Blackbourn Reports: Western Siberia

Following on from his previous article for ROGTEC, Graham Blackbourn looks at the Potential of this exciting frontier

The West Siberian Basin (WSB) occupies an area of approximately 3.5 million km2, including the offshore South Kara Basin in the north and the western part of the Yenisei-Khatanga Trough in the northeast (Figs. I.1.1 & Enclosure 1). The basin is bounded in the west and northwest by the Ural and Novaya Zemlya ridges, in the south and southeast by the North Kazakh and Altai-Sayan uplands, in the east and northeast by the Central Siberian plateau and Taimyr uplift and in the north by the North Siberian Sill. As is evident from the depth to top-Jurassic map (Enclosure 1), the Basin is deepest in the offshore area to the north; the thickness of the Phanerozoic sedimentary cover ranges from approximately 3-5 km in central parts of the basin onshore in Siberia, but reaches 8-12 km or more below the South Kara Sea in the north. The Mesozoic and Tertiary basinfill has been estimated as having a total volume of around with 16 million km3, ranging in thickness from 3-4 km in the central area to 8-10 km or more in the north (this asymmetry is clearly reflected at top-Jurassic level). The Mesozoic-Cenozoic cover is less than 1 km thick along the North Siberian Sill, which comprises a basement high of Mesozoic age extending between the northern end of Novaya Zemlya and the northwestern part of the Taimyr uplift. The basin is connected with the Ustyurt and Aral basins of Kazakhstan to the south through the narrow Turgai valley, which runs between the southern Ural Mountains and the North Kazakh uplands (Fig. I.1.1). During the latest Cretaceous and early Tertiary the West Siberian Sea was connected through this channel with the Tethys Ocean. Apart from the Kara Sea which covers its northern part, the Western Siberian Basin now lies almost wholly beneath the vast, low-lying West Siberian Plain. The plain is drained by the Ob (in the west) and Yenisei (in the east) rivers, which flow northwards into the Kara Sea, and it exhibits little topography, containing vast tracts of swampland. It is the world’s largest unbroken area of flat terrain, and elevations remain less than 100 m above sea level 1000 km upstream on the River Ob. Taiga (swampy forest) vegetation and landforms cover much of the Plain owing to the largely sub-Arctic conditions, with tundra in the Arctic regions in the north, and a cool continental climate over the southern steppe, which rises southwards towards the Kazakhstan uplands and Altai-Sayan. The entire Plain lies within Russia, apart from its southern rim which is part of Northern Kazakhstan.

Siberia is notorious for the length and severity of its winters: temperatures below -50 C are not uncommon in winter. Transport across the region is mostly by air, with many roads passable only in winter when the ground is frozen. The presence of permanently frozen ground — permafrost — causes particular difficulties both for surface construction, and for drilling. Permafrost in northern Siberia extends down to depths of 500 m or more (Fig. I.1.2). The structure of permafrost zones both laterally and vertically can be complex, with interfingering of frozen and thawed ground. Three main permafrost zones are recognised within the West Siberian Plain: northern, central and southern.

The northern zone lies to the north of a line of latitude running approximately through the centre of the Medvezh’e and Urengoi gas fields (i.e. about 66-67 N). The permafrost here is continuous both vertically and laterally, apart from below the channels of major rivers and deep lakes. The thickness of the permafrost increases from east to west along the 65 N line from 300 m to 500 m, and on the north coast it reaches 500 m to 600 m. The central permafrost zone has two separate permafrost layers, apart from below a few treeless ridges where the permafrost is vertically continuous. Elsewhere in the central zone there are upper and lower permafrost layers, separated by a layer of melting. The upper layer results from freezing in recent times, whereas the lower layer is a «fossil» relic which was not wholly melted during the most recent Holocene warm period. The thickness of the relict layer in the west is 250-300 m, reaching 300-400 m in the east. The upper permafrost layer is 30-80 m thick, and varies significantly laterally.

The intervening melted layer provides a supply of fresh water throughout the year; those working in the northern zone have no such supply of groundwater, and have to obtain water by melting snow or ice.

