The Bazhenov Horizon of West Siberia: structure, correlation, and thickness

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ScienceDirect Russian Geology and Geophysics 59 (2018) 846–863 www.elsevier.com/locate/rgg

The Bazhenov Horizon of West Siberia: structure, correlation, and thickness S.V. Ryzhkova a,b,*, L.M. Burshtein a,b, S.V. Ershov a, V.A. Kazanenkov a, A.E. Kontorovich a,b, V.A. Kontorovich a,b, A.Yu. Nekhaev a, B.L. Nikitenko a,b, M.A. Fomin a,b, B.N. Shurygin a,b, A.L. Beizel a, E.V. Borisov a, O.V. Zolotova a, L.M. Kalinina a,b, E.V. Ponomareva a a

A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 1 February 2018; accepted 1 March 2018

Abstract The type sections of the Bazhenov Horizon and formations recognized within this horizon have been identified based on a comprehensive analysis of paleontological, lithological, geophysical (well-log and CDP seismic data), and geochemical data on the West Siberian Basin. The Bazhenov Horizon was traced throughout the entire West Siberian sedimentary basin. The criteria for the recognition of the top and base of this horizon within the stratigraphic equivalents of the Bazhenov Formation were suggested. The proposed facies-stratigraphic zonation of the Bazhenov Horizon reflects the spatial location of all formations identified within this horizon. As seen on the newly proposed thickness map, the Bazhenov Horizon reaches a thickness of 15–25 m within the Bazhenov and Tutleim Formations, 30–35 m within the Mulym’ya Formation, 30–45 m within the Danilov Formation, 40–65 m within the Mar’yanovka Formation, up to 100 m within the Golchikha Formation, >350 m within the Yanovstan Formation, up to 35 m within the Bagan Formation, and 35–40 m within the Maksimkin Yar Formation. A marginal filter (according to A.P. Lisitzin) has been identified along the East Siberian land. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Bazhenov Horizon; Bazhenov Formation; well-to-well correlation; marginal filter; West Siberia

Introduction. Definition of the notion of Bazhenov Horizon Kontorovich et al. (1975) proposed to recognize horizons as one of the regional stratigraphic units used in the subdivision of the Mesozoic–Cenozoic section of the West Siberian geosyneclise. The stratigraphic subdivision “horizon” was accepted by the Fourth Interdepartmental Meeting on the development of the unified and correlation stratigraphic schemes for the West Siberian plain, held on November 17–19, 1976, and ISC Committee on the Jurassic System, held on January 25–28, 1978, and introduced into the General Stratigraphic Scale (Instructions…, 1984). Horizon is defined as a depositional unit traceable at a regional scale that was formed at a certain stage of the basin’s evolution. In accordance with these decisions, a horizon spanning the Volgian

* Corresponding author. E-mail address: [email protected] (S.V. Ryzhkova)

through the lowermost Berriasian was distinguished in in the regional stratigraphic scheme of the West Siberian Plain. This horizon was named after the Bazhenov Formation, the most widespread unit in the area at this stratigraphic level. In the 1960s and 1970s, most researchers believed that the Bazhenov Formation, with a good source rock potential, have a significant potential for generating commercial amounts of hydrocarbons (Gurari, 1961, Gurari et al., 1963; Kontorovich et al., 1975; Novikov et al., 1970; and others). This formation and its organic-rich stratigraphic equivalents have long been studied by numerous geologists (Braduchan et al., 1986; Bulynnikova et al., 1978; Chernikov and Zapivalov, 1958; Gurari et al., 1988; Kontorovich et al., 1967, 1975, 2016; Nesterov, 1976; Novikov et al., 1970; Polyakova et al., 2002; Sverchkov, 1958; Ushatinskii, 1981). Detailed studies on the geology of the Bazhenov Horizon as an independent unit deposited at a certain stage during the evolution of the Western Siberian sedimentary basin are extremely scarce (Braduchan et al., 1986; Bulynnikova et al., 1978; Golbert et al., 1968; Kontorovich et al., 1975). Since

1068-7971/$ - see front matter D 201 8, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2018.07.009 +

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the identification of the Bazhenov Horizon, the paleontological data reported on the formations comprising this horizon have been substantially supplemented, new formations have been identified, lithology, organic geochemistry, the nature of hydrocarbon reservoir rocks have been studied in much greater detail. The delineation and correlation of the Bazhenov Horizon throughout the West Siberian basin require a unified approach, which would enable us to study the depositional environments, diagenetic and catagenetic processes, and to evaluate the generative and hydrocarbon potential of the kerogen-bearing carbonate–argillaceous–siliceous sequence of the Bazhenov Horizon. However, this proved to be a difficult task because of a considerable difference among all of the correlation charts and stratigraphic subdivisions of well sections proposed by different groups of researchers. The goal of this study is to provide substantiation and unification of the methodology for the identification and correlation of the Bazhenov Horizon in different facies regions of the West Siberian basin, and to discuss some of the most important results.

Lithostratigraphy, facies stratigraphic zonation The Bazhenov Horizon is subdivided into the Bazhenov Formation, the lower units of the Tutleim and Mulym’ya Formations, the Fedorov Formation, the upper units of the Danilov and Mar’yanovka Formations, the uppermost parts of the Bagan, Golchikha, Maksimkin Yar and Yanovstan Formations (Fig. 1). The updated scheme showing the distribution of the individual formations within the Bazhenov Horizon in the West Siberian basin is presented in Fig. 2. This distribution scheme is overcomplicated. For historical reasons, at the early exploration stages, this vast area of the basin was explored by different groups of geologists, who gave different names to different formations independently from each other. In a general regional stratigraphic scheme of the entire basin, the names of the formation remained the same in accordance with the Stratigraphic Code and the priority rule. Since the boundaries of the formations (different facies zones) were originally drawn based on data from widely spaced wells, they needed refinement. Figure 2 shows the location of each formation derived from data from over 6850 wells and available seismic-stratigraphic data.

