Structure and depositional environment of the Vasyugan Horizon (Upper Bathonian–Oxfordian) in the Aleksandrovskoe arch area (West Siberia)

Russian Geology and Geophysics 52 (2011) 1212–1227 www.elsevier.com/locate/rgg

Structure and depositional environment of the Vasyugan Horizon (Upper Bathonian–Oxfordian) in the Aleksandrovskoe arch area (West Siberia) L.G. Vakulenko*, O.V. Dul’tseva, O.V. Burleva A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia Received 27 May 2011; accepted 3l May 2011

Abstract An integrated sedimentological study of the Vasyugan Horizon in the Aleksandrovskoe arch area was performed, during which its depositional environments were reconstructed and a series of paleogeographic maps was plotted. It has been established that shallow-marine, marginal-marine, transitional, and continental depositional environments existed here at various stages. Most of the arch area is characterized by sections of the transitional Naunak–Vasyugan and Vasyugan–Naunak types; the westernmost part, by sections of the Vasyugan type, and the easternmost part, by sections of the Naunak type. The sand beds of the J1 horizon are low-quality reservoirs, whereas those on the western slope and the northern pericline of the Aleksandrovskoe arch are of better quality. © 2011, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Vasyugan Horizon; types of sections; depositional environment; West Siberia

Introduction Academician Aleksandr Leonidovich Yanshin always paid great attention to various aspects of the lithofacies analysis of sedimentary deposits since the stage of paleogeographic reconstructions in geological research is one of the most interesting and creative, and its results are often of a polemical nature. Starting in the 1960s, when the first deposits of hydrocarbons were discovered in Callovian–Oxfordian sediments on the southeast of the West Siberian basin, they are a constant object of exploration and are being actively studied by experts in various fields. For the regional Vasyugan Horizon, which includes the J1 productive horizon, the composition, structure, and depositional environment have generally been established. However, the stratigraphy and correlation of this multifacial formation and the origin of its constituent sand beds associated with reservoirs of varying quality remain debatable. The Vasyugan Horizon on the Aleksandrovskoe arch has not been adequately studied by lithofacies analysis, although

* Corresponding author. E-mail address: [email protected] (L.G. Vakulenko)

19 deposits (Kondakovskoe, Protochnoe, Traigorodskoe, etc.) in the group of beds J1 have been discovered to date. Lithological studies were reported in (Belozerov, 2007; Belozerov et al., 2001; Brylina, 1987; Brylina and Danenberg, 1989; Brylina et al., 2006; Filina, 1974; Pustyl’nikova, 2008; Ul’masvai, 1974; Zonn, 1970). Another reason that makes this area interesting theoretically and practically is that it is in the zone of transition from the essentially marine Vasyugan Formation to the essentially continental Naunak Formation. The boundary between the formations is drawn in different parts of the Aleksandrovskoe arch: along its axial part (Eliseev et al., 2002), on the western slope (Grebenyuk et al., 1966; Korzh, 1978; Polkovnikova and Tat’yanin, 2006), and on the eastern slope (Belozerov et al., 1988; Gaideburova, 1977). In the latter case, most of the Aleksandrovskoe arch is differentiated as a transitional zone, whose western boundary (the outer contour of the region of marine sediments of the Vasyugan Formation or Lower Vasyugan Subformation) in on the western slope of the arch. In the opinion of Chesnokova (1989), the Naunak Formation appears further east—from the central part of the Ust’-Tym megadepression. A somewhat different point of view on the lateral transition from the Vasyugan Formation to the Naunak one was considered by Chernova (2003, 2010), who, following Brylina and Kazakov,

1068-7971/$ - see front matter D 201 1, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2011.09+.012

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Fig. 1. Location of the studied wells based on the tectonic map of the Jurassic structural stage (Kontorovich et al., 2001). 1–6, boundaries of: 1, zones of sections of different types, 2, Tomsk oblast, 3, superorder structures, 4, first-order structures, 5, second-order, 6, third-order structures; 7–16, structures: superorder: 7, positive, 8, negative, 9, monoclise; first-order: 10, positive, 11, negative; second-order: 12, positive, 13, negative, 14, mesosaddle; third-order: 15, positive, 16, negative, 17–20, zones of different types of sections: 17, I, 18, II, 19, III, 20, IV; 21–24, wells with different types of sections: 21, I, 22, II, 23, III, 24, IV; 25, sections characterized by core. Roman numerals stand for second-order structures: II, Karaminskaya mesosaddle, VIII, Traigorodskii mesoswell; Arabic numerals stand for third-order structures: 24, Okhteur dome-shaped uplift (d. u.), 25, Vakh d. u., 26, Krivolutsk swell, 27, West Aleksandrovskoe salient, 28, Poludennaya salient, 29, Okunevskaya swell.

associates the extension of the Vasyugan Formation with downward depression zones (Koltogory trench, Nyurol’ka, Ust’-Tym, and other depressions). The subcontinental Naunak Formation, in their opinion, is confined to large positive structures in the central and eastern parts of Tomsk oblast and the transition zone between these formations is characterized by complex mosaic relationships and covers the most elevated parts of the large positive structures in the western and central parts of Tomsk oblast. Thus, the issue of the position of the transition zone between the Vasyugan and Naunak Formations is still debatable. This issue can be resolved by detailed lithofacies studies of Upper Bathonian–Oxfordian deposits in the probable areas of transition, such as the Aleksandrovskoe arch. Therefore, one of the objectives of this work was to determine the structure of the Vasyugan Horizon. The second objective was to reconstruct the environments of formation of the Upper

Bathonian–Oxfordian deposits and to plot paleogeographic maps for a number of time slices corresponding to certain stages of development of the sedimentary basin. Subsequent analysis of the results of petrophysical studies allows one to determine the spatial distribution of sedimentary bodies with different reservoir properties. In this paper, the structure and sedimentogenesis of the Vasyugan Horizon 46 to 81 m thick are considered based on the result of complex lithofacies studies involving logging data (spontaneous potential, resistivity, gamma, neutron gamma, and induction logs). Analysis was made of core samples from 21 wells which penetrated most of the horizon in different parts of the Aleksandrovskoe arch (Fig. 1), and logging data from 67 wells. Differentiation and correlation of sections were performed by the authors taking into account the stratification obtained by researchers of the Institute of Petroleum Geology and Geophysics (IPGG) under the leadership of V.A. Kon-

