Application of Aromatics on Genesis of Rearranged Hopanes in Coal-bearing Source Rocks

Key words: 
• 17&alpha /
• (H)-diahopane /
• aromatics hydrocarbon /
• coal-bearing source rocks /
• Ordos Basin /
• early-eluting rearranged hopane

Abstract: Relatively high abundances of the 17α(H)-diahopanes and early-eluting rearranged hopanes were found in coal-bearing source rocks from upper palaeozoic sediments in northeast Ordos Basin. On the basis of the characteristics of rearranged hopanes and their distributions, combined with the compounds of the other biomarkers, aromatic hydrocarbons were applied to discuss the origin of rearranged hopanes. The distribution patterns of saturated hydrocarbons show that rearranged hopanes are related to terrigenous higher plants, and a weak oxidizing environment contributes to their formation. Source rocks, which contain high-abnoraml high abundances of 17α(H)-diahopanes and early-eluting rearranged hopanes (C₃₀^*/C₃₀H > 0.2, C₃₀E/C₃₀H > 0.1), are characterized by relatively high concentrations of bicyclic and tricyclic aromatic hydrocarbons (> 50 %) and relatively low concentrations of tetracyclic and pentacyclic aromatic hydrocarbons (< 35%). In addition, aromatic terpenoids and steroids are absent which shows a origin of terrigenous higher plants in source rocks. The characteristics of methylphenanthrenes (MPI₁, MPI₂ and F1) illustrate that rearranged hopanes in source rocks deposited in a weak oxidizing condition (Pr/Ph>1) mainly form in a mature stage, and their contents increase with the thermal maturity. However, source rocks deposited in a reductive environment (Pr/Ph<1) contain relatively low contents of rearranged hopanes and also increase with the thermal maturity. The parameters of dibenzothiophene, dibenzofuran and fluorine indicate that a weak oxidizing sedimentary environment is important to the formation of 17α(H)-diahopanes and early-eluting rearranged hopanes. A swamp environment is the most helpful to their formation.