Analysis of the composition of unrecovered and produced oils of Tatarstan shows that the adsorption-chromatographic process upon oil motion in the reservoir during recovery is manifested in an increase in the density and viscosity of unrecovered oils. They do not contain light hydrocarbons of the IBP -200 degrees C fraction and have dramatically smaller concentrations of both the least polar lube oil hydrocarbons and alcohol-benzene-extractable resins exhibiting the highest polarity. According to GLC data, unrecovered and produced oils are classified with different subtypes of chemical type A1 of nonbiodegraded oils: produced oils are grouped with subtype 1 and unrecovered oils are attributed to subtype 2. The dynamics of the development of areas of produced oils with the use of a technique based on the activation of reservoir microflora showed that the process of the microbiological oxidation manifests itself in the preferential ability of reservoir microflora to digest C(12)-C(34) n-alkane hydrocarbons as compared to cyclic hydrocarbons. Normal C(12)-C(20) alkanes are consumed by bacteria before hydrocarbons with a greater number of carbon atoms C(20)-C(34) of this series. An analysis of extracts from aqueous infusions of oils showed that the dissolution process leads to the transfer of 0.04-0.07 wt % hydrocarbons and heteroatomic components from oil to water.