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Fig. 2. Typical profile of the Vasyugan Horizon at the Aleksandrovskoe arch. 1–4, sequences: 1, Upper Bathonian–Callovian, 2, Callovian–Lower Oxfordian, 3, Middle Oxfordian, 4, Middle–Upper Oxfordian; 5, boundary of sequences.

torovich. Four quasi-isochronous sequences were identified which correspond to certain stages in the development of the sedimentary basin: Upper Bathonian–Callovian, Callovian– Lower Oxfordian, Middle Oxfordian, and Middle–Upper Oxfordian. In the first stage of classification of the study sections, according to the criteria considered by Vakulenko and Ryzhkova (2011), four types of sections of the Upper Bathonian– Oxfordian deposits were distinguished: I, Vasyugan; II, Naunak–Vasyugan; III, Vasyugan–Naunak; and IV, Naunak (Fig. 2). The key indicator was the thickness of low-resistivity marine clay unit of the Lower Vasyugan Subformation. In the section of type I, it is 20 m thick or more; in section of type II, 10–20 m; III, <10 m; and in IV, it is completely replaced by a carbonaceous-terrigenous-clay unit similar to the Upper

Tyumen Subformation. According to the location of wells with these types of sections, four zones of submeridional strike on the Aleksandrovskoe arch were distinguished in which is a westeastward transition from Vasyugan to Naunak type sections (see Fig. 1). Sedimentation trends in the Late Bathonian–Oxfordian depended largely on the paleorelief. It has been shown (Kontorovich, 2002; Kontorovich et al., 2009) that at the time considered, relatively elevated regions of this part of the basin were the Chebach’e and Kondakovskoe local uplifts within the Krivolutsk swell and a number of structures in the west and southwest of the arch. The Ust’-Tym megadepression had an inherited character, but its contrast was much less pronounced than that in the Lower and Middle Jurassic stage of development, and the most depressed portion Koltogorsk

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Fig. 3. Diagram of predominant depositional environments of the Upper Bathonian–Callovian sequence. 1–3, boundaries of: 1, Aleksandrovskoe arch, 2, Tomsk oblast, 3, sedimentation zones: a, based on core drilling and logging data, b, performed conventionally, 4–10, depositional environments: shallow-marine: 4, distant and transitional coastal regions, coastal-marine: 5, shoreface, 6, foreshore, 7, lagoon; transitional: 8, delta, 9, backshore (coastal plain ) continental: 10, alluvial-lacustrine-marsh, sand bodies: 11, established, 12, presumed, 13; denudation island, 14, well, 15, SP curve.

mesotrough was shifted east relative to the present position. The eastern slope of the Aleksandrovskoe arch was adjacent to the relatively elevated part of the modern Karaminsk mesosaddle.

Structure and depositional environment of the Upper Bathonian–Callovian sequence The Upper Bathonian–Callovian sequence (10–24 m) includes a large, essentially clay part of the Lower Vasyugan Subformation and its analogs. Its formation is related to an event-driven (fast) transgression at the time of transition from the Malyshevka Horizon to the Vasyugan Horizon (Beizel’ et al., 2007) and the stage of a high-standing sea level. In the zone of type I sections of the Vasyugan Horizon, adjacent to the westernmost part of the study area, the sequence has a shaly composition and a fairly uniform structure. Low apparent resistivities, weak differentiation of resistivity logs, positive anomalies, and poor differentiation of the spontaneous polarization (SP) curve are characteristic of the mudstone mass (20–24 m) formed in shallow marine

environments (Fig. 3) within the distant (below storm wave base) and transitional (between normal and storm wave base) coastal zones. Further east, in the zone of type II sections, the sequence contains thin beds of sandy siltstone (10–20 m). Their formation is related to episodes of storm sedimentation in the shallow marine zone of the basin (see Fig. 3). In these zones, the sequence is very poorly represented in core samples. In the upper half of the Protochnaya area, intervals of interbedded silty shale rocks show fine wavy, lenticular, and wavy lenticular lamination with symmetric wave ripples. Rare vertical traces of activity of benthic organisms are represented by Teichichnus ichnofossils. In more shaly portions, the texture is disturbed by mud-eating worms (Chondrites ichnofossils) up to the formation of a small-lumpy material. The bases of the interlayers and thin beds of sandy siltstone are often erosional, with intraclasts of underlying mudstones. Coarsegrained siltstones and silty sandstones are predominantly cross-stratified. Pyrite fines and concretions of various sizes and shapes are abundant. In the zone of type III sections, the sequence was formed in marginal-marine shoreface and foreshore environments (see

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Fig. 4. Sedimentological section of the Upper Bathonian–Callovian of the sequence penetrated in the Chebach’ya-219 well (type IV). 1–20, textures and texture disturbances: stratification: 1, horizontal, 2, graded (inverse/direct), 3, large-scale tabular cross, 4, small-scale tabular cross, 5, low-angle cross, 6, small-scale trough cross, 7, wavy, 8, wavy-lenticular, 9, lenticular; ripple marks: 10, with displaced crests, 11, asymmetrical, 12, massive, 13, lumpy, 14, deformative bioturbation, 15, erosion, 16, consedimentational folding, 17, intrusion 18, root traces, 19, slickensides (a), tectonic fractures (b), 20, dissolution fractures; 21–25, ichnofossils: 21, unidentified ichnofossils (vertical/horizontal), 22, Skolithos, 23, Planolites, 24, Palaeophycus, 25, Chondrites; 26–31, organic remains: 26, coalified plant detritus, 27, plant casts, 28, bitumen 29, bivalves, 30, belemnites, 31, shelly detritus; 32–35, authigenic minerals: 32, pyrite/pyrite concretions, 33, glauconite, 34, siderite/ siderite concretions, 35, calcite, 36, coals (a), carbonaceousness (b), 37, intraclasts. Here and below, the size of the scale bar is 3 cm.

