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-4oil 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 agenetic 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.