柴达木盆地北缘牛东地区煤成油及其地球化学特征

摘要

通过对柴达木盆地北缘西段牛东地区和冷湖构造带原油中生物标志物分布与组成特征的系统对比研究,首次提出牛东地区原油属于典型煤成油.其主要特征表现为具有高的Pr/Ph比值(3.0～5.5),在Pr/nC17与Ph/nC18关系图上分布在偏氧化的区域,明显富含树脂体生源的海松烷类标志物,而指示藻类贡献的C27甾烷明显偏低,表明陆源有机质是主要原始生烃母质,且总体偏低的新藿烷和重排藿烷这一特征也与煤和碳质泥岩的形成环境相吻合.而冷湖构造带原油的Pr/Ph比较低(＜3.0),在Pr/nC17与Ph/nC18关系图中分布在氧化—还原环境之间的过渡区域,树脂体生源的海松烷类标志物含量明显偏低,而与藻类输入有关的C27甾烷明显偏高,新藿烷和重排藿烷含量中等,这与淡水湖相泥岩的特征相吻合,表明它们属于淡水湖相原油.柴达木盆地北缘地区下侏罗统不同岩性烃源岩与相关地区原油中海松烷类标志物的相对组成特征表明,牛东地区原油主要来源于煤,而冷湖构造带上的原油主要来源于湖相泥岩,这与该地区下侏罗统煤具有较高的生烃潜力是吻合的,由此表明柴达木盆地北缘地区煤成油的勘探值得关注.%Based on the distribution and compositions of various biomarkers in crude oils from the Niudong area and the Lenghu structure belt in the northern Qaidam Basin,this study proposed for the first time that crude oils from the Niudong area belong to typical coal-derived oil.Those oils are characterized by higher Pr/Ph ratio (3.0～5.5) and plotted in the oxic zone in the diagram of Pr/nC17 and Ph/nCl8,and rich in resinous compounds such as isopimarane and norpimarane,but relatively low C27 steranes related to algae input,suggesting that terrigenous organic matter has made an important contribution.In addition,neohopanes and rearranged hopanes are relatively low in those oils,which is consistent with coal and carboniferous mudstone.Therefore,these oils in the Niudong area should derive from coal-bearing source rocks.As for crude oils from the Lenghu structure belt,their Pr/Ph ratios are relatively low (＜3.0) and they are plotted in transitional zone between oxidation and reduction in the diagram of Pr/nC17 and Ph/nC18.The contents of isopimarane and norpimarane are obviously low,but C27 steranes related to algae input are relatively highter,showing that planktons are an important contribution,but resinite is significantly limited.Moreover,neohopanes and rearranged hopanes are moderate in those oils,consistent with fresh-water lacustrine source rocks,suggesting that they are originally lacustrine oils.The correlation results from the relative abundances of isopimarane and norpimirane between the lithologically different J1 source rocks and the related oils in the study area show that the crude oils from the Niudong area are mainly derived from coals,not from carboniferous mudstones,and consistent with higher hydrocarbon potential of J1 coals in the study area.But the crude oils from the Lenghu structure belt were mainly generated by lacustrine mudstones.Therefore,much attention should be paid to coal-derived oil exploration in the northern Qaidam basin.