利用芳烃参数研究煤系烃源岩中重排藿烷成因

研究发现鄂尔多斯盆地部分地区上古生界煤系烃源岩存在较高丰度的17α(H)-重排藿烷和早洗脱重排藿烷,在对重排藿烷分布、组成特征及生标组成特征研究的基础上,应用芳烃参数对高丰度重排藿烷的成因进行了探讨。饱和烃生物标志物组成特征显示,高重排藿烷与陆源高等植物生源关系密切,主要在弱氧化沉积环境下演化形成。具有高-异常高丰度17α(H)-重排藿烷和早洗脱重排藿烷(C₃₀*/C₃₀H>0.2、C₃₀E/C₃₀H>0.1)的烃源岩二环+三环芳烃相对含量一般大于50%,四环、五环芳烃含量相对较低,普遍低于35%,未见芳香甾萜类,研究认为其生源主要为陆生高等植物;甲基菲指数MPI₁、MPI₂和甲基菲比值F1研究结果显示,Pr/Ph>1的弱氧化环境烃源岩17α(H)-重排藿烷和早洗脱重排藿烷在进入成熟阶段后开始大量形成,并随成熟度升高而增大;Pr/Ph<1还原环境下的烃源岩重排藿烷丰度相对较低,但在进入成熟阶段后仍有随成熟度升高而增大的趋势;三芴系列化合物参数表明,沉积环境的弱氧化性质对17α(H)-重排藿烷和早洗脱重排藿烷的形成有较大的影响,沼泽相沉积环境最有利于重排藿烷的形成。

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

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.