塔里木盆地玉北地区奥陶系原油成因特征分析

Genesis features of crude oil in Ordovician,Yubei area,Tarim Basin

玉北地区鹰山组原油为低凝固点、含硫、高蜡的中质油,饱和烃色谱正构烷烃分布完整,但色谱基线出现不同程度抬升,且原油非烃和沥青质碳同位素偏轻,族组分碳同位素发生倒转,表明该区原油经历了至少2期成藏过程,早期充注原油遭受生物降解作用后又与后期充注的高成熟原油相混合。地化分析主要表现为后期充注原油特征:弱植烷优势(Pr/Ph≤1.0),较高含量的伽马蜡烷(G/C30藿烷为0.24～0.4)及C35升藿烷/C34升藿烷>1.0的"翘尾巴式"特征,表明后期原油主要来源于还原—强还原生烃环境;正庚烷含量为46.47%～48.57%,C23-三环萜烷优势,较高的三环萜烷/藿烷,指示其生油母质为菌藻类;较高的庚烷值及四甲基萘指数表明原油处于高成熟演化阶段,推测来源于寒武系烃源岩。

The Ordovician oil in Yubei area has middle density, low freezing point and high waxy content. The distribution of normal alkane is complete while the baseline of saturated hydrocarbon chromatogram shows various degrees of excursion. It shows upside-down phenomenon that the δ13C value of nonhydrocarbon and asphaltene is smaller than that of saturated and aromatic hydrocarbon. All of these indicate that the reservoir underwent no less than two periods of oil charging. The early charging oil experienced strong biodegradation and mixed with the lat- er higher mature charging oil. Geochemical properties are mainly attributed to the late charging oil with weak phytane predominance （ Pr/Ph ≤ 1.0）, abundance of gammacerne （ G/C30-hopanes 0.14-0.24 ） , and higher C35-hopane/ C34-hopane value （〉1.0） characteristics, showing reduction-strong reduction sedimentary envi- ronment. The predominance of n-heptane （46.47%-48.57% ）, and C23-tricyclic terpane and higher trieyclic terpane/hopane indicate that the original source materials of later oil are homonemeaes. High heptane value and tetramethylnaphthalene index suggest that the oil is in high mature stage and probably generated from the Cambri- an source rock.