The ratio K

reported by Mango(1987)

ranges from 1.09 to 1.16 in the oils produced in north slope of Tazhong area shows relative invariant;the K of oils trapped in the east part of Tazhong fault uplift shows not only high value(1.20～1.54)

but also displays regular changes in the reservoirs.The index of methylcyclohexane shows similar variance to the ratio K.It indicates that the oils trapped in the north slope are different from the oils of light hydrocarbons in the east part of Tazhong fault uplift

and that some of the oils with high K value are formed by mixing procession.