Fig. 3). It is characterized by a marked differentiation of resistivity logs, sometimes with levels of elevated values. Based on observations of core samples from the Kondakovskaya area, it differs from the above types in an increased proportion of sandy siltstones, appearance of carbonaceousness, consedimentational folding, and weaker pyritization. The exception in this zone is the section of the Chebach’ya-219 well (Naunak type). In the above-mentioned area, the Bazhenovka Formation in the section of well 217 lies on the basement, which suggests the presence of insular land masses in the Late Bathonian–Kimmeridgian (see Fig. 3). The essentially clay section of the sequence, containing carbonaceous mudstones, coal seams, thin-bedded silty sandstones, pyritized and sideritized intervals, and bioturbation levels was formed in an island lagoon environment (Fig. 4). In the easternmost part (Vartovskaya area), characterized by type IV sections, the sequence consists of irregularly interbedded sandstone, siltstone, and mudstone with interlayers of carbonaceous rocks and coals. The resistivity curves are highly differentiated. In the Vartovskaya-330, -331 well, two

sand beds (2–4 m) are found. The formation of the sequence is related to coastal-continental environments: a coastal plain with shallow channels and wave-cut bars (see Fig. 3). Presumably, in the northeastern part of the study area, there was a medium-sized river system with a delta complex at that time.

Structure and depositional environment of the Callovian–Lower Oksfordian sequence The Callovian–Lower Oxfordian sequence (9–41 m) includes the subcoal strata of the J1 horizon and its analogs. Its development is related to the migrational (gradual) regression of the sea. The westernmost part of the study area at that time was dominated by marginal-marine shoreface and foreshore environments (Fig. 5). The J41 bed (4–5 to 12–13 m) formed mainly in an environment of submerged bars/shoals of the shoreface. Its electric model, according to Muromtsev (1984) shows a block- and bell-shaped negative SP anomaly. A series

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Fig. 5. Diagram of predominant depositional environments of the Callovian–Lower Oxfordian sequence. For notation, see Fig. 3.

of sand bodies extends from north and northeast (Protochnaya area) to southwest and south (Aleksandrovskaya, Pankovskaya, Yuzhno-Aleksandrovskaya areas) of the study region. Sandstones of the Protochnaya area were probably formed in the zone of influence of a presumed river. The sand bodies of the J31 bed (2–3 to 16–17 m) are associated with the appearance of barrier and longshore bars in place of existing submerged shoals, which developed due to the supply and redistribution of material by longshore currents and sea waves. Thicker beds were penetrated in the Protochnaya-2 and Sredneprotochnaya-9 wells in the zone of increased influence of the river. The electric model of thin beds (2–5 m) consists of variously elongated triangles attributed to rip current channels (distributary and tidal channels) (Aleksandrovskaya-3 and Protochnaya-4 wells). The thickest single sand bed J3–4 1 (25 m) was penetrated in the Aleksandrovskaya-10 well near the junction of the Aleksandrovskoe arch with the Koltogory– Nyurol’ka trench. The bed is characterized by a general upward coarsening of deposits and by the presence of finer-grained interlayers and carbonatization levels in sandstones. Its development was related to a thick system of the bar type, which continues to grow. At the crest of the local uplift, the Aleksandrovskaya-1 well penetrated a single sandy siltstone bed of much smaller thickness (9 m), which was likely formed within a small submerged shoal. In some sections of the Aleksandrovskaya and Yuzno-Aleksandrovskaya areas, one of the beds is replaced by a silty shale unit

formed within submarine depressions. A section different from the other was penetrated in the Kichanovskaya-100 well, where the essentially clay strata of the shoreface included thin sandy silt beds of storm wave crests characterized by negative SP anomalies in the form of narrow, often right triangles. The sequence described here was cored in the YuzhnoAleksandrovskaya and Protochnaya areas. In the YuzhnoAleksandrovskaya area, the J41 bed (10 m) with graded base and top is composed of massive fine-grained sandstones, more rarely silty sandstones and coarse-grained siltstones, with intervals of finely horizontally and wavy laminated silty shale rocks. This bed contains calcitization levels, pyrite concretions, and rare trace fossils. The thick silty shale unit (up to 10 m) overlying the reservoir was formed under low-energy conditions of shoreface troughs. The J31 bed (up to 4 m) with a graded base is composed of silty sandstones and fine-grained sandstone with a slight upward coarsening of grain size. The rocks are massive, rarely horizontally and wavy laminated, and sometimes low-angle cross-stratified. In the upper part, the amount of coalified plant detritus (CPD) increases and plant root debris are present, indicating a subaerial depositional setting. The formation of this bed could be related to a tidal delta environment within a coastal bar-lagoon area. In the Protochnaya area, the J41 bed (4–5.5 m) is close in composition, structure, and depositional settings to the samename bed of the Yuzhno-Aleksandrovskaya area. The thicker J31 bed (7.8–11.5 m), whose formation is related to sand bodies

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Fig. 6. Sedimentary section of the Callovian–Lower Oxfordian sequence penetrated in the Protochnaya-6 well. For notation, see Fig. 4.

of the delta front, is well represented in core samples (Fig. 6). The fine- and medium-grained sandstones composing it are variously cross-bedded or rarely wavy and horizontally laminated due to the concentration of CPD and siderite nodules, and in places massive. At the top of the bed, there is a graded transition to the overlying intercoal strata. Intervals of interbedded silty shale are encountered in the Protochnaya-3 well, which is probably due to its position on the slope of the shoal. A more homogeneous sand bed is distinguished in the section of the Protochnaya-6 well. The rocks contain pyrite fines in sandstones and concretions in silty shale interbeds and levels of intense calcitization. In the Protochnaya-7 well, the sequence consists of a J41 sand bed (6 m) and an overlying essentially clay strata (12 m) formed within the delta. The

latter is composed of more-or-less silty mudstones with mutual transitions into fine-grained clayey siltstones with pyrite concretions. The rocks are massive, in places horizontally, lenticular, and wavy laminated, with displaced ripple-wave crests, and are rarely disturbed by traces of folding (small-pillow structures), intrusions of coarser-grained sediments into fine-grained ones, and bioturbation. Further east, in the area of type II sections, the thickness of the sequence ranges from 13–23 to 30–36 m in the north and from 11 to 27 m in the south. In the thickest sections in most wells of the Gorstovaya and Nadezhdinskaya areas, the electric model of the J3−4 single bed is represented by a 1 complex block-shaped SP anomaly. Increased thicknesses, undifferentiation of the bed, and the multistage shape of the

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Fig. 7. Sedimentary sections of the Callovian–Lower Oxfordian sequence, penetrated: a, Traigorodskaya-5 well (transitional genetic sequence); b, Chebach’ya-219 well (marginal-marine genetic sequence). For notation, see Fig. 4.

