Permafrost, Phillips, Springman & Arenson (eds) © 2003 Swets & Zeitlinger, Lisse, ISBN 90 5809 582 7 Relict permafrost in the central part of Western Siberia G.V. Ananjeva (Malkova), E.S. Melnikov & O.E. Ponomareva Earth Cryosphere Institute, Siberian Division, Russian Academy of Sciences: Russia, Moscow ABSTRACT: Relict permafrost has been encountered during the investigation of numerous oil and gas fields in the central part of Western Siberia at depths ranging from 100–150 m (top of permafrost) to 250–400 m (base) below ground surface. We present results of ongoing geological and hydrogeological research and provide additional information on the geocryological conditions prevalent in this territory. The new data have modified our scien- tific views on the character of relict permafrost in the central part of Western Siberia. Computer maps depicting the extent of relict permafrost and the accompanying database are available based on GIS – technology. 1 INTRODUCTION A map of Western Siberian permafrost thickness and structure (scale 1:2 500 000; V. Baulin editor) accompa- The cryolithozone in the central part of Western Siberia nied by explanations and borehole catalogues was is characterized by the presence of relict permafrost at issued in 1985 as a result of this research (Baulin & depth. The relict permafrost was formed as a result of Dubikov 1982). severe climatic conditions in the Pleistocene. Huge Furthermore, geologists and geographers from soils masses froze which were both on land and under Moscow State University under the supervision of V. the shallow sea at that time. With subsequent Holocene Trofimov were engaged in the exploration of per- climate warming the permafrost soils thawed from the mafrost occurrences in Western Siberia in 1970–1985 ground surface, but persisted at depth as relics. (Trofimov 1977). Relict permafrost bodies are found in Western We collected all separate data sources in which the Siberia between 60° N and 64° N at depths between relict permafrost in the central part of Western Siberia 100 and 300 m, sometimes to 400 m below the surface. are mentioned, and have augmented this with new data The area of relict permafrost coincides with the ter- on permafrost occurrences derived from geological ritory of the main oil- and gas-fields in Western surveys of oil- and gas-fields and prospective drilling Siberia. The development of this territory during the for potable water, which were carried out in last decades last 40 years was accompanied by extensive drilling (1980–2000). and the application of a variety of geophysical methods. The available borehole data and survey results enabled us to work out an approximate representation of the 3 METHODOLOGY distribution of relict permafrost. The processing of our data is based on GIS-technology. We created a computer database, including all archival 2 DATA SOURCES and literature data on the depth of permafrost top and base using the results of drilling boreholes and of Most research on permafrost occurrence in the West interpretation of geophysical surveys. The GIS-data- Siberian lowland was carried out by V. Baulin’s scien- base used PARADOX software. tific group (E. Belopuchova, L. Shmelev, G. Dubikov, It contains the results of testing 1100 geological and A. Chekhovskiy etc.), and also by V. Solov’ev and G. hydro-geological, 120 –3000 m deep boreholes. More Ginsburg. These researchers were the first to bring than 900 boreholes show evidence of relict permafrost. attention to the occurrence of deep permafrost hori- For each borehole, the coordinates, elevation and depth zons so far south in Western Siberia. They described of drilling were entered into the database. The depth the methods of discovering deep frozen horizons, of the top and base of the relict permafrost and the made the first conclusions on the deposition laws of permafrost thickness were stored. All data were placed the relict permafrost, and developed a hypothesis for in *.db or *.dbf formatted tables. its origin. The map of relict permafrost distribution was cre- Their research is based on analysis of data from sev- ated with GEOGRAPH software on the basis of these eral hundred boreholes and vertical electrical sounding data. A Gauss-Kruger projection with 72° axial merid- (VES) profiles. They compiled summary catalogues ian was used (GeoGraph 1996). of prospecting boreholes which detected permafrost Finally, all maps were transferred to ARCINFO for- in Western Siberia (Baulin 1985). mat by means of so-called exchange gen-files. 