Graham Blackbourn: Blackbourn Geoconsulting
II.3.1 Main elements of the Jurassic hydrocarbon system
Jurassic sandstone and siltstone reservoirs are productive within very numerous oil and gas fields in many parts of the West Siberian Basin (WSB). Several fields also produce oil from fractured reservoirs in the highly organic-rich latest Jurassic argillaceous Bazhenov Suite, the main source horizon in the basin. Up to twenty Jurassic reservoir horizons have been identified (Yermakov and Skorobogatov, 1984).
Jurassic reservoirs mostly host oil accumulations, although gas is produced over a wide area of the Berezov region on the western side of the basin (the first hydrocarbons discovered in the WSB), and associated or free gas is common in many of the oil fields. Gas and some oil have also been discovered in Jurassic sandstone and siltstone reservoirs in a few Northern WSB fields, all of which also produce gas-condensate from Cretaceous reservoirs. The Jurassic in the Northern WSB has not been thoroughly explored owing to its depth in many areas, and because of an abundance of developed gas reserves in shallower Cretaceous reservoirs.
Many more accumulations have been discovered to date within Late Jurassic reservoirs than in the Early or Middle Jurassic. Three main Late Jurassic plays are recognized: (1) the Late Jurassic Vasyugan Suite sandstones and its lateral equivalents; (2) the Vogulka Suite sandstones and limestones; and (3) the Bazhenov Suite shale. Vasyugan Suite sandstone and siltstone beds are important reservoirs in the southeastern, central, and northern regions of the basin. The Bazhenov Suite is commonly 10-40 m or more thick and is widespread over much of the WSB. Significant production is obtained from fractured Bazhenov reservoirs in parts of the central basin, especially in the Salym and adjacent areas. Bazhenov Suite limestones are productive in the Surgut area (Usmanov et al., 2005). However, Bazhenov discoveries have also been made in the Tomsk region, in the Northern WSB, and in several other areas. Bazhenov reservoirs are fractured and generally overpressured, expecially in areas of higher temperature. Production problems have not yet been thoroughly resolved, and partly for this reason the Bazhenov has not been fully explored. Horizontal drilling has been suggested as a method of increasing production significantly from the Bazhenov Suite. The Salym region and adjacent areas, the Khanty-Mansi Trough, the northern Middle Ob region, and the Yamal Peninsula are thought to be prospective.
Jurassic sandstones are moderately rich in feldspar (30-50% or more), with 20-45% quartz, and 1-12% mica, becoming slightly more micaceous in the Northern WSB. Late Jurassic sandstone reservoirs in the southeastern part of the basin are associated mainly with an elongate south-north trend of deltaic and fore-deltaic facies extending along the eastern border of the Middle Ob region. The thickest and most porous sandstone reservoirs have been discovered within this zone, many with a north-south trending channel orientation. The deltaic facies grade westward into marine organic-rich shale beds. A similar deltaic facies occurs within the Early to Middle Jurassic Tyumen Suite, although this unit is much richer in lacustrine and swamp deposits, and the sandstones are generally of low permeability and porosity, although some higher-porosity channel sandstones do occur.
II.3.1.2 Source Rocks
The Jurassic succession, especially the Upper Jurassic, is exceptionally rich in organic material. Early and Middle Jurassic rocks are characterized by mainly humic kerogen, with coal beds prominent in the Central and Southern WSB where lacustrine and swamp beds are common. These are thought to be the source of Jurassic gas accumulations in the south of the Kaimysov and Vasyugansk areas (e.g. Myl’dzhino and North Vasyugan fields; Enclosure II.1), and in the northern Pre-Urals (e.g. Berezov). Marine sapropelic material is particularly abundant in Late Jurassic rocks over much of the basin, except for the eastern and southern areas where dispersed humic material and some coals
It has previously been proposed that the Callovian to Kimmeridgian marine claystones of the Abalak Suite (and possibly their lateral equivalents in the Lower Vasyugan Suite) comprised an oil source, especially for accumulations in the Shaim Arch area and other parts of west-central Western Siberia, including the sub-unconformity accumulations (e.g. the area illustrated in cross-section in Fig. II.3.1). Geochemical analyses, however (e.g. Moskvin et al., 2004), appear to show that oils within the Shaim Arch reservoirs and residual oil within the Abalak Suite are both sourced from the overlying Bazhenov Suite. This might suggest that the Abalak Suite cannot be regarded as a source rock, although as noted in Section I.3.2.2 the Abalak and Vasyugan suites do contain quite extensive bituminous beds. It is perhaps more likely that the overlying Bazhenov Suite is such a prolific source that its signature swamps that of any contribution from the older marine claystones.