SP curve suggest that the reservoir was formed in the delta complex (Fig. 7). The development of Oxfordian delta fronts on the western side of the Aleksandrovskoe arch was previously noted by Filina (1974). Belozerov et al., (2001) reconstructed the delta complex in the subcoal strata of the J1 horizon on the northern pericline of the Aleksandrovskoe arch within Vakh zone of petroleum accumulation. They assume the development of the submarine part of the delta (delta front) in the Okhteurskaya and Prigranichnaya areas. The electric model of the sequence penetrated by the Ohteurskaya-103 well shows a series of elongated right triangles of negative PC anomalies, can be correlated with the sand bodies of the slopes of the delta. An upward increase in the thickness of these bodies and a general coarsening of clastics indicate delta progradation during the formation of the sequence. The sharp

decrease in the thickness of the sequence in the Prigranichnaya area is probably due to the existence of shallow submerged shoals in the upper shoreface (J41 bed) and sandstones of a secondary distributary (up to 10 m—J31 bed). Sufficiently stable thicknesses of the sequence are observed in the south of this zone: in the Ob’, Chapaevskaya, and Poludennaya areas (23–27 m). Electric models of the J41 bed (from 5–6 to 13–15 m) show bell-shaped and, more rarely, block and triangular negative SP anomalies associated with sand bodies of regressive bars (Chapaevskaya and Poludennaya areas) and rip current channels (Obskaya area). The overlying J31 bed (from 6–7 to 15–16 m) is separated by a thin (1–2 m, sometimes 4–5 m) silty shale unit. Electric models of the thicker sand bodies show a block negative SP anomaly associated with the deposition of barrier bars/islands recon-

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structed on the Obskaya and Chapaevskaya areas. The sand bodies penetrated in the Poludennaya area were formed in regressive bar and tidal microdelta environments. In the Poludennaya-222 well, located in the marginal part of the southern slope of the Poludennaya uplift, the sequence is represented by the J3−4 1 single bed (25 m) formed in a barrier bar environment. Such a bed (27 m) was also penetrated in the Chapaevskaya-2 well. From the results of core tests of the Obskaya and Chapaevskaya areas, the J41 bed was formed in a longshore bar environment in the foreshore is composed of fine-to-mediumgrained and fine-grained sandstones. Coarser-grained rocks are massive, and fine-grained rocks exhibit low-angle and large-scale tabular cross-stratification and horizontal lamination due to the concentration of siderite nodules, in some places ample, and more rarely, CPD. Levels of calcitization, rare concretions, and pyrite fines are noted. The unit separating the sand beds (2 m) was formed in a muddy foreshore shallow and consists of low-angle wavy and lenticular silty shale interbeds, disturbed by intense bioturbation (Chondrites ichnofossils) with sideritization intervals. The formation of the J31 bed (13.5–16 m) is related to a barrier bar. It is composed of medium- and fine-grained sandstones, and rarely coarse siltstones and silty sandstones with an upward increase in grain size and a high content of authigenic siderite and pyrite. At the top, the CPD content increases, and plant root debris and thin coal lenses are found. A decrease in the thickness of the sequence is observed on the southern slopes of the Krivolutsk swell (up to 18.5–24 m) and Traigorodskii mesoswell (up to 11–13.5 m) within the Nazinskaya area. The thickest J3−4 bed in the Nazinskaya-4 1 well is interpreted as a distributary of a small river. It is characterized by a block-shaped negative SP anomaly with saw-toothed base and top lines and a wavy sideline. In the Nazinskaya-3 and -95 wells, the bell-shaped anomaly has an inclined, highly dissected sideline, which reflects the coarsening of deposits due to delta progradation. In the Yuzno-Nazinskaya area, thin silty sandstone bodies were formed presumably in the coastal part of the lagoon (see Fig. 5). In the area of sections of type III, the Callovian–Lower Oxfordian sequence (15–40 m) was examined mainly based on core and logging data in the well-drilled portions at the crest and on the northern slope of the Krivolutsk swell. The J3−4 single bed penetrated in the Traigorodskaya area (16– 1 20 m) is characterized by block-shaped SP curves (see Fig. 7, a). The bed is composed of sandstones, in places calcitized, with grain size often varying along the bed from fine- to medium. In the bottom half of the reservoir, the rock texture is massive with intervals of low-angle cross-stratification, sometimes large-scale tabular cross-stratification with multidirectional series, and horizontal lamination due to grain size variations and, more rarely, the presence of CPD, siderite nodules, and sometimes clay intraclasts. The upper half of the bed displays wavy lamination due to the inwash of inequigranular CPD with intervals of upward current ripple; in the Traigorodskaya-3 well, rhizoids, isolated fragments of