5 Figure 1. The relict permafrost map of the central part of Western Siberia: 1 – the isograms of permafrost base (m, with reference to sea level); 2 – the same for permafrost roof; 3 – the probable southern boundary of the cryolitozone; 4 – the open taliks. I-I – section line Noyabrsk-Surgut. The position of the top of the permafrost and the The relics are found at depths of 80–Ͼ400 m below base are plotted on the maps as isolines (with the special surface. graphic software GEOFIT). This GEOFIT-software The depth of the top of the relict permafrost depends allowed us to automatically draw the isolines of depth upon exogenous factors such as the age of the geo- of the top of the relict permafrost and its base, with morphologic expression and hydro-geological, hydro- reference to sea level, using borehole coordinates. We logical and landscape conditions of the site. chose an isoline interval of 50 m. The depth with The depth of the relict permafrost base also reference to sea level is displayed on the map. If the depends on these factors and in addition on endoge- depth of the top of the permafrost and base relative to nous factors: depth to bedrock, neo-tectonic pro- the surface had been used then the picture would have cesses, the geothermal gradient etc. (Geocryology depended very much on relief. USSR 1989) Isolines of the top of the permafrost and base depth The greatest depth and the thickest relict permafrost (relative to sea level) show the wavy character of the are found under the oldest and highest geo- relict permafrost. They define the upward and down- morphologic terrains (marine, glacial and lacustrine- ward anomalies and allow an estimation of the per- alluvial high plains with elevations of 100–150 m mafrost thickness at any point (Fig. 1). above sea level). Shallower and thinner relict per- Unfortunately, the borehole distribution in the mafrost is prevalent at sites with young and low geo- research area is not uniform. Most data are available morphologic terrain (lacustrine-alluvial and alluvial from developed oil- and gas-fields, river valleys and low plains and floodplains with elevations of 10–30 m vicinities of large settlements. Thus the reliability of the above sea level). plotted isolines depends on these factors, and some- The depth of the relict permafrost base ranges times the isolines are approximate. from Ϫ300 m below sea level (on the left bank of the Ob River, near Khanty-Mansiysk city) up to 0 m in the west of the area (near Igrim city). 4 RESULTS AND DISCUSSION The top of the relict permafrost in one spot is about Ϫ180 m (below sea level) near Kogalym city. The data obtained during the last two decades have Usually it is Ϫ50 to Ϫ100 m below sea level and only significantly enlarged the information on relict per- in the western part of the area does it rise above sea mafrost and improved our knowledge of this subject. level (up to ϩ50 m). 6 Figure 2. Permafrost relic along the section Noyabrsk-Surgut. 1 – overburden: sands, sandyloam and loam; 2 – Oligocene: interleaving sands, clay and argillites; 3 – Eocene: clay; 4 – contemporary permafrost; 5 – relict permafrost. The data show that the maximum thickness of the geocryological features, but this territory is tectoni- relict permafrost is 345 m. This thickness is from a cally non-uniform. At the same time, the highest ele- borehole on the right bank of the Vakh River’s mouth, vation of permafrost is found at the southern city of near Nizhnevartovsk city. The most common thickness Megion. Here the permafrost is found from 0 to Ϫ50 m ranges from 100 to 200 m. (relative to sea level). The thickness of the relict permafrost depends The cross-section shown in Fig. 2 represents the mostly on the geothermal gradient. For example, the character of discontinuous permafrost in the central geothermal gradient varies near Nizhnevartovsk city part of Western Siberia. As can be seen on the cross- from 2.5 to 3.5°C/100 m (Mizerov 1988). In the south- section between Noyabrsk, Kogalym, and Surgut, the western part of the West Siberian cryolithozone thickness of relict permafrost decreases gradually (located within neo-tectonic zones) the observed from north to south. increased geothermal gradient is more than 4°C/100 m. Besides the wide-spread relict permafrost horizon, Here, the depth of the base of the relict permafrost some frozen masses can be found 100–150 m below decreases and a reduction of permafrost thickness surface. They are relics of another cold climate of the ranging to a complete pinching-out is observed. Pleistocene epoch (Zemtsov 1972). Isolated masses of It is worth paying attention to the relict permafrost modern (Holocene) permafrost can be found to the in the western area: the permafrost depth is at about north of Kogalym city. They are just subsurface and 100 m below surface, while the top of the permafrost their thickness is 20–50 m. Peat bog or thick conifer- is found at depths between 0 and 50 m (relative to sea ous wood landscapes are on the surface. level). Neo-tectonic domes or ledges of platform Usually open taliks underlay the rivers. Taliks are mantle are found in these sites and geothermal gradi- also formed under favorable geological and hydro- ents are up to 4.8°C/100 m.