Composition Characteristics and Genesis of four types of high-abundance Rearranged Hopanes in Coal-measure Source Rocks: A case Study of Upper Paleozoic in Ordos Basin

Key words: 
• Rearranged hopane /
• main controlling factors /
• sedimentary environment /
• coal measure source rock /
• Ordos Basin

Abstract: [Objective]In order to explore the composition characteristics and main controlling factors of high-abnormally high abundant rearranged hopanes in coal-measure source rocks. [Methods]The saturated hydrocarbons and aromatic hydrocarbons of 29 coal measure source rocks in the Upper Paleozoic of Ordos Basin were analyzed in detail by gas chromatography-mass spectrometry (GC-MS). [Results] According to the peak order and retention time of compounds, GC-MS analysis of coal measure source rocks in Ordos Basin was carried out. Four types of rearranged hopanes with different abundances were systematically identified, which were 17α (H) -rearranged hopane series (C30*), 18α(H) -neohopane series (Ts and C29Ts), early-eluting rearranged hopane series (C30E) and 21-methyl-28-norhopane series (29Nsp). And the peak order of the four types of rearranged hopane compounds is: early-eluting rearranged hopane series > 17α (H) -rearranged hopane series > 18α (H) -neohopane series > 21-methyl-28-norhopane series. The study of the internal composition of four types of rearranged hopane compounds in the coal-bearing source rocks in the study area shows that there is a good correlation between the same series of rearranged hopane compounds; the correlation between different series of rearranged hopanes is different. The correlation between C30*/C30 hopane, C30E/C30 hopane and 29Nsp/C29 hopane is good, but the correlation between C29Ts/C29 hopane and the above three is poor, indicating that the formation mechanism of different series of rearranged hopanes may be different. In addition, The Pr/Ph values of the high-abnormally high abundant of 17α (H)- rearranged hopane series, early-eluting rearranged hopane series, 21-methyl-28-norhopane series in the coal source rock samples in the study area are distributed between 1.0 and 2.0, the Gammacerane（G）/C30 hopane values are mostly distributed around 0.13, and the relative content of dibenzofuran is between 11.67 % and 55.26 %, with an average value of 35.51%. To put it another way, the sedimentary environment of coal-measure source rocks has a great influence on the relative abundance of these rearranged hopane compounds. With the increase of thermal evolution degree of organic matter, the relative abundance of these rearranged hopane compounds shows an approximate normal distribution. In the peak stage of oil generation (Ro is between 0.8%and 0.9%), the relative abundance of high-abnormally high abundant rearranged hopanes compounds reaches the peak. What’s more, The ratios of (C28+C29) tricyclic terpanes（TT）/C30 hopanes and ∑regular steranes / ∑C30-35 hopanes in high-abnormally high abundant rearranged hopanes samples were significantly positively correlated with the ratios of four types of rearranged hopanes, revealing that the biogenic parent materials of high-abnormally high-abundant rearranged hopanes are mainly lower aquatic organisms and algae compounds.[Conclusion] Through the same evolution path and formation mechanism of the same series of rearranged hopanes, it is speculated that the main controlling factors for the formation of 17α (H) -rearranged hopane series, early-eluting rearranged hopane series, 21-methyl-28-norhopane series high-abnormally high rearranged hopanes in coal-measure source rocks are mainly weak oxidation-weak reduction, bacterial hopane precursors developed in brackish water sedimentary environment ; the formation of high-abnormal high-abundance 18α (H)-neohopanes is mainly controlled by the biogenic parent material of organic matter, which may be limonene or C29 hopane compounds.