Graham Blackbourn: Blackbourn Geoconsulting
In some central and southern parts of the West Siberian Basin (WSB), oil and gas shows are widespread and numerous accumulations have been discovered in weathered and fractured mid and late Palaeozoic rocks below the Jurassic unconformity, especially on structurally high areas adjacent to Late Permian and Triassic grabens, and including Palaeozoic reefs and other carbonate rocks (Karnyushina, 2005). Most of these rocks are metamorphosed to varying degrees, but areas of relatively low-grade Palaeozoic lithologies have been encountered.
Lithologies hosting Triassic and Palaeozoic sub-unconformity reservoirs within the Middle Ob region include (Myasnikova et al., 2005):
» Basic volcanics: basaltic rocks and associated tuffs (Surgut Arch, Fedorovsk crest).
» Intermediate volcanics: andesite-porphyries, andesite-basalt-porphyries, brecciated rhyolitic porphyries.
» Acid volcanics: dacites, rhyolites, ignimbrites (North Danilovsk area) and associated welded tuffs.
» Ultrabasic rocks over small areas.
» Metamorphic slates, gneisses, hornfelses, charnokites (Vengapurovsk field).
» Intrusions: plagiogranites, granites, serpentinites forming dykes, sills and small intrusions.
» Lightly deformed and metamorphosed clastic and carbonate rocks, almost horizontal, of Carboniferous and Devonian age (Khanty-Mansi area, Gorelaya field).
Zadoenko et al. (2004) list test results from about 35 wells drilled into the Palaeozoic beneath the base-Jurassic unconformity in the east-central part of the WSB which yielded hydrocarbons. Typical flow rates were several tonnes per day of oil, with a maximum of 90.4 m3/day from vuggy carbonates over the 2690-2734 m interval of the Medvedevskaya-6 well.
Analysis of over 50 such oil and gas accumulations discovered immediately below the base-Jurassic unconformity by Surkov & Smirnov (2003) showed to their satisfaction that, in all cases, they are in direct communication with hydrocarbon-bearing Early, Middle or Late Jurassic reservoirs, onlapping the underlying topography. These authors concluded that, although Palaeozoic reservoirs exist, the only significant source rocks (apart from in the Nyurol’ Basin, Section II.2.1.1) lie within the Mesozoic section. Factors thought to affect hydrocarbon accumulation adversely within the pre-Jurassic section include: 1) high temperatures, with the probability that there would have been widespread thermal destruction of oil; potential oil source rocks are overmature in most areas; 2) active Palaeozoic and Triassic tectonism and widespread igneous activity, which would have caused the escape or destruction of earlier accumulations; 3) generally poor reservoir quality (most of the pre-Jurassic accumulations mentioned above are in zones of fracturing, weathering or leaching associated with the sub-unconformity surface); and 4) a scarcity of good-quality seals, such as evaporites and good clays.
Bokcharev et al. (2003) have listed several hundred deep wells from various parts of the West Siberian Basin which penetrated the Palaeozoic (in addition to the Triassic) section. They argue strongly for the presence of potential reservoirs, and also for organic-rich Palaeozoic potential source rocks, and therefore suggest that the Palaeozoic prospectivity has been grossly underestimated. Although they make a plausible case for the presence of occasional gas accumulations within the Palaeozoic, they do not address the points listed above which adversely affect the likelihood of oil.
This view is supported by Fomin (2004), who studied maturity levels of Palaeozoic kerogens from across much of the West Siberian Basin, and reported that most of the Palaeozoic had reached deep into the gas window before the uplift which preceded the Mesozoic depositional cycle. Fomin concluded that most of the hydrocarbons generated during the Palaeozoic would have been lost to the atmosphere before the Jurassic, that later recharge is likely to have been limited to relatively small amounts of gas, and that the chances of finding any large oil accumulations within the Palaeozoic (or sourced from the Palaeozoic within the Mesozoic) were small. Fomin did, however, note that maturity levels were lowest within the Nyurol’ Basin, where thermal gradients are lower, and his conclusions appear to be consistent with the conjecture in Section II.2.1.1 for the origins of the Palaeozoic oil there.
It should be noted however that E. A. Kostyreva (2004) undertook a biomarker study of oils within Palaeozoic (including sub-unconformity) and near-base Mesozoic accumulations in the vicinity of the Nyurol’ Basin, and concluded that they group into three families:
1) Genetically related to Palaeozoic marine source rocks.
