风化作用对川西北剖面露头固体沥青有机地球化学特征的影响

Impact of natural weathering on the organic geochemical characteristics of solid bitumen in an outcrop profile in the Northwestern Sichuan Basin

在野外露头剖面上沥青会受到风化作用影响,包括生物降解、淋滤和光氧化降解等,这些风化作用会不同程度地改造沥青的有机地球化学特征。目前对沥青的有机地球化学特征在风化过程中的变化过程及其机理还缺乏系统的认识,尤其是多环芳香烃类和含杂原子极性化合物,用常规手段很难深入研究。本研究在川西北地区油坊沟组出露地表的软沥青丘钻了一口0.80 m浅钻,利用岩石热解(Rock-Eval)、气相色谱-质谱(GC-MS)和傅里叶变换离子回旋共振质谱(FT-ICR MS)等手段,研究了剖面沥青的有机地球化学特征在风化过程中的变化。结果显示,在剖面顶部(0.05~0.15 m),风化作用使沥青的总有机碳(TOC)和非极性族组分损失明显,极性族组分相对含量升高,O/C原子比的增加表明,剖面顶部沥青受到了较强的氧化作用。正构烷烃、C_(27)~C_(29)规则甾烷、藿烷、二苯并噻吩和甲基二苯并噻吩等分布特征的变化显示,剖面顶部沥青同时遭受了显著的生物降解和淋滤作用。奇偶优势指数(OEP)、C_(29)甾烷异构化参数及甲基菲指数(MPI_(1))等成熟度参数指示剖面沥青进入成熟阶段,但藿烷类成熟度参数(Ts/(Ts+Tm))和烷基萘指数(DNR和TNR)指示的成熟度偏低。FT-ICR MS结果表明,多环芳烃和含硫多环芳烃在风化过程中损失,其风化产物主要是含氧的杂原子多环芳烃。在0.15~0.80 m,多环芳烃和含硫多环芳烃无明显降解特征;在0.10~0.15 m,碳数较低且缩合度(DBE)较低的多环芳烃被生物降解为碳数更低的酚、醇类化合物和羧酸;在深度≤0.10 m,碳数较高且DBE较高的多环芳烃和含硫多环芳烃被光氧化,主要产生相似碳数和DBE的醛、酮类化合物和羧酸。

Natural weathering alters the organic geochemical characteristics of solid bitumen in outcrop profile to various extents,including biodegradation,leaching,and photooxidation.However,the impacts of natural weathering on the organic geochemical characteristics of bitumen have not been systematically investigated.A soft bitumen mound in the Youfanggou Formation in the northwestern Sichuan Basin provides a perfect opportunity to study the natural weathering process.In this study,a 0.80 m deep well was drilled in this bitumen mound.The organic geochemical characteristics of the solid bitumen in this well were investigated using Rock-Eval,gas chromatogram-mass spectrometry(GC-MS),and Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS).The results showed that the contents of the total organic carbon(TOC)and nonpolar oil fractions decreased with the increasing polar oil fractions as the weathering progressed at the top of the profile(0.05 m to 0.15 m).The increase in the O/C ratio also indicated that the bitumen on the top suffered from extensive oxidation.In addition,severe biodegradation and leaching were illustrated by the consumption of relatively lower carbon number molecules(short chain n-alkanes,C_(27)-C_(29)regular steranes,hopanes and dibenzothiophene,methyldibenzothiophenes,etc.)of bitumen in this interval.Thermal maturity parameters,such as Ts/(Ts+Tm),DNR,and TNR,indicated lower maturity than odd-to-even predominance(OEP),C_(29) sterane isomerization,and MPI_(1).The FT-ICR MS results showed the loss of PAHs and polycyclic aromatic sulfur heterocycles(PASHs)and the relative enrichment of oxygen-containing polycyclic aromatic hydrocarbons during weathering.No obvious degradation of PAHs and PASHs appeared in the 0.15-0.80 m interval,but in the 0.10-0.15 m interval,PAHs and PASHs with less carbon number and condensation(DBE)were gradually biodegraded to phenols,alcohols,and carboxylic acids with shorter carbon chains.In 0-0.10 m,PAHs and PASHs with more carbon numbers and DBE were transited to aldehydes,ketones,and carboxylic acids with similar carbon numbers and DBE distributions by photooxidation.