Estimates of the Catagenesis Grades of Rocks in Different Modeling Systems: An Example of the Sedimentary Sequence of Well Ostaninsky 438, Tomsk Region, West Siberian Basin

Calculations of the vitrinite reflectance (%Ro) and the hydrocarbon generation by the probable source rocks in the Ostaninsky area of the West Siberian basin demonstrate noticeable differences in TeploDialog and GALO estimates of the maturation degree of the organic matter, hydrocarbon generation, and the chronological sequence of the generation of liquid and gaseous hydrocarbons. It is important that both the TeploDialog and the GALO modeling systems make use of the same data on the modern sedimentary sequence of the basin and the same measurements of rock temperatures and vitrinite reflectance in this sequence. Calculations with the application the thermochronologic evolution of the formations reconstructed by the paleotemperature method in the TeploDialog system suggest that the generated liquid hydrocarbons had completely degraded during the period of high heat flow from the Silurian to the Permian and that the rocks of the Larinskaya, Mirnaya, and Chuzik formations are gas-generating in terms of generation characteristics. In the thermochronologic evolution of these formations reconstructed by the GALO system, the main thermal impulse comes from the emplacement of a sill near the basement subsurface in the Jurassic. Because of this, HC were generated mostly during and/or after sill emplacement. The maturity of organic matter calculated for all the formations in the GALO system from the thermochronologic evolution is noticeably lower than that calculated from the temperature histories in the TeploDialog system. As a result, the GALO modeling predicts that even the rocks of the Mirnaya Formation are capable of oil generating according to their generation characteristics, the rocks of the Chuzik Formation can be considered as predominantly oil-generating, and the rocks of the Chaga Formation are interpreted as oil-generating. The TeploDialog modeling of the thermochronologic evolution assumes that the Jurassic “Bazhenov oil” is the main source of hydrocarbons for crustal and Paleozoic reservoirs, whereas the GALO modeling of this history and the calculations of hydrocarbon generation indicate that oil was generation by Domanik-like rocks (for example, the rocks of the Chaga Formation). The examples of modeling discussed in this paper demonstrate that the oil and gas potentials of basins should be assessed with caution when using modeling systems in which the heat flow is specified for relatively shallow depths (in the bottom part of the sedimentary sequence).