charred wood, and thin coal seams were found, indicating subaerial deposition in certain time intervals. Pyrite in the form of concretions of different sizes and shapes is common. The formation of the deposits described above is probably related to a delta complex that prograded throughout the Late Callovian–Early Oxfordian. However, in contrast to the delta complex penetrated in the Gorstovaya area, these bodies thinner and more uniform in structure. Their formation can be attributed to a distributary environment. The great thickness of the sequence in the Taezhnaya-211 well (34 m) is due to its confinement to the main distributary of the ground part of the delta. The electric model shows a negative SP anomaly of a complex block shape with a stepped sideline reflecting the different stages of development of the delta complex. The sand bodies penetrated in the Chebach’ya (18–26 m) and Kondakovskaya (15–20 m) areas, based on logging data and results of sedimentological studies are interpreted as deposits of barrier bars/islands. From the core samples of the Chebach’ya-219 well, most of the J3−4 1 bed consists mainly of medium-size massive sandstones (see Fig. 7, b). Some areas exhibit low-angle and large-scale tabular cross-stratification, horizontal lamination, and rarely wavy lamination, sometimes with intraclasts of argillaceous and sideritized rocks. Sandstones are in places pyritized and calcitized, enriched in CPD, with single layers of clayey and sideritized mudstones with abundant fine rhizoids. The upper third of the bed consists of two sand units (2 and 4.5 m) composed of cross-bedded medium-fine-grained sandstones with inclusions of argillaceous intraclasts and an upward reduction of grain size. Their formation is associated with environments of the lobe and channel of a tidal delta. In the Poiskovaya area, the sequence is represented by two sand beds separated by a silty shale unit (1–4 m). The electric model of the J41 bed (5–10 m) is typical of environments of regressive longshore bars of the foreshore, and that of the J31 reservoir (10–15 m) is typical of a distributary environment. In the eastern part of this zone, the sequence was penetrated with four boreholes (see Fig. 1), in three of them, the sand beds are characterized by a block-shaped SP curve with horizontal base and top lines and a vertical wavy sideline. This corresponds to the bodies formed in channel environments of an alluvial complex. The thickest section in the Mygytinskaya-306 well is consists of two beds (14 and 22 m) separated by a carbonaceous silty shale unit (up to 4 m) formed in a floodplain environment. Undifferentiated beds 18 and 25 m thick were distinguished in the Nan’-Yakhskaya and Noyabr’skaya areas. In the Kruglozernaya area, only one sand bed J41 (11–12 m) was penetarted, characterized by a bell-shaped SP curve typical of a channel bank. An analog of the J31 bed is an argillaceous-siltstone strata (8.5–9.5 m) formed in a floodplain valley environment. Within the Vartovskaya area, the Callovian–Lower Oxfordian sequence (10.5–19 m) is represented by sections of type IV. The electric model of the deposits is represented by a spiked SP curve located in the zone of positive anomalies. Negative SP anomalies in the form of elongated triangles

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correspond to thin sandy silt beds (2–3 m). Strong differentiation and increased resistivities are due to the presence of carbonaceous rock and coal strata. The sequence was formed in environments of a periodically inundated floodplain with thin accumulative sandy silt bodies of fractured alluvial fans (flood sands), more pronounced in the Vartovskaya-331 well. Structure and depositional environments of the Middle Oxfordian sequence The Middle Oxfordian sequence includes an intercoal strata and its analogs. Minimum thicknesses (2–4 m) were recorded in the Gorstovaya and Taezhnaya areas, and maximum thicknesses (15–17 m) in the Vartovskaya, Mygytinskaya, Nan-Yakhskaya, Traigorodskaya, Okhteurskaya areas. In general, the thicknesses of this sequence increase eastward, but within the zones of various types of sections of the Vasyugan Horizon, they can vary considerably, being determined by paleolandscapes in the sedimentary basin, local subsidence, and erosion that preceded the deposition of the Middle–Upper Oxfordian sequence. This sequence, examined from cores of wells in the western and central parts of the Aleksandrovskoe arch, was accumulated in a lagoon-marsh environment. It consists of irregularly interbedded mudstones and siltstones with different grain sizes and clay contents with subordinate interlayers and thin beds of silty sandstones. Rocks are more or less carbonaceous and contain coal seams and coalified root traces, indicating periodic waterlogging of the area. The most clayey and carbonaceous deposits formed in coastal marshes. Finegrained rocks have textures indicating low-energy depositional conditions, with wavy, horizontal, and lenticular bedding, often disturbed by bioturbation, sometimes intense. Increased thicknesses of the sequence are often due to the presence (usually in the middle part) of silty sandstones with low-angle and horizontal cross-bedding, apparently formed in environments of small river beds and tidal deltas, and with wavy lamination in coastal lagoons. The sequence cannot be considered purely continental because of the presence of frequent levels of bioturbation, including ichnofossils of the Cruziana ichnofacies, marine trace fossils, persistent pyrite concretions, and abundant wavy-laminated textures with buried symmetric wave ripple marks. In the east of the research area, the complex was studied only by log analysis. Its electric models are similar to the facies of behind-bar lagoons and coastal marshes. However, considering the formation of underlying sediments in terrestrial delta and alluvial plain environments, the Middle Oxfordian sequence in this region was associated with inundated floodplain and lacustrine-boggy environments. Structure and depositional environments of the Middle–Upper Oxfordian sequence The Middle–Upper Oxfordian sequence (5–7 to 25–29 m) includes the coal-overlaying strata of the J1 horizon and its

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analogs. Its formation is due to the migrational (progressive) transgression of the sea, complicated by repeated short-term regressions and rewashings of the underlying sediments under conditions of insufficient supply of terrigenous material. Sand beds are thinner, laterally discontinuous, and probably timetransgressive, becoming younger toward paleoelevations. The paleolandscape in the stage was derived from the Callovian– Early Oxfordian. The westernmost sections of the sequence (Aleksandrovskaya-2 and -10 wells) are composed of silty shale rocks (10–10.5 m) characterized by low differentiation, positive SP anomalies, and reduced resistivities, and apparently formed in the transitional zone of the coast (Fig. 8). The western, northwestern, and, to a lesser extent, central parts of the Aleksandrovskoe arch are dominated by sections in which the lower part of the sequence formed in a foreshore environment, and the upper part in a shoreface environment, while the roof contains a more or less pronounced layer (0.5–5 m) calcitized sandstones with marine trace fossils—a lithofacial association of shell banks within submerged shoals (see Fig. 8). The thickest sections of the sequence were identified in the Yuzhno-Aleksandrovskaya area (22–25 m). The J21 bed (5.5–14 m) is characterized by a block-shaped SP curve and is interpreted as a series of longshore bars. Core samples indicate that the base of the bed is erosional with intraclasts of clay and carbonaceous-clay rocks. The grain size and texture of rocks vary vertically through the section with an upward coarsening of clastic material. The J11 bed (4–10 m) developed in a submerged bar environment. It is characterized by an abundance broad development of wavy-laminated and massive textures, and significant calcitization of rock. In the Aleksandrovskaya area (wells 1 and 3), the J11 bed is replaced by a silty shale unit (11–14.5 m) formed in interbar troughs in a shoreface environment or in the transition zone. In the Pankovskaya-1 well, the sequence (29 m) is represented by the J1−2 1 single bed characterized by a negative SP anomaly with a strongly dissected side line and inclined base and top lines, assigned to the facies of the heads of rip currents acting for a long time in the marginal-marine part of the basin. In the Protochnaya and Kichanovskaya areas, the sequence (13–21 m) includes one to three silty sandstone beds. The negative SP anomaly of the lower bed (2–5 m) usually has the shape of a right triangle of varying size, typical of beach sandstones. Thicker bodies could form in an environment of regressive longshore bars. Core samples from a number of wells indicate that the bed is composed of silty sandstones and fine-grained and fine-to-medium-grained sandstones, with a coarsening and then fining of clastic material. Finer-grained rocks exhibit low-angle cross-bedding and, more rarely, small-scale trough cross-bedding, and coarser-grained rocks are massive with series of large-scale low-angle and largescale tabular cross-bedding due to the inwash of CPD and intercalations of clay intraclasts, siderite nodules, and sometimes clay material. The middle bed (3–5 m) is separated by a poorly defined break and has close electric characteristics, likely being a series of thin bar sandstones of the fore-

