The paper presents the first results of a detailed study of hydrocarbons of the oil series, characteristic of oil and gas production facilities, which is carried out in the laboratory of reservoir fluids research of RN-BashNIPIneft LLC as part of the development of a new direction: "Geochemical studies of organic matter and oil associated with domanic deposits". Peculiarities of the composition of high-molecular alkyltoluols (AT) with an alkyl chain of regular structure in the oil of the Timan-Pechora region are considered. Earlier studies allowed to conditionally divide paraffins into six types based on the molecular mass distribution. In this paper, one of the selected types, specific to the Lower Devonian - Silurian deposits, is analyzed. An attempt is made to consider general patterns in the composition of this type of oil without reference to a specific well. For detailed studies, one of the aromatic groups was taken, which is high-molecular AT. The patterns in their composition duplicate the patterns of n-paraffins and at the same time carry additional geochemical information. High-molecular AT has become more often used to solve various geochemical problems, of particular interest are compounds with a normal alkyl chain (nAT), the structure and composition of which is associated with both the genetic type of the original organic substance, and a variety of its transformation processes. The high informational value of nAT is due to the presence of three main isomers in them. The content of isomers makes it possible to establish the thermodynamic state (stability) in each isomers group and for oil as a whole. This greatly expands the possibilities for establishing the thermocatalytic transformation of oil along with other methods. At the same time, in oils there is a significant difference in the thermodynamic state between the groups of alkyltoluene isomers of even and odd compositions. Thus, the ratio of isomers in groups of even composition is more thermodynamically transformed with respect to compounds of odd composition. This was facilitated by a longer time period of their stabilization than that of compounds of odd composition. The presence of different time periods indicates different stages of the formation of the composition of AT in oil.
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