Development of the West Siberian Basin during the Mesozoic and Tertiary: Palaeogeography and Stratigraphy
Following on from the excerpt in the last issue of ROGTEC Magazine, we look at the Late Jurassic period and the overlying Neocomian
Late Jurassic sediments, which lie disconformably on the Middle Jurassic in parts of Western Siberia, are 500 m or more thick in northern parts of the basin and in the Khatanga region to the northeast. Marine incursions from the Arctic basin penetrated as far south as 54° N at this time. Most of the basin was characterized by clastic deposition in a shallow to deep shelf environment (Fig. I.3.9). Shales, locally bituminous, began to be deposited in the more rapidly subsiding central and western parts of the basin, corresponding with the Abalak Suite and its lateral equivalent the Vasyugan Suite. Ammonite-bearing marine glauconitic shelf sandstones and shales accumulated along the eastern and western flanks of the basin, with continental fluvio-deltaic sediments and red beds grading seaward into shelfal shales and sandstones on the southeastern and southern margins. Some conglomeratic and limestone beds occur locally, especially in the northeast of the basin.
The Late Jurassic section below the Tithonian Bazhenov suite can be divided in very general terms into two parts: a lower part including coastal shallow-water deposits of Callovian to Oxfordian age, and an upper part represented mainly by marine sands and clays of Kimmeridgian age, belonging to the upper sub-suite of the Sigovo Suite. Facies variations are quite substantial, however, and many individual sections vary from this simple pattern (Figs. I.3.3 and I.3.4).
In the northeast of the basin the Callovian to Late Oxfordian deposits comprise the Vasyugan Suite and its lateral equivalent, the Naunaksk Suite. The Vasyugan Suite includes sand and clay beds that are widely developed across the central area of Western Siberia. The Lower Vasyugan Sub-suite is of Late Callovian to Early Oxfordian age and is composed mainly of compact black and dark-grey clays with 2-3 cm bituminous beds. Thin beds of grey quartz-feldspar and quartz-glauconite sandstone and siltstone occur, with discontinuous lenticular bedding. The sediments are locally slightly calcareous, and occasional beds of argillaceous limestone occur. Carbonaceous material occurs along bedding planes. The proportion of sand and silt increases towards the base of the unit, and coaly material and pyritised woody fragments occur.
To the east the Lower Vasyugan Sub-Suite passes into the Naunaksk Suite, which was formed in coastal and alluvial plain environments, with occasional marine incursions, and a shallow shelf. These lay adjacent to a depositional plain in the east and southeast with a fluvial network. Sands and silts represent deltaic deposits, including coastal barriers and bars, pools and beach sands. The different sand bodies occur in distinct zones, and permeable beds (horizon JuV1) form mainly north-south-trending zones lying en echelon with one another from east to west.
The Sigovo Suite or the Lower Mar’yanovsk Sub-Suite was developed on the eastern side of the basin during the Kimmeridgian. To the west the Upper Sigovo Sub-Suite passes into the mainly argillaceous succession of the Georgievsk Suite, characterised by glauconite and siderite-manganese-calcite concretions, and then in turn into the upper part of the Abalak Suite. In the Pre-Ural area the latter, within the regional pinch-out zone of the Kimmeridgian, is represented at the top by the lower sub-suite of the Danilov Suite (Fig. I.3.4).
At the top of the Jurassic section is the highly bituminous Bazhenov Suite, which was deposited over much of the basin area, grading to thin deposits of shelf sandstone and shale around the basin margins (Fig. I.3.12). The Bazhenov Suite forms the most important source rock in the WSB. It is described in greater detail in Section II.3.1.2. Clastic influx at this time was low, and the basin was semi-starved, especially in its central and western parts. At about the Jurassic-Cretaceous boundary the basin margins were uplifted, possibly in response to “Laramide” folding on the eastern flank of the Siberian craton. Uplift and local erosion is particularly marked in the Khatanga region and in the south of the Yamal Peninsula. The basin south of about 68° N is likely therefore to have become partially restricted at this time, forming a sub-basin particularly favourable for the accumulation of organic matter. Marine regression at the end of the Jurassic may have further restricted this southern basinal area. At the same time, the more uplifted parts of the basin may have become emergent. Geophysical data suggest the presence of erosional top-Jurassic unconformities within several large structures in northern parts of the basin. The presence of highly bituminous shales in the Bazhenov Suite on the Yamal Peninsula and probably in the South Kara Sea sub-basin may have been associated with further restriction of basin circulation, resulting from the presence of the North Siberian sill.
I.3.3 Cretaceous
I.3.3.1 Neocomian
During the Cretaceous the WSB retained its connection with the Arctic basin, except briefly at times when it may have been partly or entirely cut off by the presence of the North Siberian Sill. The amount and rate of clastic deposition increased at this time owing to rapid uplift of the basin margins. Climate change, with increasing rainfall, may also have accounted partly for an increased proportion of coarse clastics.
Figures I.3.13, I.3.14 and I.3.15 illustrate palaeogeographies during the Berriasian- Valanginian, Hauterivian and Barremian stages respectively. The Neocomian reaches 1500 m or more thick in the northeast of the basin, approximately 1000 m in the central basin, and typically 500 m or less in more marginal areas. The general pattern of sedimentation from the Berriasian to the Hauterivian was as follows: (1) marine bituminous shale and grey shale deposition in the more rapidly subsiding part of the basin (Khanty-Mansi Trough); (2) marine shelf sand and clay deposits along a wide northsouth- trending belt to the east of the trough; (3) coastal plain deposits of sand and clay on the east-central basin flank; and (4) continental red beds along the basin-margins in the southeast, which increased in width throughout the Neocomian. A relatively narrow zone of shallow marine and coastal plain sand and clay facies is also present along the western margin of the basin, adjacent to the Urals uplift.
The most widespread stratigraphic units recognised are the Megion Suite (Berriasian and most of Valanginian) and the Vartov Suite (late Valanginian, Hauterivian, and Barremian) and their lateral equivalents (Fig. I.3.3). Marine deposition during the Hauterivian and Barremian was increasingly restricted to deeper parts of the basin as a late Neocomian regression proceeded. Red beds became dominant in the southern and southeastern parts of the basin at this time, while coastal plain, coal-bearing sandstone, siltstone, and shale deposits dominated the eastern flank. By the Barremian, marine deposition was more or less confined to the Khanty-Mansi Trough, and the West Siberian sea was largely isolated from the Arctic basin.
Owing to the interaction between continuing basin-floor subsidence, a steady sediment supply from the east and southeast, and a repeated pattern of marine transgression and regression, westward progradation of sandy marine clinoforms during regressive phases was repeatedly interrupted by transgressive episodes in which the sandy clinoforms were blanketed by marine shales – sometimes including potential source facies. The resulting interfingering of clinoformal reservoirs with marine shales, all overlying the rich Bazhenov source rock, provides the setting for the most prolific hydrocarbon system in the basin – and indeed one of the most prolific in the world. Clinoforms also prograded, though to a much lower extent, from the western, Uralian, margin of the basin. This system is considered in further detail in the section on Neocomian petroleum geology (Section II.4.1).
