E.A. Vtorushina, T.D. Bulatov, E.V. Kozlova, M.G. Kul’kov
Pyrolysis criteria for assessing thermal maturity of organic matter in the Bazhenov Formation
Key words: Organic Matter; kerogen; catagenesis, Bazhenov Formation; triangular diagram; Rock-Eval pyrolysis; kinetics; molecular parameters; biomarkers.
For citation: Vtorushina E.A., Bulatov T.D., Kozlova E.V., Kul’kov M.G. Pyrolysis criteria for assessing thermal maturity of organic matter in the Bazhenov Formation. Geologiya nefti i gaza. 2022;(4):53–63. DOI: 10.31087/0016-7894-2022-4-53-63. In Russ.
For the purposes of resource development and commercial development of kerogen-saturated deposits of the Bazhenov Formation in the West Siberian Petroleum Basin, a reliable evaluation of organic matter thermal maturity is required. Catagenesis evaluation based on Rock-Eval pyrolysis parameters providing good repeatability and promptness of analysis is best possible for “oil shales”. The authors present pyrolysis data for five wells drilled on the territory of Krasnoleninsky Arch and Frolovsky Megadepression. Studies of rock samples were carried out using HAWK RW pyrolyzer; the following main parameters and indices were determined: S0, S1, S2, Tmax, PI, GOC, NGOC, TOC, HI, OSI, and Kgoc. With the purpose of improving reliability of organic matter thermal maturity determination for the Bazhenov rocks using a set of pyrolysis parameters, the authors propose a new “tool” (triangular diagram in Tmax, HI, and Kgoc coordinates. This approach allows determining catagenesis stage for the certain Bazhenov sections through pyrolysis studies of rock samples before and after exhaustive chloroform extraction. Zones corresponding to different catagenesis stages (from PС3 to MС3 according to N.B. Vassoevich scale) are delineated in triangular diagram created using pyrolysis data after extraction. In the case the pyrolysis data before extraction are used, position of points outside the zones selected on triangular diagram will indicate an increased oil saturation of the section intervals. However, thermal maturity of oil saturated deposits, which is revealed using data before extraction, is not correct because of the presence of newly generated hydrocarbons in the zone of S2 parameter determination. For the wells outside oil saturation zones, linear nature and region of catagenesis stages occurrence will be virtually the same as those in triangular diagrams before and after extraction. Evaluations of organic matter catagenesis stages obtained using triangular diagram for the Bazhenov Formation in five wells under studies are confirmed by the results of time-consuming kinetic and molecular studies.