2) Polygenetic, formed by mixing of oils from Palaeozoic and Mesozoic source rocks.
3) Genetically related to continental source rocks of the Early Jurassic Togur and Tyumen suites, formed in lacustrine, swamp and fluvial conditions.
It was also concluded that most of the oils within sub-unconformity accumulations were of Group 1; i.e. sourced from the Palaeozoic. It remains likely that the sub-unconformity accumulations throughout most of the WSB are sourced from the Mesozoic, although Kostyreva’s work supports the view that those in the Nyurol’ Basin area are of Palaeozoic origin.
Some Russian geologists regard Palaeozoic sedimentary successions on the Khanty-Mansi basement block (Section I.2.2.3) as prospective. However, although these successions may contain viable reservoirs, no significant source-rock intervals have yet been shown to exist. The Frolov-1 well, drilled to the southeast of Khanty-Mansiisk (Chuvashov and Yatskanich, 2003) penetrated up to 300 m of Early Devonian limestones, which core studies suggest were deposited in very shallow water. They are therefore unlikely to have source potential like that reported for the Devonian limestones of the Nyurol’ Basin (Section II.2.1.1).
The sub-unconformity accumulations mentioned above may add value to Jurassic accumulations, but are unlikely to be economic in their own right. Surkov and Smirnov (2003) have produced a map of the WSB showing where they consider the circumstances are favourable for the development of such accumulations, based on the distribution of hydrocarbon-bearing Jurassic reservoirs onlapping basement highs in areas of possible sub-unconformity porosity (Fig. II.2.1).
II.2.1.1 The Nyurol’ Basin
This basin lies in the southeast of the SWB (Fig. II.2.1). Most of the oil and gas fields it hosts are sub-unconformity accumulations like those discussed above, lying within weathered, dolomitised, fractured and brecciated carbonates (Fig. II.2.2). As discussed above, these would ordinarily be assumed to have been sourced from the Mesozoic, although there are indications here of a Palaeozoic source. Oil here has been encountered within deeper Devonian and even Silurian carbonate traps (in the Maloichsk field), at depths of up to around 4600 m. It is difficult to see how these oils could have been generated from a Mesozoic source – the base-Jurassic unconformity lies at up to 1000 m above these accumulations, and it is assumed that an intra-Palaeozoic source rock was responsible for their generation. As noted above, this is reported to be supported by biomarker evidence.
The deeper accumulations within the Maloichsk field (i.e. those below the subunconformity accumulations) lie within a succession composed mainly of bioclastic limestones, claystones and marls. These are reported to have a total stratigraphic thickness of up to around 3000 m (comprising the >1000-m thick Early Devonian succession (Kyshtovsk, Armichevsk, Solonovsk and Nadezhda suites) and the 2000-m thick Middle Devonian to Early Carboniferous succession (Luginetsk and Tabagansk suites). However, owing to erosion and intense folding, drilling has reached the underlying top-Silurian with a maximum Palaeozoic penetration of around 1000 m. The succession is poorly known, but Kostyreva (2004) concluded that the thick Devonian marine carbonate-dominated succession had a considerable initial source-rock potential.
The reservoirs are reported to depend on secondary porosity, resulting from fracturing and leaching (and dolomitisation) of carbonates, including possible biohermal structures. The accumulations illustrated from the Maloichsk field in Fig. II.2.2 are fault-related, and the secondary porosity may relate to fluid movements along these faults.
There is little direct evidence for the burial history of the Palaeozoic in the Nyurol’ Basin. Danilkin (2005) has suggested that oil began to be generated within the succession during the Palaeozoic, but that the depth of burial did not exceed the oil window at that time, and that Permian uplift and Triassic rifting destroyed any accumulations which had formed. Danilkin further suggests that, after the onset of Mesozoic subsidence, the Palaeozoic of the Nyurol’ Basin re-entered the oil window, and that there was sufficient residual oilsource potential to generate the relatively small amounts of oil now known to exist within the Palaeozoic reservoirs (including the sub-unconformity reservoirs) in this area.
Reserves of oil in the Archinsk field (the largest in the Nyurol’ Basin) are stated by Zapivalov (2004) as 12.85 million tonnes (around 85 million barrels, category C1 + C2 reserves), but the 3 production wells produced only 5.6 thousand tonnes between them in 2003 (average of about 36 barrels per day per well). Productivity from the other fields in the area is of the same order of magnitude. These are not highly productive fields.