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Fig. 8. Diagram of predominant depositional environments of the Middle–Upper Oxfordian sequence. For notation, see Fig. 4.

shore. The upper J11 bed (2–7 m) is characterized by two types of electric models. Thinner beds (Protochnaya-2 and -7 wells) are similar in log characteristics to underlying ones. Thicker beds (Sredneprotochnaya-9 and Protochnaya-4, -8 well) are interpreted as submerged bar deposits. They are characterized by a block-shaped negative SP anomaly and are composed of fine-grained and fine-to-medium-grained, inhomogeneously argillaceous, calcitized, and pyritized sandstones. The rocks show irregular wavy lamination and, more rarely, low-angle cross-bedding, disturbed by bioturbation, sometimes with vertical burrows of burrowing organisms (Skolithos ichnofossils) at the top, along which authigenic glauconite develops. In the Protochnaya-6, -3 and Kichanovskaya-100 wells, the J11 bed is replaced by a silty shale unit formed in interbar troughs (see Fig. 8). In the Gorstovaya area, the thicknesses of the sequence decrease to 12–16 m. The J21 bed (3–4 m) is characterized by a small triangular negative SP anomaly and was probably formed in a foreshore environment. In some wells, it is replaced by a silty shale unit (6–9 m) of interbar troughs of the foreshore. The J11 bed (3–9 m) is characterized by bellshaped negative SP anomalies. It is composed of silty sand bodies of the slopes of regressive bars or submerged shoals (see Fig. 8). In the Nadezhdinskaya and Novonadezhdinskaya areas, the thickness of the sequence is 21–22 m. The J21 bed (3.5–6 m)

is characterized by low-amplitude bell-shaped negative SP anomalies corresponding to regressive sandy silt beach deposits. In the Nadezhdinskaya-13 well, it has an erosional base and is composed of sandstones with an upward decrease in grain size (from coarse-to-medium-grained to fine-grained) (Fig. 9). The textures are mostly cross-bedded, and at the top, wavy-bedded. Pyrite concretions are common. The carbonaceous sandy silty argillaceous composition of the overlaying 7-m thick unit, probably of lagoon origin, suggests that the layer was formed in a tidal delta environment within a lagoon foreshore. The thick clay unit (12.5 m) separating the beds in the Nadezhdinskaya-14 well is composed of mudstones and, more rarely, poorly-sorted argillaceous siltstones, in places with abundant Chondrites ichnofossils and pyrite, and is interpreted as marginal-marine deposits of a silty-clay trough. The J11 sand bed (4.5–10 m) is characterized by a larger-amplitude block-shaped SP anomaly with inclined top and base lines and a saw-toothed sideline, indicating a change in environmental energy in the sedimentary basin. Based on core samples from of the Nadezhdinskaya-13 well, it can be divided into two halves (see Fig. 9). The bottom shows a change from wavy-bedded coarse-grained siltstones to low-angle cross-bedded fine-grained sandstones. The top consists of fine-grained sandstones, more or less calcitized, interbedded with silty sandstones. The bedding is irregular wavy, with symmetric ripple marks, contorted, disturbed by numerous ichnofossils (including ichnofossils Skolithos). Among the re-

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Fig. 9. Sedimentary section of the Middle–Upper Oxfordian sequence penetrated in the Nadezhdinskaya-13 well. For notation, see Fig. 4.

mains of marine fauna are bivalves, belemnites, and, in the Nadezhdinskaya-14, abundant scaphopods. In the initial stage, the bed was probably formed in a barrier bar environment, that evolved, during transgression, into a marginal-marine submerged shoal (see Fig. 8). In the Prigranichnaya area (14.5–18 m), the J21 bed (3 m) of beach origin is characterized by a negative SP anomaly in the form of an elongated triangle. It is separated by a silty shale unit (5 m) from the thicker (up to 10 m) J11 bed with a negative SP anomaly of a complex block shape. From the

results of core tests, the bed consists of several cyclically arranged units in which mudstones are replaced by sandstones. Fine-grained rocks are massive or lenticular-, horizontally-, and wavy-bedded, disturbed by bioturbation (Teichichnus, Planolites, and Chondrites ichnofossils). Silty sandstones show irregular horizontal and low-angle cross-stratification and sometimes small-scale trough cross-stratification. The boundaries between strata are often erosional and exhibit traces of intrusions and consedimentational folding. The formation of the bed can be related to the head of rip currents in the upper shoreface (see Fig. 8).

