Blackbourn Reports: Petroleum Geology of the West Siberian Basin
Chapter 1.
Introduction and Brief Review of the Petroleum Geology
II.1.1 Introduction
As noted in Section I.1.3, the West Siberian Basin has been sub-divided into distinct petroleum-geological regions by various authors, and there is no wholly consistent terminology. The ten regions illustrated in Enclosure II.1 are widely used in the petroleum geology literature (after e.g. Maximov, 1987). These differ from the administrative regions illustrated in Figure. I.1.1.
For the purposes of this discussion of the petroleum geology of the WSB, an approach similar to that used by Peterson and Clarke (1991) has been adopted, in which Maximov’s ten regions are grouped into four blocks on the basis of the age of the main reservoirs, the dominant trap type and the nature of the hydrocarbons produced. They are described in outline below, and comprise: (1) the Middle Ob region; (2) the Pre-Ural region; (3) the Southern WSB; and (4) the Northern WSB. The individual regions defined by Maximov are, however, referred to where greater precision is required.
II.1.1.1 The Middle Ob Region
The Middle Ob region as defined here is the same as that of Maximov (1987) depicted in Enclosure II.1. Most of the production in this region is oil from Early Cretaceous clastics, mainly in anticlinal and stratigraphic traps within a series of progradational marine clinoforms. Two large regional uplifts dominate this area, the Surgut and Nizhnevartov arches (Enclosure II.1, Enclosure 5), which together account for a substantial proportion of the oil produced in the WSB. The Samotlor field and other giant oil fields are located in this area.
II.1.1.2 The Pre-Ural Region
This includes both the Pre-Ural and Frolov regions shown on Enclosure II.1. Production in the Pre-Ural region in the western part of the basin is primarily from Late Jurassic clastics. There is also some production from Early Cretaceous clastic reservoirs and from weathered basement. The traps are mainly of a combined structural and stratigraphic type, with reservoirs pinching out against uplifted basement rocks. The Pre-Ural area is mostly oil-producing in the south and gas-producing in the north.
II.1.1.3 The Southern WSB
This includes the Kaimysov, Vasyugan, and Paidugin regions (Enclosure II.1). Most of the producing fields here are of oil and oil-and-gas within Jurassic clastics, and are mainly trapped within anticlines or uplifts overlying basement highs. Some production is also obtained from middle and late Palaeozoic carbonate reservoirs in the Nyurol’ Basin.
II.1.1.4 The Northern WSB
This area comprises the Yamal, Gyda, Nadym-Pur, and Pur-Taz regions (Enclosure II.1). Gas and gas-condensate are produced from mainly Late Cretaceous (Cenomanian) clastics within anticlinal traps. Urengoi, one of the world’s largest gas fields, and several other giant gas fields are located in this area.
II.1.2 Main Features of the Petroleum Geology of the West Siberian Basin
The timing of trap development differed slightly within each of the four major producing areas defined above. Throughout most of the WSB, uplifted basement blocks were present prior to the deposition of Jurassic and Cretaceous reservoirs. In the Pre-Ural and Southern WSB areas, Jurassic reservoirs commonly pinch out against these blocks. Structures responsible for trapping hydrocarbons in the Northern WSB are thought to have formed early in the Mesozoic and were enhanced by regional uplift in the Tertiary, presumably a far-field effect of the collision of the Indian continent with Eurasia. Structures in the Middle Ob region commonly formed at approximately the same time as deposition of the reservoirs. Differential compaction may also have had a significant role in trap development in most areas. A substantial stratigraphic component is important in reservoir, source rock and trap development in almost all parts of the basin.