Although partly speculative, Danilkin’s proposal for their formation does provide a plausible explanation for the quite numerous but mostly relatively small accumulations within the Palaeozoic of the Nyurol’ Basin. There is no reason why similar accumulations may not be discovered within other thick Palaeozoic successions within the basement of the West Siberian Basin which were not deeply buried prior to the Mesozoic, although the number of areas fulfilling these criteria is unlikely to be large. In any event, the accumulations will almost certainly be small, and it is difficult to identify any areas in the WSB where exploration directed specifically at the Palaeozoic would be justified.
II.2.2 (Permo-) Triassic Rifting
As described in Section I.3.1, a major episode of rifting, volcanism and trap formation occurred at around the Permo-Triassic boundary. The rifts locally include Late Permian sediments below the oldest volcanic horizons; most of the volcanics are of Early Triassic age, although some volcanic horizons occur within the Middle Triassic.
By the Late Triassic, deposition had largely overstepped the rift margins and was taking place over a wide area of the northern WSB (Fig, I.3.2). It probably continued in places to include basal Jurassic continental deposits.
This essentially “syn-rift” stratigraphic interval, from the latest Permian to the earliest Jurassic, lying between the eroded Palaeozoic basement and the overlying post-rift succession, is widely termed the “intermediate complex” in the Russian-language literature. For simplicity it is here referred to most commonly simply as the Triassic interval, since the overwhelming majority of the deposits it includes are of that age.
These Triassic rifts are filled with thick deposits of volcanics and interbedded sediments, and some fluvio-lacustrine rocks. They are generally not considered to be prospective for hydrocarbons owing to the absence of source rocks, although there is a slight possibility that oil and gas may have migrated into these rocks in places from Palaeozoic carbonate source rocks. As noted in Section II.2.1, some of the sub-unconformity accumulations sourced from the Jurassic do occur within the Triassic, including within volcanics (Myasnikova et al., 2005). Accumulations in such locations are likely to be small.
Triassic rocks with vitrinite reflectance values indicating palaeo-temperatures not exceeding the oil window have been reported along the Irtysh River near Omsk. Potential Carboniferous source rocks have also been reported to the southwest in the vicinity of Kurgan (Fomin, 1987).
II.2.3 Triassic (to Earliest Jurassic) Platformal Succession
As noted above, deposition later in the Triassic extended beyond the original rift margins to form a Triassic “platformal” succession comprising thick lagoonal and marine sedimentary rocks, which extend over much of the Northern WSB (Fig. I.3.2). These platformal Triassic rocks thicken rapidly northward to more than 2-3 km. They are buried deeply beneath as much as 6 km or more of Jurassic and younger sediments north of the Urengoi-Yamburg region (Enclosure 6). The lithologies of the Triassic rocks here are not well known. Occasional wells drilled into the Triassic in this area have penetrated a section of alternating dark-grey shales, siltstones, sandstones, and tuffaceous sandstones (Kontorovich et at., 1975). The presence of extensive Triassic clastic deposits in the Yenisei-Khatanga trough near the Taimyr uplift indicates that clastic reservoirs, in part marine, may occur to the northeast, and perhaps in other parts of the northern region, although the occurrence of source rocks is uncertain. A relatively thick section of Triassic rocks is identified on seismic sections across the South Kara basin to the northwest. The possibility of good-quality Triassic clastic reservoirs and source rocks in the north of the basin, especially in the present offshore area, appears plausible. However, apart from in the northeast close to the Taimyr uplift and in the Yenisei-Khatanga trough, these rocks may be buried in most places to depths within or even below the gas window. Good shale seals would be expected to be present within and above the Triassic horizons over this whole area, and adequate source rocks may be present within Triassic marine, lagoonal or even lacustrine units. Yermakov et al. (1979) considered the platformal Triassic in these northern areas to be favourable for gas.
In summary, the prospectivity of the Triassic – both rift-filling and platformal – is similar to that of the Palaeozoic succession in the West Siberian Basin. Sub-unconformity accumulations are known to occur associated with adjacent Jurassic accumulations, but the absence of any known source rocks considerably diminishes the likelihood of discovering oil at any greater depths.