The melted layer has also acted as a fluid conduit on occasions when wells drilling below it have unexpectedly encountered pockets of gas. The gas may reach the surface at a considerable distance from the well. For example, such a situation arose on one occasion while drilling on the Urengoi field, when gas was observed to be emitted from the bed of a fluvial floodplain and from lakes up to 1500 m from the well (Medvedskii, 1987). Only the relict frozen layer occurs in the southern zone. This is observed both to the north and south of the east-west-trending section of the River Ob, where it typically lies at depths between 150-300 m (e.g. in the Ust-Balyksk, Pravdinsk and Mamontovsk areas), although its upper surface is occasionally encountered at depths as shallow as 80 m. Its thickness varies considerably, however, depending on surface conditions. For example, it is 150 m thick in the Chernogorsk area, whereas in the Samotlor area, immediately to the northeast, it is considerably reduced owing to the large number of surface lakes and swamps here.

The relict layer in the southern zone does not cause any particular problems for drilling. It can, however, cause problems in the interpretation of seismic surveys owing to its very variable thickness.

No permafrost has been encountered at latitudes south of about 59-60 N (Fig. I.1.2). The permafrost below the West Siberian Plain incorporates considerable volumes of gas hydrates. Environmentalists have expressed concern that global warming could release huge volumes of the bound methane into the atmosphere. Methane is a potent «greenhouse gas», and its release could engender further warming. Russian occupation of Siberia began in 1581, when a Cossack expedition overthrew the small khanate of Sibir, which gave its name to the entire region. During the late 16th and 17th centuries, Russian fur trappers and traders and Cossack explorers penetrated all of Siberia, reaching a border treaty with china in 1689 (although they advanced further east, into the Amur basin, in contravention of the treaty, in the 1860s). Although a place of exile for criminals and political prisoners, Russian settlements were of little significance until the building of the Trans-Siberian railway across the southern part of the West Siberian Plain in 1891-1905. Industrial growth along the railway and in the Kuznetsk Basin coalfield was considerable after the first Soviet Five-Year Plan (1928-32).

The population began to fall again during the 1960s. The discovery of hydrocarbons in 1953, and especially that of the giant Samotlor oil field in 1965, however, was the major impetus for a redevelopment of the area, especially its northern regions, which reached a peak during the 1980s. Figure I.1.3 illustrates the average size of oil discoveries in Western Siberia from the 1970s, compared with those from Russia as a whole. It is clear that the average field size in the WSB has consistently been significantly higher than that of Russia as a whole, but that discovery sizes in both areas have fallen steadily and dramatically. Nonetheless, although Western Siberia is now a mature province, it covers an immense area, and there is plenty of scope for further discoveries, even if no «supergiants» remain, in addition to development and rehabilitation of existing fields. Some of the latter hold very considerable remaining reserves. The early years of the 21st century, with steadily increasing oil and gas prices, have seen a significant increase in the levels of activity.

I.1.3 Brief Historical Review of the Hydrocarbon Industry of Western Siberia, and a Short Introduction to the Petroleum Geology of the West Siberian Basin.
The first field to be discovered in Western Siberia was the Berezov gas field in 1953, in the northern Pre-Urals area on the western margin of the Basin (Enclosure II.1). Owing to the remote location and the absence of infrastructure to the field, which has a Jurassic reservoir, it was not brought on-stream until 1963. In the meantime, the Megion oil field in the Middle Ob Region was discovered in 1961, followed by the giant Samotlor field in 1965. Samotlor was one of the largest oil fields in the world: ultimately recoverable oil has been estimated as 24.7 billion barrels, and it immediately drew the attention of Soviet planners to the West Siberian Basin, and drew investment away from almost all other hydrocarbon provinces in the FSU. The Samotlor field was brought on-stream in 1964. Reservoirs vary in age from Late Jurassic to Cenomanian, but the large majority of the oil lay within Neocomian marine sandstones, which have proved to be the most prolific oil reservoir in the entire West Siberian province.

Further development of oil fields within central parts of the West Siberian Basin during the 1970s was followed by the development during the 1980s of massive gas fields in the north, mostly within Cenomanian reservoirs, first discovered in the 1960s. Oil production in the basin has however declined since 1988, and gas production since 1991, and recent production has exceeded the reserve replacement rate.

