用高分辨率质谱揭示塔中4油田原油成因机制

塔里木盆地塔中4油田位于塔中构造较高部位, 具有显著不同于周边原油的特征.采用高分辨率质谱、色谱-质谱等对原油成因进行了调查.GC/MS分析表明, 塔中4油田石炭系原油中链烷烃(指示未降解-轻度降解)极其发育, 同时检测到丰富的降解三环萜烷及25-降藿烷系列(指示强烈生物降解), 反映该区油气具有多期充注特征.高分辨率质谱分析显示, 塔中4油田原油中硫化物分布正常, 低等效双键数(DBE)硫化物不太发育, 天然气组成与碳同位素也未显示异常, 反映石炭系原油可能未受TSR作用影响.观察到塔中4油田原油中硫化物组成与分布特征多数与下奥陶统原油相近, 指示两者可能有一定成因联系.混源模拟实验显示, 下奥陶统原油混入可导致塔中4油田原油出现高芳香硫特征.地质地球化学综合研究认为, 塔中4油田原油中芳香硫异常与深源油气混合有关, 与TSR作用关系不明显. 

Formation Mechanism of the Oils in Tazhong-4 Oilfield Analyzed by High Resolution Mass Spectrum

The Carboniferous oil in the Tazhong-4 Oilfield situated in the tectonic high of the Tazhong uplift, is quite different from the surrounding oil. High resolution mass spectrum and GC-MS is utilized to reveal the genetic mechanism of the oil. It is observed that the Tazhong-4 oil are abundant in both n-alkanes (suggesting no or little biodegradation) and biodegraded tricyclic terpanes as well as 25-norhopanes (indicating strong biodegradation) based on GC-MS analysis, which suggests multiple phases of petroleum charging in the area. No abnormal sulfur compounds are detected from the Tazhong-4 oils by FT-ICR MS, namely little sulfur compounds with low values of double-bond equivalent (DBE) detected, and no abnormality is observed for the gases associated with the Tazhong oil from views of both the composition as well as distribution and the stable carbon isotope, which suggests no or little TSR alteration of the Tazhong-4 Carboniferous oil. It is also found that the composition and relative distribution of the sulfur compounds in the Tazhong-4 oilfield is similar to that of the majority of the Lower Ordovician oil, suggesting a genetic affinity between them. It has been testified by experiments that oil-mixing of deep oil such as the Low Ordovician oil with the shallow oil could result in a kind of high dibenzothiophen (DBT) oil like the Tazhong-4 oil. After a comprehensive geochemical and geological investigation, we believe that it was deep oil-mixing that caused the unusually high DBT oil in the Tazhong-4 oilfield rather than TSR.