In addition to the main hydrocarbon accumulations within Jurassic and Cretaceous reservoirs, oil and gas have also been found in several areas within pre-Jurassic (largely middle and late Palaeozoic) sediments, with commercial accumulations within the Nyurol’ Basin (Fig. II.2.1). The oils within Palaeozoic reservoirs here are chemically distinct from the Mesozoic oils, and are thought to have been generated from one or more Palaeozoic sources (Section II.2.1.1). Seismic data indicate the presence of a substantial thickness of unmetamorphosed or lightly metamorphosed Palaeozoic sediments in several parts of the basin (Section I.2.2). In the Middle Ob region they are known to reach 3-4 km thick, and in several other parts of the basin they may reach 5-7 km. The main hydrocarbon-bearing successions within the WSB are therefore clastic deposits of Jurassic, Neocomian, and Aptian to Cenomanian age. These successions are overlain by thick regional shale deposits of Kimmeridgian to Valanginian (Bazhenov Suite), Aptian to Albian (Alym Suite and equivalents), and Turonian (Kuznetsov Suite) ages respectively (Fig. I.3.3). Further potential seals exist up to the Oligocene, but these are generally impersistent and unreliable and are not known to trap commercial hydrocarbons. There appears to be quite a simple relationship between the distribution of oil and gas and the nature of the organic material present within the various source-rock successions. Thick and widespread source-rock units of either sapropelic or the humic type, or mixtures of the two, are present in both the Jurassic and Neocomian hydrocarbon systems. Accumulations of predominantly gas or gas-condensate are found in the Northern WSB and northern Pre-Ural regions; predominantly oil is found in the Middle Ob and southern Pre-Ural regions (including the Frolov region) and the Kaimysov region; and mixed oil and gas are found in the Vasyugan and Paidugin regions.
Additional underexplored potential hydrocarbon-bearing successions are present within the Triassic clastic succession in the Northern WSB, and in the Palaeozoic basement in various areas. Whether source rocks are present and not overmature appears to be the major risk factor in most of these regions.
Geothermal gradients are close to normal in most parts of the WSB. The lowest values (2.8—3.0° C/100 m) occur in the southeastern part of the basin, with highest values (>4.5° C/100 m) in the Pre-Urals region in the west. Values of 3.3—4.2° C/100 m are typical of the remainder of the basin (Duchkov et al., 1995). Present-day temperatures at the top of the Middle to Late Jurassic Tyumen Suite, based on 3200 wells, range from 30- 50° C where it pinches out around the margins of the basin, to 100-130° C in central and northern parts of the basin. Corresponding temperatures at the top of the Neocomian range from 30-60° C to 80-100° C, and at the top of the Cenomanian from 10-20° C to 55° C. Temperatures within shallow parts of the succession are depressed over most of the WSB owing to the thick permafrost layer (Section I.1.2).
Heat flow throughout the basin has been considered in detail by Kurchikov (2001). The following sequence of episodes of petroleum generation and accumulation has been proposed for the Mesozoic and Tertiary successions:
1. Biogenic gas began to be generated in the Lower and Middle Jurassic in the Northern WSB during the Middle Jurassic. Much of this gas was lost owing to an absence of seals. Some lower parts of the Jurassic section may already have been in the oil window by the Late Jurassic time.
2. Source rocks within the lower part of the Jurassic were generating oil and gas over much of the WSB area during the Early Cretaceous.
3. Oil generation within some Jurassic source rocks probably diminished after the Cenomanian, especially over much of the Northern WSB where they would have reached temperatures greater than 145-150° C. Peak oil generation from the Bazhenov Suite may have occurred during the early- to mid-Teritary.
4. Regional uplift during the Neogene caused a drop in reservoir pressures; some gas was released from solution, and the formation of new traps and the redistribution of old accumulations occurred.
The petroleum geology of the WSB is described further in the remainder of Part II of this report, which considers the known and potential hydrocarbon systems in stratigraphic order, from the Palaeozoic up. The Cenozoic of the WSB is thin, shallow, and almost devoid of potential seals. The Cenozoic succession was described in Section I.3.4. However, it is not regarded as prospective, and is not considered here in relation to the petroleum geology of the basin.
In the next excerpt, we will look at the Petroleum geology of the pre-Jurassic period.
