2.1 Geological setting
Three structural-tectonic stages are distinguished in the geological section of the West Siberian plate: the lower, intermediate, and upper. The lower stage is a folded basement formed during the Proterozoic and Paleozoic under the impact of various tectonic-magmatic events. It is composed of effusive, intrusive and sedimentary strongly dislocated and metamorphozed rocks. Later on, sustained mersion of the crust occurred, accompanied by the accumulation of sedimentary terrigenous and terrigenous-chemogenic rocks. The intermediate structural level was formed in the immersed parts of the basement and is related to the sediments of the Permian-Triassic age. According to the data of seismic surveying, the sediments of this stage are composed of the Tampey series of the Triassic age (Fig. 2). The upper structural-tectonic level is composed of a thick mass of Mesozoic and Cenozoic formations accumulated under the conditions of long-term and persistent warping of the basement. This layer, or exactly the sedimentary cover of the plate, has been most thoroughly investigated (Fig. 2). The thickness of the sedimentary cover may reach 6.5–7.5 km. During the Late Eocene, the rise of the Earth’s crust started in the Arctic sector of the northern regions of Siberia, which involved the formation and re-formation of hydrocarbon pools in in the sedimentary cover of the northern part of West Siberia. The major hydrocarbon resources are concentrated in the Cretaceous and Jurassic reservoirs. Two productive complexes are distinguished in the Cretaceous section: the Lower Cretaceous (the Barremian – Lower Aptian, the Berriasian-Hauterivian) and the Aptian-Albian-Cenomanian, while the productive complexes distinguished in the Jurassic section include the Oxfordian, Bathonian, Aalenian-Bajocian, Toarcian, Pliensbachian, Hettengian-Sinemurian ones. The structure of the sedimentary cover is clearly seen in the deep seismogeological sections along the regional profiles (common depth point, CDP). The zone of commercial oil and gas occurrence is more than 2.5 km thick and includes the sediments of a broad stratigraphic range – from the zone of contact of the Paleozoic basement with the sedimentary cover up to the Upper Cretaceous. The largest hydrocarbon fields are the Urengoy, Medvezhye, Yamburg, Bovanenkovo, Vankorskoye and some other.
The Urengoy oil and gas condensate field is situated in the north-eastern part of the Nadym-Pur oil and gas bearing area. It was discovered in 1966 by the first exploratory well No. 2, the test of which produced a gas spouter from the Cenomanian sediments, with the gas flow rate of 995 thousand m3/day at the orifice of 25 mm. The commercial oil and gas bearing potential of the lower bedded sediments was confirmed in 1968 (Brekhuntsov, Bityukov 2005).
The Urengoy multilayered oil and gas condensate field is unique in the assured hydrocarbon resources (measured reserves: 16 trillion m3 of natural gas and 1.2 billion tons of gas condensate). Hydrocarbon pools were discovered at this field in the sediments from the Cenomanian to the Lower Jurassic inclusive. The traps of the pools are structural and structural-lithologic (Fig. 3).
The Cenomanian pool is a massive and floating gas pool. The size of the Urengoy pool itself is 30.0×120.0 km, its height is 230 m; the size of the En-Yakhin pool is 55×64 km, its height is about 100 m, the size of the Pestrsovaya pool is 29×85 km, and its height is 88 m. The free gas contains mainly methane (average methane content is 98.5%). With respect to gas resources, the Cenomanian pool is the major object of the Urengoy field. Reservoir rocks are sandstone and siltstone, their effective gas saturated thickness varies from 6.6 m to 200.0 m, the average open porosity is 18–38%, permeability is 5.0-1000·10− 3 µm2, gas saturation factor is 69–74%. The pools of the BU group beds are gas condensate with oil rims, gas condensate bedded and roof-type, sometimes lithologically screened. The major beds with respect to the resources and the oil and gas productive area are the productive bends of the BU group bedded within the depth range of 2500–3100 m. The pools of the group Yu beds are bedded, lithologically screened or lithologically limited, and contain oil. Reservoir gas in them contains mainly methane. The Jurassic pools are characterized by anomalously high formation pressures [Brekhuntsov, Bityukov 2005].
The Bovanenkovskoye oil and gas condensate field is situated in the central part of the Yamal Peninsula; it was discovered in 1971. The thickness of the Mesozoic-Cenozoic sediments in the sedimentary cover is 3200–3600 m (Brekhuntsov, Bityukov 2007). The rocks of the basement are argillites, clay slate, basalts and siltstone of the Permian age, with the signs of oil and condensate. The field is unique in the revealed resources of raw hydrocarbons (measured reserves: 4.9 trillion m3 of natural gas). The commercial oil and gas bearing potential involves the cross-section from the Upper Cretaceous to the Lower Jurassic: Marressalynsky (PK1, PK9, PK10 and KhM1, KhM2), Tanopchinsky (TP1-TP18), Akhsky (BYa1-BYa5), Malyshevsky (Yu2, Yu3), Dzhangodsky (Yu10), and Levinsky (Yu12) formations (Fig. 4).
