Modeling the Evolution of the Permafrost Zone in the Southern Part of the West Siberian Basin and Comparison With Its Evolution in the Urengoi Area

ABSTRACT Thermal evolution of the cryolithozone in the Pliocene–Quaternary time has been numerically reconstructed for the sedimentary section of the Ostanin‐438 well (southern part of the West Siberian Basin, Tomsk Region). The modeling considers the evolution of the sedimentary section over the last 3.5 million years as the final stage of modeling the evolution of the basin, starting with continental rifting in the Lower Silurian. The modeling results for the Ostaninskaya area are compared with the modeling in the area of the superdeep borehole SG‐6, located 8.6° north of the first area. The difference in the position of the studied areas by 8.6° corresponds to a change in the average annual air temperature values of approximately 7°C. Comparison of the modeling results for the two areas shows that a shift of 8.6° to the south is accompanied by a decrease in the thickness of the cryolithozone from a value of about 311 m in the SG‐6 area to the complete degradation of the cryolithozone in the Ostanin area. Despite the complete degradation of the permafrost zone during the last climatic optimum, the influence of past periods of cold climate remains noticeable in the distribution of the modern heat flow of the Ostanin area with depth. The flow increases from 15 mW/m 2 at the surface of the sedimentary cover to 66.5 mW/m 2 at a depth of about 1 km, after which it decreases monotonically with depth, approaching deep regional values. This behavior of heat flow is also characteristic of many sedimentary sections of the West Siberian Basin.