Graham Blackbourn: Blackbourn Geoconsulting
Neocomian reservoirs are the main oil producers in the West Siberian Basin. Owing to the nature of associated source rocks, they are mainly oil-bearing in the central part of the WSB (Middle Ob region), but are largely gas-bearing in the Northern WSB.
The Neocomian section varies in thickness from around 800 m in the Southern WSB to more than 1500 m in the north. At least thirty productive sandstone and siltstone horizons, separated by claystone or shale units, have been designated. Attempts to correlate these horizons across the basin during the early years of exploration and production activity gave rise to numerous inconsistencies, as it became clear that sandstone beds which appeared to correlate on a “layer-cake” model of deposition, counting up from the underlying Bazhenov Suite, differed significantly in age.
Recognition of productive beds led to the designation of horizons B1 to B22 within the Valanginian to Hauterivian section, and horizons A1 to A12 in the late Hauterivian to Barremian section. However, high-resolution biostratigraphic analyses showed that most reservoir beds tended to shale out from east to west. The proportion of sandstone within the Neocomian succession reaches 60% or more on the eastern side of the basin, 25-40% in the Middle Ob region, and less than 10% in the west of the basin.
Examination of good-quality seismic data available over the past 10-20 years has made it clear that most of the Neocomian succession within the WSB comprises prograding clinoforms, which ultimately filled virtually the entire basin (Fig. II.4.1). This model was first suggested by Naumov (1977), but was not widely recognised until nearly 20 years later (Karagodin, 1994; e.g. Fig. II.4.1). The direction of progradation was broadly from the ESE, the source of most of the sediment, with some minor progradation of a sedimentary wedge from the Urals towards the east. Progradation was interrupted on a number of occasions by rises in sea level which caused a blanket of marine shale to be deposited over the progradational surface, before progradation commenced again.
The topset of each progradational unit was deposited in a variety of environments from continental (including red beds), through fluvio-deltaic to littoral and shallow-shelf environments. The slope led down into deeper water, although as described below there was quite a substantial accumulation of base-of-slope sands (Achimov Suite), probably deposited as distinct basin-floor fans (Ukhlova et al., 2004) forming separate mappable sand-mounds (Salmin, 2006). A scarcity of core has limited the scope for detailed sedimentological examination of the clinoformal deposits; Ershov et al. (2001) have perhaps provided one of the best core-based descriptions to date.
The Neocomian clinoformal succession has been described from a sequence stratigraphic standpoint by Pinous et al. (2001).
The sand-rich clinoformal structures, sealed up-dip both stratigraphically and by transgressive shales, and “dipping their toes” in the rich Bazhenov Suite source rocks, obviously represent very prospective exploration targets, and have been prolific producers.
Because of the complexity of the sand-body geometry, the earlier attempt to create a basin-wide stratigraphy for the Neocomian was largely abandoned, with separate systems for designating productive beds being established for individual areas or fields. Fig. II.4.2 provides an example of the reservoir stratigraphies established for the Surgut and Nizhnevartovsk arch areas respectively (the main area of production from Neocomian reservoirs). Note that the division into upper Neocomian “A” units and lower Neocomian “B” units has been retained in both areas, but that the bed designations are otherwise quite distinct from one another.
Quite thick, generally good-quality sands found in wells in many parts of the WSB low in the Neocomian, a little above the Bazhenov Suite, were originally named the Achimov Suite. It was later recognised that these sands comprise a series of base-of-slope sands formed at the foot of successive clinoforms, and that they do not form a single continuous sand sheet. Commonly being in communication with the Bazhenov Suite source rock, they comprise a significant exploration target, although in some cases any oil has passed through them into up-dip sands. The Achimov sands are usually given reservoir-bed designations beginning “Ach” – e.g. Ach1 (Fig. II.4.2). The Achimov beds are of course generally laterally equivalent to reservoir beds deposited on the shelf or top-of-slope, in shallower water. E.g., Fig. II.4.2 makes it clear that base-of-slope bed Ach4 in the Surgut Arch area is the lateral equivalent of bed BV13, deposited on the shelf in the Nizhnevartovsk Arch area.