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In the Traigorodskaya-5 well, the silty sand bed J21 (5.5 m), interpreted as a foreshore bar, is expressed in a SP curve as a negative anomaly in the form of a narrow triangle. It is characterized by an erosional base, a heterogeneous structure with a general upward fining of the material, and the presence of carbonaceous silty shale rocks. The argillaceous siltstone layers in the lower half of the bed contain deformative bioturbation and a variety of ichnofossils: Skolithos, Planolites, Terebellina, Palaeophycus, Chondrites. The J11 bed (5 m), characterized by a bell-shaped negative SP anomaly, was formed in a submerged bar environment. It consists of fine-to-medium-grained sandstones of fine-spotted appearance due to deformative bioturbation. Coarser-grained areas are massive and with irregular and contorted bedding. Small Skolithos and Palaeophycus ichnofossils and remains of large bivalves are encountered throughout the bed, and belemnite rostra are found in the upper third of the bed. In the Obskaya area, the thickness of the sequence is 10–13 m. The J21 bed (2.5 m) is characterized by a negative SP anomaly in the form of an elongated triangle, has an erosional base, composed of fine-grained sandstones, in places calcitized, and at the bottom with thin interlayers of carbonaceous mudstone. Given that the carbonaceous clay unit (4.5 m) separating the beds is of lagoon origin, the formation of the sequence can be related to a tidal delta or a lagoon foreshore environment. The J11 bed (3–4 m), with a triangular negative SP anomaly, is made up of fine-grained and fine-tomedium-grained calcitized sandstones, massive and wavy-bedded, in places bioturbated, and with bivalve remains. Its formation is confined to a shallow shoreface (see Fig. 8). The shoal crest was located near the Obskaya-1 well, which penetrated a thicker (up to 13 m) undifferentiated bed J1−2 1 , characterized by a SP curve with a more complex high-amplitude negative anomaly of triangular shape. In the Kondakovskaya area, the sequence is rather thin— 6–9.5 m and is poorly represented in cores. Sand beds (2–3, more rarely 5 m) in SP curves are characterized mostly by low-amplitude simple triangular anomalies with slightly inclined base and top lines. The J21 sandy silt bed is weakly expressed in core samples, and in some wells, it is represented by an essentially clay unit with remains of small bivalves and thin sand interlayers. Its development is related to foreshore environments: small bars and interbar troughs. The silty shale unit separating the sand beds contains remains of large and small bivalves, including normal marine (by B.N. Shurygin’s definition). Mclearnia and Meleagrinella. The J11 bed sequence is made up of fine-to-medium grained sandstones formed in a shallow shoreface environment. Rocks are wavy-, swaley-, and contorted-bedded and spotted due to uneven bioturbation, and calcitization with pyrite concretions and bivalve shells. In the Chebach’ya area, the sections of the sequence (7–13 m) are greatly different due to the local “patchiness” of settings (see Fig. 8). Thus, the Chebach’ya-218 well penetrated a carbonaceous silty shale strata (9 m) formed in an

island lagoon. In the 220 well, the lower part of the sequence contains a silty shale unit of an interbar trough in a foreshore, and the J11 layer (6 m) with a bell-shaped negative SP anomaly bar is interpreted as a foreshore bar. In the Chebach’ya-219 well, the J1−2 1 bed (7 m), with a similar electric model is made up of siltstones at the base and, in the upper pat, of fine-grained sandstones with irregular wavy lamination, in places calcitized, with remains of bivalves (Fig. 10, b). In the Poiskovaya area, the sequence in the well 2 is characterized by a positive PC anomaly, poorly differentiated due to the presence of siltstone layers, and in the well 1, it is characterized by a block-shaped negative SP anomaly (11 m). The deposits are interpreted as a foreshore interbar trough and sandy bar, respectively. The above characteristics of the Middle–Upper Oxfordian sequence in most of the western half of the study area indicate that, compared with the Callovian–Lower Oxfordian, it has significantly a increased proportion of marginal-marine sediments and a decreased amount of delta sediments. The latter were reconstructed in several areas in the central part of the arch. In the Traigorodskaya area (wells 2 and 3), the sequence 1−2 (J1 bed 11–13-m) is characterized by a block-shaped and bell-shaped SP curve and is well-represented in core samples (see Fig. 10, a). The base of the sequence is erosional with intraclastic breccia levels. The lower half is composed of fineto medium-coarse-grained sandstones, interbedded with siltstones that are massive or display horizontal, low-angle, and tabular cross-stratification of graded type, accentuated by inwash of fine CPD. The change in rock structures and textures reflects changes in the strength of the fluvial flow that supplied the sedimentary basin, which allows the bed to be interpreted as distributary deposits. The upper half of the sequence, which was probably accumulated in a mouth bar environment, is composed of fine-grained sandstones and in places silt-sandstones (see Fig. 8) with low-angle cross-stratification and sometimes horizontal and wavy stratification due to an admixture of clay-carbonaceous material. Abundant large bivalves and belemnite rostra were found near the top, indicating a progressive transgression and the formation of shell banks in place of mouth bars in shallow shoreface environments. In the Taezhnaya area, the sequence (14 m) is represented by two silty sandstone beds (5 and 6 m) separated by a thin clay unit. They are expressed in the SP curve by block-shaped negative anomalies from slightly inclined top and base lines and wavy sidelines, indicating the impermanence of environmental hydrodynamic activity, and in this area, the beds under consideration can be interpreted as sand bars of the delta front exposed to ocean currents and waves (see Fig. 8). In the southern part of the Aleksandrovskoe arch, the formation of the sequence is inherited related to lagoon environments. In the Chapaevskaya and Poludennaya areas (9.5–10.5 m), the sequence is characterized by a poorly differentiated SP curve in the range of positive values indicative of an essentially clay composition. Significant dif-

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Fig. 10. Sedimentary sections of the Middle–Upper Oxfordian sequence, penetrated. a, Traigorodskaya-2 well (transitional genetic sequence), b, Chebach’ya-219 well (marginal-marine genetic sequence). For notation, see Fig. 4.