The Tithonian (to early Berriasian) Bazhenov Suite (Section I 3.2.2) is the source of most of the Jurassic oil (in addition to that of the Cretaceous; (Section II.3.1)). Tables II.3.1 and II.3.2 show typical TOC contents of Jurassic rocks in the West Siberia basin. Polyakova et al. (2002) describe a 32-m thick type section of the Bazhenov Suite from the Egansk-1 well in the Middle Ob region. It is composed largely of homogeneous sapropelic claystones, with thin beds of laminated siltstone. Nodules of silica, chalcedony and calcite are common, and pyritisation is widespread. The most organic rich section, between 2866-2885 m, is described as a black siliceous-argillaceous sapropelic bituminous rock with a banded to nodular fabric. Lamination is accentuated by beds of very fine silt, and with reddish colloalginite which forms 30-50% in thin section. Fish-bone debris, recrystallised shelly material (foraminifera etc.), pyrite and radiolaria are common throughout. The average TOC is 11%, with individual samples up to 18%.
The Bazhenov Suite is readily identifiable on wireline logs by its high gamma-ray and resistivity response.
Borisova and Fursenko (2004) have examined the geochemistry of the C5-C8 fraction of numerous oil and gas-condensate samples from various parts of the West Siberian Basin. They found that the fluids cluster in three genetic groups: marine, continental and mixed, each of which was found in reservoirs of varying age. The “marine” oils and condensates are more common in the Middle Ob Region, and also in the west and south of the WSB. They derive from a source which accumulated in relatively deep water, rich in planktonic and bacteriogenic organic matter, which Borisova and Fursenko identified as the Bazhenov Suite.
The fluids of “continental” origin also occur in reservoirs of different ages, but they are most common in the north and northeast of the WSB. They are thought to be sourced from fluvio-lacustrine coal-bearing deposits such as the Tyumen, Togur and similar suites. Borisova and Fursenko call these the “Lower-Middle Jurassic” source, although there appears no reason why this group should not include Cretaceous coal-bearing continental successions.
In addition to the “mixed” genetic group, comprising a combination of these two sources, Borisova and Fursenko also recognised a “biodegraded” group, which occurs locally in the shallower reservoirs.
Figure II.3.5 illustrates the distribution of these genetic types within Early/Middle Jurassic, Late Jurassic and Cretaceous reservoirs respectively. It is apparent, as would be expected, that Bazhenov-sourced fluids are concentrated where the Bazhenov Suite is know to be well-developed, especially around the Middle Ob Region. Fluids of “continental” origin occur mostly to the north and west, with mixed types lying between these main areas. Perhaps the most striking feature of these three figures is that the distribution of the genetic types is similar in each, indicating that there was substantial vertical movement of hydrocarbons in both directions (i.e. not only up from the Lower and Middle Jurassic into shallower horizons, but also down from the Bazhenov Suite into the Lower and Middle Jurassic).
Regional shale seals are present in the upper part of the Upper Jurassic section (Bazhenov and Georgiev Suites and the upper part of the Tyumen Suite). Seals tend to be discontinuous in the Lower and Middle Jurassic section, although persistent clay or shale intervals are present in the lower and middle parts of the Tyumen Suite. Because of highquality source rocks, the Jurassic section has a high generating capacity, but it contains few large fields compared with the Cretaceous section owing to poorer reservoir development. Larger reserves are found in the Late Jurassic Vasyugan Suite reservoirs owing to better-quality reservoirs and a more effective seal (Bazhenov Suite and early Neocomian shales), compared with those of the Early and Middle Jurassic Tyumen Suite reservoirs.
II.3.2 Kaimysov, Vasyugan, and Paidugin Regions
Sixty-five or more oil and oil & gas fields that produce from Jurassic reservoirs, or from associated weathered granitic or metamorphic rocks, are located in these three regions in the southeastern part of the basin (Enclosure II.1). Most of the fields are located on domal or anticlinal structures that reflect basement highs or buried hills beneath the Jurassic unconformity. Production is mainly from sandstone and siltstone reservoirs in the Vasyugan Suite (Late Jurassic) and from Middle Jurassic sandstone or siltstone reservoirs in the Tyumen Suite. Reservoirs are also present as updip stratigraphic trap pinch-outs on weathered granite hills beneath the unconformity (like those described from the Shaim Arch area, Section II.3.4). A few fields, particularly in the Nyurol’ and southern Vasyugan regions, are also productive from Palaeozoic carbonate reservoirs and weathered basement rocks lying in contact with Jurassic reservoirs (Section II.2.1).