The beds of the PK group (Upper Albian - Cenomanian) are represented by the alternating sand-siltstone and clay rocks. Reservoir rocks are sandstone and siltstone, with the effective gas-saturated thickness varying from 7.26 m to 20.33 m; their average open porosity is 24.6–25.0%, gas saturation factor is 55.7–56%. Methane is the prevailing component in gas composition (95.4–98.8%). The pools are floating and massive, they contain gas condensate and gas. Pool sizes are 3–7 × 8–14 km, their height is 16 to 51 m. The beds of the KhM group (the Lower to Middle Albian) are represented by alternating sandstone, siltstone, and clay rocks. Reservoir rocks are sandstone and siltstone, with their effective gas saturation thickness within the range 2.81–14.08 m, average open porosity 24.6%, gas saturation factor 57.2%. Methane dominates in gas composition (95.7–96.2%). The pools are bedded (sheet-type) and roof-type, and contain gas condensate. Pool sizes are 20.5–25 × 50–56 km, the height is 216–265 m. The beds of the TP group (the Aptian) are composed of unevenly alternating sandstone, siltstone, and clay rocks. Reservoir rocks are sandstone and siltstone, their effective gas saturation thickness varies from 2.0 m to 48.37 m. The average open porosity is 20-26.5%, gas saturation factor is 59-77.6%. Oil is high-paraffinic, low-sulfur, low-resin, with the density of 835 kg/m3 (Table 6.1). The reservoir gas is mainly methane (91,3–95,3%). The pools contain gas, gas condensate, as well as oil-and-gas condensate (the ТP18 bed). The pools are bedded, roof-type and massive (the ТP1 − 6 bed) (Fig. 4). The sizes of the pools are 5.5–27.5 × 6.5–57 km, their height is 13–297 m. The beds of the BYa group (the Lower Hauterivian) are composed of unevenly alternating sandstones, siltstone, and argillites. Reservoir rocks are sandstone and siltstone, the average open porosity is 20%, gas saturation factor is 73%. Methane dominates in the gas composition (89.0–91.0%). The pools are bedded and roof-type; they contain gas condensate. The beds of the Yu group (the Lower and Middle Jurassic) are composed of interbedding sandstones, siltstone, and argillites. Reservoir rocks are sandstone and siltstone; the average open porosity is 14–15%, gas saturation factor is 65–70%. The pools are characterized by anomalously high formation pressure. Methane is the dominating gas component. The pools are bedded and roof-type, containing gas condensate. Results of physicochemical analyses reveal the presence of oil with medium density, containing large amounts of paraffin and low amounts of resins (Brekhuntsov and Bityukov 2007).
2.2. Hydrogeological setting
According to the accepted pattern of the hydrogeological differentiation of the Mesozoic-Cenozoic water-drive system of the West Siberian artesian basin, the following aquifer systems are distinguished (from top to bottom): the Paleogene - Quaternary; the Upper Cretaceous; the Aptian-Albian-Cenomanian; the Berriasian-Hauterivian; the Upper Jurassic; the Lower and Middle Jurassic; the Triassic, and undifferentiated Paleozoic (Novikov 2020a). The Mesozoic systems, to which the commercial oil and gas content is related, are composed mainly of permeable sandstone-siltstone rocks separated by argillite-clay reservoir rocks. In general, the porosity of sandstone/siltstone within the boundaries of the oil and gas bearing sediments varies within a broad range from 0.70 to 42.55%, decreasing regularly from the Aptian-Albian-Cenomanian system to the Pre-Jurassic reservoirs. Hydrocarbon-productive systems are isolated from the hypergenesis zone by the regional Turonian-Oligocene confining bed; its screening capacity is distorted only with the lithologic substitution in the near-edge parts of the basin (Shvartsev and Novikov 2004; Novikov and Lepokurov 2005; Novikov and Sukhorukova 2015). The major hydrodynamic feature of the region under investigation is a broad occurrence of increased and anomalously high formation pressure at a depth of 2.8-6.0 km, both in the Jurassic aquifer systems and in overlying beds up to the Berriasian-Hauterivian ones (Novikov 2017), which decreases to the hydrostatic values while approaching the periphery of the West Siberian artesian basin. Two types of natural water-drive systems are observed at present: elision-based in the internal regions and infiltration-based in the external marginal regions (Novikov 2019).
The Arctic regions of the West Siberian artesian basin are distinguished by rather low content of total dissolved solids (TDS) in groundwaters and brines. The waters dominating in all aquifer complexes are (according to the classification proposed by S.A. Shchukarev) of sodium chloride, sodium chloride-hydrocarbonate, sodium hydrocarbonate-chloride types with TDS varying from 2–5 g/dm3 in the near-edge regions to 63.3 g/dm3 in the central parts (Novikov 2020b; Novikov et al. 2019). The groundwaters of the Mesozoic stage are distinguished by the absence of sulfates and a substantial content of dissolved organic substances and hydrocarbon gases.
The saturation of groundwaters with gas exhibits different trends and may vary by a factor of two or more within a single bed. However, a general trend is conserved: an increase in gas saturation with the depth, from 0.3-3.0 L/L in the Aptian-Albian-Cenomanian system to 0.9–5.7 in the Lower and Middle Jurassic system (Novikov and Borisov 2021).