors rearranged hopanes;however,the content differences of hopanes affected by different pyrolysis rates at high maturity must be considered.