塔河油田原油稳定碳同位素特征及其成藏意义

Stable Carbon Isotope and Its Significance in Hydrocarbon Accumulation in Tahe Oilfield,Tarim Basin

塔河油田田油族组分中烷烃δ13C值为-33.3‰--31.5‰,差异不明显,但全油、芳烃、非烃、沥青质δ13C值的差别却逐渐增大,特别是4、6、7区部分奥陶系原油出现沥青质δ13C值低于非烃、芳烃组分,甚至低于全油的"倒转"现象,详细的地球化学分析表明,这种同位素"倒转"现象主要是由于生物降解造成的;塔河油田存在两期原油充注的成藏历史,早期成藏原油成熟度较低,具有相对较低的碳同位素值,在经历了生物降解后残留以非烃,特别是沥青质为主的族组分,与后期充注来的原油相混合,呈现复杂的稳定碳同位素值分布特征。

Oils from different blocks and horizons in Tahe oilfield belong to the same oil group.The difference of stable carbon isotope is not resulted from the parent source.The oils have less distinct differentiation with alkane δ13C ranging from-31.8‰to-33.2‰,while the δ13C value distinction of whole oil,aromatic,nonhydrocarbon and asphaltene fractions of diverse reservoirs become bigger and bigger,especially the Ordovician oils of 4,6and 7 districts,which has a lower asphaltene δ13C value than the aroma,nonhydrocarbonand furthermore the whole oil of itself.This upside-down phenomenon is due to biodegradation by detailed geochemical analysis.Study shows that there exist two stages of recharging events,with low maturity of oils at early reservoir accumulation and lower carbon isotope value;following the biodegradation,nonhydrocarbon-and especially asphaltene-dominated group fraction are remained and mixed with late stage of recharging oil,indicating a complex feature of the stable carbon isotope distribution.