II.3.3 Middle Ob and Southern Parts of the Nadym-Pur and Pur-Taz Regions
A large number of fields produce oil or oil & gas from Jurassic sandstone and siltstone reservoirs in these regions of the central basin area. The Early and Middle Jurassic is mainly of continental origin in this region, and accumulations are small. The Upper Jurassic is mainly marine, with littoral, deltaic and foredeltaic deposits with relatively persistent sandstones, many with good porosity and permeability, interbedded with shale. Shale content increases rapidly from east to west across the region. The sandy facies is most prevalent on the Nizhnevartovsk Arch and on other structures to the north and south, becoming increasingly discontinuous and more shaly toward the west in the area of the Surgut arch (Enclosures II.1 & 5). Most of the oil accumulations are on the flanks of the Nizhnevartov Arch, and traps are mostly combined stratigraphic and structural. Sandstone reservoirs commonly wedge out updip onto arches or anticlines, and at least half of the traps are stratigraphic.
Organic-rich marine sapropelic beds are prominent in most of the Jurassic section across the entire Middle Ob area, especially in the Upper Jurassic, where the Bazhenov Suite forms a regional seal as well as a rich source rock for Jurassic oil. The relatively thick overlying early Neocomian shale section (Frolov Suite) also provides an additional regional seal for Jurassic sandstone accumulations, as well as for fractured shale accumulations in the Bazhenov Suite.
II.3.4 Pre-Ural and Frolov Regions
These productive regions lie in the western part of the West Siberian province. The first discoveries in this province were made here during the early 1950s. The prolific deltaic and other nearshore sandstones of the central and northern regions of the WSB are not present here, having shaled out westward into the Khanty-Mansi Trough. A widespread Early and Middle Jurassic western sandy reservoir facies of the Tyumen Suite occurs here, however, pinching out against erosional highs on the pre- Jurassic surface. These deposits are continental coal-bearing sandstones and claystones derived from the Ural uplands in the west (the Bogulkinsk unit). Towards the east, they grade into coastal plain and marine deposits (Fig. II.3.2). Overlying the Tyumen Suite are Late Jurassic argillaceous sediments of the Vasyugan, Georgiev, and Bazhenov suites and equivalents. Dark-grey, commonly sapropelic claystones and shales of Neocomian age rest conformably on the Bazhenov Suite in most of the area. Most oil and gas fields in this area occur in stratigraphic pinch-out and drape traps over the basement highs. The Berezov gas area contains twenty or more gas fields. Most reservoirs are basal sandstones of Late Jurassic age, which pinch out against basement highs. Weathered basement rocks are productive in a few fields.
The Shaim oil area includes the Shaim Arch and a large region of smaller highs to the northwest. Oil accumulations occur in Late Jurassic basal sandstones, onlapping and pinching-out against basement uplifts that were islands in the Late Jurassic sea. The Krasnoleninsk Arch in the Frolov region also hosts several oil fields (Fig. II.3.1).
II.3.5 Northern WSB, (Yamal, Gyda, Nadym-Pur, Pur-Taz, Ust-Yenisei, and Taimyr Regions)
The Jurassic is quite poorly known in this region owing to its depth of burial. It has been penetrated by 50 or so wells. Good-quality reservoir sandstones appear to be common over the Ust-Yenisei, Taimyr, Yamal and Pur-Taz regions. Sapropelic marine shales are also widespread, especially in the central and eastern parts of the northern area, grading into and inter-tonguing with the adjacent sandstone facies. Significant hydrocarbons, including oil, may therefore be expected, although further exploration drilling would be needed to confirm this. Gas production is reported from Middle and Late Jurassic sandstones in the southern part of the Ust-Yenisei region, and gas has been encountered in at least one field in the Yamal region. Gas-condensate and some oil are found in Jurassic reservoirs in other parts of the northern basin. Oil accumulations are reported in the Vasyugan Suite in the Gubkin, North- and South-Kharampur, and Verkhnechasel’ka fields, and in the Tyumen Suite in the South-Kharampur field. Yermakov and Skorobogatov (1984) consider that the Nadym-Pur and Pur-Taz regions, between the Nadym and Taz Rivers, are prospective for exploration within the Jurassic section, although they believe that prospects are likely to host only small to medium-sized gas accumulations. Early and Middle Jurassic reservoirs are thought to be favourable for gas-condensate accumulations, and Late Jurassic reservoirs for light oil.
Yermakov and Skorobogatov (1984) also consider that the most probable time of formation of oil and gas in the Jurassic reservoirs of the northern area was during the Neocomian to Albian period. They suggest that the potential for Jurassic reservoirs to accumulate hydrocarbons later than this decreased considerably owing to deterioration of reservoir quality associated with subsidence and compaction. Opportunities for hydrocarbon migration reduced sharply, and hydrocarbons generated later than the Albian were not able to move into larger discrete accumulations, but are spread over numerous non-commercial accumulations.
Fomin et al. (2001), on the basis of a major review of vitrinite reflectance measurements, consider that Jurassic source rocks in the Northern WSB remain in the oil window down to depths of 4.0 km.
In the next issue of ROGTEC Magazine, we will study the Petroleum Geology of the Cretaceous.