Most of the known and potential hydrocarbons in the basin lie within the Mesozoic succession. The Palaeozoic and older basement was formed by a complex array of microplates and continental fragments brought together by ocean closure and strike-slip faulting during the mid- to late Palaeozoic (Section I.2.1). Following Triassic rifting and igneous activity, the basin as a whole began to subside in the Early Jurassic and to fill with sediments sourced from the surrounding uplands, lying primarily to the southeast and northeast. Once the erosional topography had been blanketed, deposition occurred across a very extensive platformal area. Owing primarily to the vast extent of the basin, sediment input did not keep pace with subsidence, and the western half of the basin in particular was at times sediment-starved, leading to the deposition of up to 2500-3000 m of dark marine shales, commonly rich in organic matter, being deposited there between the Middle Jurassic and early Tertiary, with the latest-Jurassic Bazhenov Suite being of particular importance as a source rock. During most of the Early Cretaceous the transition between deltaic and open-marine deposition lay approximately mid-way across the basin. Repeated transgressive-regressive cycles in this environment provided optimum conditions for the reworking and winnowing of feldspathic deltaic and interdeltaic sands, leading to an improvement in their reservoir potential. Shelf deposits prograded westwards and northwestwards across the basin during regressive phases, creating clinoformal structures with distinct sand accumulations in the upper, shelf, environments, on the slope, and at the base-of-slope. During transgressive phases the clinoforms were draped by marine sapropelic muds of excellent source-rock quality, which encased the sand-rich clinoformal structures and these, together with the underlying Bazhenov Suite source, created a remarkably efficient source rock — reservoir — seal relationship.

A delicate balance between sediment input, sea-level fluctuations and basin-floor subsidence provided a combination of circumstances in which this interfingering of reservoir sands and source rocks continued to form throughout the Early Cretaceous over the extensive central-southern area of the basin, in particular within the Nizhnevartov, Surgut, Urengoi, Yamburg, and other regions (Enclosure II.1). The high concentrations of organic matter within the basin may have been related partly to its palaeogeography, and especially to its restricted connection over the North Siberian Sill with the Arctic basin to the north. Southward circulation of cooler nutrient-rich marine waters across the North Siberian Sill into the warmer epicontinental basin may have stimulated the production and accumulation of planktonic organic matter to an unusually large extent. The oil-prone marine source-rocks did not extend over the substantial eastern and northeastern areas of the basin, and other parts of the basin margins, which had become the sites mainly of fluvio-deltaic and lacustrine deposition. Significant coals accumulated within these environments, however, which constitute a substantial gas source. This is the main reason for a general transition from oil fields within central and southern parts of the WSB, to gas in the north and east, and along its western margin (Enclosure II.1).

Another factor which makes the WSB such a prolific hydrocarbon province is that there has been very little tectonic activity within the area since hydrocarbon emplacement, so that early accumulations of hydrocarbons have been preserved within structural and stratigraphic traps which have remained relatively undisturbed.

Over the past 30 years or more, the WSB, particularly its central-southern part (either side of the Ob-river where it runs in an approximately east-west direction before turning northward in the Khanty-Mansi area — the Russians call this the «latitudinal Ob» region (Fig. I.1.1) has been quite thoroughly explored. However, parts of the basin remain under-explored, particularly the South Kara Sea and the Yenisei-Khatanga Trough. Both of these areas appear prospective, especially for natural gas and possibly for oil. The major Jurassic and Cretaceous source-rock facies are thought to extend northwards into both these regions: the gas-prone source may become more extensive, although the oilprone Bazhenov Suite may well extend into the South Kara Basin. The Taimyr uplift, and probably Novaya Zemlya, were sources of clastic material during most of the Mesozoic, and could have provided good-quality reservoirs.

The West Siberian Basin is of enormous extent, and the Russian-language geoscience literature uses a variety of systems for sub-dividing it into geographic regions. One of the most common subdivisions is illustrated on Enclosure I.1 (after e.g. Maximov, 1987). The ten regions depicted are somewhat arbitrary, apparently being defined on the basis of a variety of geological, geographic and administrative criteria. These regions differ from the purely administrative divisions, illustrated in Fig. I.1.1. A pragmatic approach is adopted in this report, switching between the different terminologies according to what is most appropriate to the discussion in hand.