Although individual sand beds have limited lateral continuity, the shale beds deposited during periods of elevated sea level, draping the clinoformal structures, are of far greater extent, and some can be recognised over much of the basin. Each of these major transgressive shale beds has an individual horizon name (“Sarmonov”, “Cheuska”, “Pokachev” etc.). They form useful marker horizons between individual hydrocarbon areas (Fig. II.4.2).
Figure II.4.3 is an attempt based on numerous well sections and seismic data to construct two realistic cross sections through the Neocomian clinoforms in the central WSB area (over the northern flank of the Surgut Arch). Although various authors have attempted to count and “pin down” the individual clinoformal structures (11 or 12 are commonly enumerated), it is evident from Figure II.4.3 that each so called clinoform is itself a package of clinoformal units, and that there is a great deal of objectivity involved in distinguishing one from another. Comparison of the internal structure of individual clinoformal packages between the two sections, which vary along their length from around 30 to 100 km apart, makes it clear that there is considerable variation along strike within each package. Most of the clinoformal packages are defined at the top by one of the transgressive shale units, although some of the packages contain several such shale units. It is common to name each clinoformal package according to the name of the associated blanketing shale bed.
Although this approach, of naming around 11 or 12 individual Neocomian clinoforms across the entire WSB, has tended to play down the considerable complexity of the clinoformal system, it does provide a useful basis for considering the development of the system in simple terms. Enclosure 7 (based on a map by Surkov et al., 2001) attempts to map out the clinoformal packages across the basin, distinguishing those prograding toward the WNW from those prograding towards the east. The line designating each clinoformal package marks the position of the top of the slope at the time it was buried below its blanketing shale. The position of this shelf-break is not always well-defined, as is clear from Figure II.4.3, but nonetheless the general pattern of progradation through time is evident.
During the Barremian there was a fall in sea level which exposed much of the shelf, leading to continental and red-bed deposition, although marine progradation may have continued within the deeper-water parts of the basin in the area of the Khanty-Mansi Trough. This was succeeded in the later Aptian by a significant transgression, depositing the thick shales of the Alym Suite, which blanketed most of the basin apart from the southeastern shelf, and marked the end of the progradational phase of basin filling. The Alym Suite also forms a regional seal to the Bazhenov-Neocomian hydrocarbon system within the West Siberian Basin.
Hydrocarbons occur within the reservoirs associated with the Neocomian clinoforms in two main horizons. The first comprises the beds on the shelf as far as the shelf-break, including equivalent fluvio-deltaic deposits to the east. The shelfal area has a reliable seal and a large number of structural traps. The beds here contain generally the most a really extensive and potentially the most productive oil accumulations. The second main reservoir horizon comprises the permeable parts of the Achimov base-of-slope beds. The Achimov section is generally composed of a large number of lenticular permeable beds, and commonly lacks any single reliable seal. The Achimov Suite is therefore much more commonly associated with combined structural-stratigraphic and purely stratigraphic traps. In a few areas, small oil accumulations have also been discovered within lenticular sands and silts on the slope below the shelf-break.
Primary migration of oil from the Bazhenov Suite into the Neocomian clinoforms was assisted by the presence of so-called “anomalous” Bazhenov Suite sections, recognised both on seismic sections and in wells. In these sections the bituminous beds are broken up, deformed and intermingled with permeable beds of the Achimov succession. This probably resulted where the Achimov was deposited in a series of high-energy massflows (turbidites, and possibly secondary slumps and slides), which ripped up the unconsolidated Bazhenov claystones that constituted the basin floor. As a result, the area where the bituminous rocks are in direct contact with the permeable beds was increased, greatly improving the drainage of the fluids generated.