ferentiation and increased resistivities indicate the presence of carbonaceous interlayers. In the Poludennaya-222 well, the bell-shaped low-amplitude negative anomaly of the J11 bed (6 m) corresponds to a regressive barrier bar which separated the reconstructed lagoon from the sea. In the Nazinskaya areas, the thickness of the sequence shows significant variations (5–14 m). The thick silty sandstone beds, represented by narrow triangular negative SP anomalies, was formed in a lagoon foreshore environment complicated by a small delta. In the Yuzhno-Nazinskaya area, sandy siltstone beds (up to 5 m thick) are characterized by block- and bell-shaped low-amplitude negative SP anomalies and highly differentiated and increased resistivities indicative of carbonaceous deposits, suggesting a coastal lagoon environment of their formation. During the development of the sequence considered, the eastern side of the Aleksandrovskoe arch was dominated by coastal-continental and continental settings. In the Noyabr’skaya-1 well, the sequence (9 m) is characterized by a positive SP anomaly, with low-amplitude saw-toothed deflections toward negative values being attributed to siltstone beds formed within a nearshore plain. In sections of the NanYahskaya-1 and Kruglozernaya-1 wells, the thickness of the sequence increases significantly—to 18 and 26 m, respectively. The J21 bed (10 and 17 m) commonly shows a block-

shaped negative SP anomaly which corresponds to fluvial sandstones. They are overlain by a carbonaceous silty shale strata (8 and 9 m), interpreted as deposits of a waterlogged floodplain. In the Mygytinskaya area (8 m), the sequence consists of two thin (2.5 m each) sand beds characterized by triangular and block anomalies, probably due to fractured alluvial fans (flood sands) within a floodplain valley. In the Vartovskaya area, the investigated sequence has increased thickness (20–25.5 m) and shows irregular carbonaceous sandy shale interbedding formed in continental settings, mainly in floodplain-lacustrine-marsh areas (see Fig. 8). The J21 sand bed (5–12 m) at the bottom of the sections, characterized by a block- and bell-shaped negative SP anomaly was probably formed in a channel setting.

Discussion and conclusions The depositional environments of the Vasyugan Horizon at the Aleksandrovskoe arch were reconstructed as a result of the sedimentological studies performed. Regular lateral and vertical changes in the environments was shown to be related to the regressive and transgressive phases of the development of the sedimentary basin. Most of the sedimentary material was brought by rivers from the southeastern and eastern

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margin of the West Siberian Plain. In addition, material came from the erosion of Middle Jurassic deposits and erosion-tectonic protrusions of the pre-Jurassic basement (Krivolutsk batholith). The start of the accumulation of the Vasyugan Horizon is related to an event-driven regional transgression that led to the establishment of a normal marine sedimentation regime in most parts of the West Siberian sedimentary basin. In the Late Bathonian–Callovian, the area of the Aleksandrovskoe arch from west to east successively changed from shallow-marine to marginal-marine and coastal-continental depositional environments. During the accumulation of the Callovian–Lower Oxfordian sequence, the migration regression was accompanied by is a shift of different facies zones to the west. In the area of the Aleksandrovskoe arch, shallowmarine depositional environments were absent, and periodically waterlogged alluvial plain environments developed in the eastern part. In the north, the reconstruction revealed an actively prograded delta complex, which was replaced southward by a lagoon-bar coastline. The Middle Oxfordian terrigenous-carbonaceous-shaly sequence was characterized by the development of lagoon coast, swamped coastal plain, and alluvial-lacustrine-marsh plain environments. The transgression of the Middle–Upper Oxfordian stage was accompanied by smaller-scale episodes of regression. It was characterized by insufficient detritus influx, erosion, and redistribution of underlying sediment. During the sedimentation of the Vasyugan Horizon, the marine coastline changed its position in accordance with the development of transgressions and regressions. During the accumulation of the Upper Bathonian–Callovian sequence, it was on the eastern slope of Aleksandrovskoe arch, and during the accumulation of the essentially sandy Callovian–Lower Oxfordian and Middle–Upper Oxfordian sequences, it was close to its central part. It seems that the boundaries of the Vasyugan and Naunak Formations, as well as the zones of transitional types of sections should be identified based on the types of sections of the Vasyugan Horizon. In this case, most of the Aleksandrovskoe arch is characterized by sections of the transitional Naunak–Vasyugan and Vasyugan–Naunak types; the westernmost part, by sections of the Vasyugan type; and the easternmost part, by sections of the Naunak type. The reconstruction confirmed the extensive development of coastal coal-overlaying sand beds on the west side of Aleksandrovskoe arch. Analysis of the reservoir properties (RP) of rocks performed at the IPGG RAS and JSC “TomskNIPIneft” and literature data on the reservoirs of the J1 horizon of the Aleksandrovskoe arch leads to the conclusion they are predominantly of low quality—reservoirs of class VI, less often V, locally interbedded with class IV, according to A.A. Khanin (1969). The silty sandstones studied in a number of areas of the central part of the arch are characterized by porosity of 4 to 23.4% (predominantly 9–14%), permeability from thousandths to 5.2 × 10−3 µm2 (mostly (0.1–0.6) × 10−3 µm2). The poor reservoir properties of rocks at the Krivolutsk swell and a number of local uplifts is probably related to the consedimentation growth of these structures in

the Upper Jurassic (Kontorovich, 2002). Higher permeability and porosity values were recorded on the western slope of the Aleksandrovskoe arch (Protochnaya area), where subcoal strata show permeability values of 29.1 × 10−3 µm2 (mean 4.9) and coal-overlaying strata, 307.8 × 10−3 µm2 (mean 48.9). In the northern part (Prigranichnaya area), increased permeabilities were recorded for subcoal strata—up to 283.7 × 10−3 µm2 (mean 98.3). High porosity (>15%) is most often noted in subcoal strata (Callovian–Lower Oxfordian sequence) formed in barrier bar and foreshore environments, with the reservoir properties of the J31 being somewhat better than those of the reservoir J41. The rock composition is characterized by a predominance of clastic quartz, a high content of kaolinite, and an almost complete absence of carbonates in the cement. According to the data of Pustyl’nikova (2008), a number of areas (Mygytinskaya, Poiskovaya, Yuzhno-Okhteurskaya), the J31 bed contains reservoirs of class IV. In the lower coal-overlaying bed J21 (Middle–Upper Oxfordian sequence), the reservoir properties are significantly reduced. Interbedded reservoirs of class IV were encountered mainly in the eastern and northern parts of the Aleksandrovskoe arch. The J11 bed is characterized by improved reservoir qualities and local manifestations of reservoirs of class IV in many areas of the research territory. According to Belozerov et al. (2001), the subcoal sand beds in the Vakhsakaya area have low porosity and permeability values and coal-overlaying beds show significant heterogeneity due to postsedimentation processes (primarily calcitization). Oil shows and small oil flows from coal-overlaying beds were noted marked in the Yuzhno-Aleksandrovskaya area (Brylina et al., 2006).

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