Oil migration was assisted by the slope of the permeable beds towards the oil-generating source rocks. Furthermore, distinct “channels” with improved reservoir quality were formed with an approximately east-west orientation, associated with the transport of clastic material from the east. These channels assisted the oil to migrate up-dip towards more elevated structural elements. There is evidence for relatively long-distance migration of oil from the Bazhenov Suite, over distances of several tens of kilometres. The Neocomian reservoir sandstones in the Middle Ob region are composed of 25-40% quartz, 30-55% feldspar, and 3-6% mica, and are typically very fine to fine-grained sands. To the north (in the Urengoi and Gubkin fields), equivalent sands contain 25-50% quartz, 30-50% feldspar, and 3-10% mica, with a similar grain-size range.
Porosity and permeability generally increase upwards in individual reservoir horizons, because of lower clay content and better sorting within the shallower and shelfal positions.
Shale (clay) units of both marine and non-marine origin and of both regional and local extent are common within and overlying the Neocomian section. Most widespread are the marine shale units deposited during significant transgressions which interrupted deposition of the progradation units, as described above (Fig. II.4.2). Regional sealing of the Neocomian sandstone complex was completed by deposition of the thick transgressive marine shale of the Alym Suite.
Almost all hydrocarbon accumulations in Neocomian rocks found to date have been on structural anticlines or arches, many of them inherited from older basement uplifts. However, few of the reservoir units are laterally continuous over long distances; they tend to be highly irregular in thickness, extent, grain size, and clay content within a given horizon. A strong stratigraphic trapping element is therefore present in most fields, and many traps are located on the flanks of structures in addition to the crests. In the area of the Surgut Arch, Mamleyev (1976) has recognized 19 stratigraphic accumulations containing three giant, two large, and one medium-sized accumulation. 23% of the reserves on the Surgut Arch are estimated to lie within such stratigraphic traps. At least twenty productive sand units have been recognised within horizon BV10 on the Nizhnevartov Arch (Fig. II.4.2), most of which shale-out over short distances. The thickness of individual sand beds is highly variable, and clay sections occur between the sands. In a single stratigraphic interval within the Surgut Arch region (Horizon BS10; Fig. II.4.2), an extensive zone of potential stratigraphically trapped hydrocarbons has been delineated in a north-northeast-trending belt that is 30-80 km wide and 300 km long, lying between the Mamontov field in the south and the Muravlenkovsk field in the north. Within this belt there are 16 Early Cretaceous hydrocarbon-bearing zones, in addition to Jurassic zones. The combination of the continuous tectonic growth and shifting facies zones which characterised the WSB combine to make stratigraphic traps of particular significance to this basin.
II.4.1.4 Source Rocks and hydrocarbon generation and accumulation
The most important source rock for Neocomian oil accumulations is the Bazhenov Suite of mainly Tithonian age. The upper parts of the Bazhenov Suite are of early Berriasian age, and the suite continues upwards in places into the Valanginian to Hauterivian Tutleim Suite, but the most prolific Bazhenov source rocks occur in the Late Jurassic.
Neocomian claystones and shales are dark-coloured and bituminous, especially in the lower part of the section, over much of the western and central basin areas. Humic organic material, including coals, is present in variable amounts in the eastern and southern parts of the basin and in parts of the Pre-Ural western margin of the basin. TOC values within shale sections are reported to be higher within the Berriasian and Valanginian section than in the Hauterivian to Barremian, where values reach a little over 1%. Values are also reported to be generally higher in the Northern WSB than the Middle Ob. Yermakov and Skorobogatov (1984) comment as follows on the distribution of organic matter:
Berriasian to Valanginian
Western and central regions of WSB (predominantly sapropelic and mixed humicsapropelic kerogen): TOC values of 0.5-2.0% (average 0.66%) in clays; 0.3-0.6% (average 0.5%) in sandstones and siltstones.
Northern WSB (humic components are greater, including coal beds): 1.05% average in clays; 0.65% average in sandstones and siltstones.
Hauterivian to Barremian
Middle Ob and Mansi region (mixed sapropelic-humic kerogens, grading to sapropelic): 0.3% in east to 1.0% in west (average 0.49%) in clays; 0.2-0.6% (average 0.35%) in sandstones and siltstones.
Northern region: 0.89% average in clays; 0.70% average in sandstones and siltstones.
The timing of hydrocarbon generation and accumulation within the Neocomian reservoirs of the Middle Ob region has been interpreted by Ozeranskaya (1979) and Schepetkin (1980) as follows:
First stage: early Turonian; first phase of hydrocarbon accumulation in the Middle Ob region.
Second stage: oil charge complete by the end of the Mesozoic.
Third stage: migration of gas-condensate and gas from deeper parts of the basin to the north of the Middle Ob region, related to mid-Tertiary uplift and expulsion of gas from solution in formation waters, resulting in the formation of gas caps.
The combined thickness of coal beds in the Neocomian to Cenomanian sediments in the Northern WSB ranges from 10-15 m along the margins of the basin to 30 m in the central parts. The total mass of organic matter in the Cretaceous section in Western Siberia has been calculated as follows: Berriasian to Valanginian, 6.7 x 1012 million tonnes; Hauterivian to Barremian, 10.0 x 1012 MT; and Aptian to Cenomanian, 48.4 x 1012 million tonnes. Table II.4.1 shows the calculations of Yermakov and Skorobogatov (1984) for the type and distribution of organic matter in the basin.
According to Yermakov and Skorobogatov (1984), maximum temperatures attained by the Neocomian beds were 90-140° C, and the hydrocarbon type was controlled largely by the type of organic matter. From south to north in the central basin area, the kerogen composition changes from 60-90% sapropelic and mixed humic-sapropelic in the Middle Ob area, to 30-40% in the Nadym-Pur and Pur-Taz regions, and 20-30% on the Yamal and Gyda peninsulas, accompanied by a northward change to less marine material. A shift occurs in the same direction in the main hydrocarbon type from predominantly oil in the Middle Ob region, to gas-condensate and oil immediately north of the Middle Ob area, to gas-condensate farther north. Small oil rims are present in gas fields in the Urengoi, Taz, and Yamburg areas.
II.4.2 Aptian to Cenomanian
Hydrocarbons produced from post-Neocomian, reservoirs are almost entirely gas or gascondensate in fields located north of the Middle Ob region, in reservoirs mostly of the Cenomanian Pokur Suite. Approximately 60 gas and gas-condensate fields have been discovered, many of them of giant or supergiant size. The approximate areal extent of the largest are as follows: Urengoi, 200 x 20-30 km; Medvezh’ye, 120 x 25 km; Yamburg, 170 x 45 km; Zapolyar, 50 x 30 km; and Taz, 26 x 15 km. Production is from thick, loosely compacted, friable sandstone and siltstone horizons interbedded with silty clays containing terrestrial plant remains and coals. The combined net thickness of reservoir beds reaches 500-800 m. The sandstone proportion increases from west to east, reaching 60%, and locally up to 80%, over a wide area of the eastern basin. Eighteen main reservoir horizons have been recognized (PK1—PK18) at depths from 500-1800 m. Most of the largest accumulations lie within the massive PK1-PK6 reservoir units in the upper part of the Pokur Suite. The gas column within horizon PK1 can reach over 200 m high.
These Cenomanian reservoirs are massive blanket sandstone deposits laid down during the major Albian to Cenomanian regressive phase of basin development, prior to the Turonian transgression. Around 62% of the known initial reserves of free gas in the WSB were in these reservoirs. The gas composition here differs significantly from that of the Jurassic and Early Cretaceous accumulations, which contain a wide range of heavier hydrocarbons, reaching up to 30% or more of ethane in the Middle Ob region. Gases in the post-Neocomian reservoirs are mostly dry methane with only a small percentage of higher gases: mainly ethane with almost no nitrogen, CO2, or H2S. The gases are low in condensate, (e.g. 0.20 g/m3 gas in the Urengoi field and 0.25 g/m3 in the Medvezh’ye field). In contrast, the gases in the Neocomian are very rich in condensate, with proportions ranging from 56 to 610 g/m3.
Neocomian accumulations within the Middle Ob region are almost all of oil or oil-and-gas, and accumulations of dry gas in this area are found only in the Cenomanian reservoirs (e.g. in the Samotlor field). Residual oil within these gas accumulations is interpreted as indicating that the traps were once filled entirely with oil, which was subsequently displaced from the traps by thermal gas resulting from greater subsidence of the source beds, or by gas expelled from solution as a result of Tertiary uplift, erosion, and reduction of formation pressure. There is a general decrease in oil density with depth, with a corresponding decrease in tar and sulphur content, and an increase in wax (Ozeranskaya, 1979). Most stratigraphic traps occur where sand beds pinch-out and are replaced by shales.
According to Yermakov and Skorobogatov (1984), the main factors accounting for the huge reserves of gas in the Northern WSB are as follows:
1. High contents of humic kerogen and coal beds within the succession have generated large quantities of gas.
2. Large volumes of source and reservoir rocks in which migration capacity was high.
3. Large structures with great closure far from the borders of the basin.
4. The presence of thick seals.
5. The relatively young age of formation of the gas accumulations.
These genetic conditions are best for the Hauterivian to Cenomanian sediments in the central part of the northern region (Urengoi, Yamburg, Taz, and Nadym) and to a lesser extent for the Valanginian sediments of the Nadym-Taz area and also for the Neocomian to Aptian and Albian to Cenomanian sediments in the Yamal and Gyda hydrocarbon regions. The differences in vertical distribution are governed by the presence of seals in the section (Yermakov and Skorobogatov, 1984).
Aptian – Cenomanian source rocks contain mainly humic kerogens, including coals. Calculations show that by the end of the Cenomanian, the high concentrations of coaly organic matter in the Hauterivian to Cenomanian sediments had begun to generate large volumes of methane. Most of this gas was lost because no seal was yet in place. A second stage of generation began in the Turonian, after the Cenomanian and older reservoirs had been effectively sealed by the overlying regional Turonian clays. Gas did not migrate from deeper horizons at this time because it was sealed by Neocomian and Aptian to Albian clays. Between the Turonian and the mid-Oligocene, gas generation from the humic organic matter continued. This gas was trapped in Cenomanian and older reservoir sands. The lower part of the thick Turonian clays may have generated substantial amounts of biogenic gas which, unable to escape upwards, migrated downwards to contribute to the almost pure methane in the Cenomanian reservoirs. In the mid-Oligocene and Neogene, gas generation essentially ceased because of regional uplift, but redistribution of the earlier trapped gas accumulations occurred. The decrease in pressure accompanying the late Tertiary uplift and erosion led to the release of gas from solution in the formation waters of the Mesozoic sediments, thereby adding yet more gas to that already present in the reservoirs (Littke et al., 1999). The uplift also had the effect of moving water from southern to northern parts of the basin, so enlarging the effective catchment area of the gas field. Further volumes of gas may have been supplied from the formation waters as a result of freezing to great depth during glacial times. As the formation waters became frozen, the gas in solution would have passed into the gas hydrate stage. On subsequent thawing during interglacial time, part of the gas released by melting of the hydrates would remain as free gas to collect in the reservoirs.
Karagodin (2004), considering the factors influencing the filling of the major Cenomanian reservoirs in the north of the West Siberian Basin, reckoned that there was a high probability of a similar very large gas accumulation occurring within the South Kara Sea, to the west of the southern part of the Yamal Peninsula.