Criteria for the recognition of the Bazhenov Horizon based on well data The Bazhenov Formation (and its stratigraphic equivalents) represents the time of maximum marine transgression in the Late Jurassic and is interpreted as the product (at least Bazhenov Formation and the lower unit of the Tutleim Formation) of predominantly biogenic sedimentation (Braduchan et al., 1986; Bulynnikova et al., 1978; Golbert et al., 1968; Gurova and Kazarinov, 1962; Kontorovich et al., 2016;

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Ushatinskii, 1981; Zubkov, 2001; Zubkov and Mormyshev, 1987; and others). Based on its lithology, chemistry (in particular, U content), and physical parameters, the Bazhenov Formation differs considerably from the overlying and underlying strata. These properties make it a key regional marker for subdivision of the section using well-log and CDP seismic data. The subdivision and correlation of polyfacies sediments of the Bazhenov Horizon was performed using an integrated approach. The identification of the Bazhenov Horizon in well sections was based on paleontological data because of lithological and facies changes. For this purpose, the existing fossil (both macro- and microfaunal) identifications for the Upper Jurassic and lowermost Lower Cretaceous were compiled and reinterpreted at the Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences. In addition, macro- and microfaunal analysis was carried out on core samples collected from well sections that have not been previously dated paleontologically (Bazhenov, Mar’yanovka, Golchikha, and Yanovstan Formations). A number of type sections were identified based on recently available geological and geophysical data and re-examination of the stratotypes of the formations (Fig. 2) (Borisov et al., 2017). At the same time, the previous classification of the Bazhenov Formation sections across the entire West Siberian basin or its parts were taken into account (Braduchan et al., 1986; Bulynnikova et al., 1978; Gaideburova, 1982; Kontorovich et al., 1975; Mukher et al., 2016; Polyakova et al., 2002; Ushatinskii, 1981). The boundaries of the Bazhenov Horizon and areas where these rocks are missing were specified. The so-called anomalous sections of the Bazhenov Formation are not considered in this study. Facies equivalents of the kerogen-bearing carbonate–argillaceous–siliceous sequence were identified and correlated through the well sections using lithological and geochemical data from 15 cored wells. The identification of Bazhenov Horizon and detailed correlation of well sections at this stratigraphic level were preformed using a unified approach to the analysis of well-log data. For these purposes, we used the following formation of well-logs: electrical log (resistivity and spontaneous potential logs (RL, SP), induction logs (IL), laterologs (LL)), caliper and sonic logs (CL and SL), radioactivity logs (gamma-ray, neutron gamma-ray logs (GR, NGR) and their modifications). At the first stages of the analysis, preference was given to wells drilled to the Bazhenov Horizon for which a complete suite of logs is available. It should also be noted that our study was based on well-log data on the Bazhenov Horizon that have been acquired over a period of more than 60 years. Over this period, considerable advances have been made in the development of new logging techniques and tools, which enabled improvements in instrumental errors and resolution. In connection with this, no quantitative criteria have been established for individual types of well logs used for recognition and delineation of the upper (top) and lower (base) surfaces of the Bazhenov Horizon

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Fig. 1. Formations distinguished within the Callovian and Upper Jurassic sections of the West Siberian sedimentary basin. 1–3, horizons: 1, Bazhenov; 2, Georgiev; 3, Vasyugan.

across the entire West Siberian basin and within individual facies zones. Special attention was paid in this study to the delineation of bounding marker beds of different rank within the Bazhenov Horizon. Since facies changes identified from logs are not synchronous through the entire section, the position of individual members in each specific section can be determined by means of well-to-well correlation. At the same time, cyclic stacking pattern analysis was carried out to study areas along the periphery of the West Siberian basin (Danilov, Mar’yanovka, Maksimkin Yar, and Bagan Formations), and sequence stratigraphic analysis was applied to study the Golchikha and Yanovstan Formations. The application of the integrated approach to study 6850 wells allowed us to draw the following conclusions on correlation of rocks of the Bazhenov Horizon. At a regional scale, induction log data provide the most robust geophysical criterion to delineate the Bazhenov Horizon within the Bazhenov, Tutleim, Mulym’ya, and Mar’yanovka Formations. However, induction log readings are not very useful for delineating the Bazhenov Horizon within the Danilov, Maksimkin Yar, and Bagan Formations, where it can be reliably traced on other log curves (RL, LL, GR, NGR, SL, CL). The subdivision and correlation of well sections of the Yanovstan and Golchikha Formation was inhibited by clinoform geometries of the Callovian and Upper Jurassic strata (Nezhdanov and Garelin, 1987; Shestakova and Ershov, 2016). The well correlation shows that the most discriminating

individual well logs are conventional (RL, SP, and IL), nuclear (GR, NGR), and caliper (CL) measurements, which allow a reliable subdivision of the Golchikha and Yanovstan Formations. At the same time, nuclear measurements, as well as caliper and induction logging (CL and IL) proved to be useful in the distal offshore zone of the Upper Jurassic basin. The integration of paleontological data from core samples and interpretations of seismic data proved to be fairly successful with respect to the unambiguous identification of the lateral extent of the Bazhenov Horizon.

Method of interregional correlation Interregional correlation was performed in two steps. The first step involved the correlation of the stratotype sections of the Bazhenov, Tutleim, Mulym’ya, Danilov, Mar’yanovka, Maksimkin Yar, Bagan, Yanovstan, and Golchikha Formations. A comprehensive analysis of the geological and geophysical data on individual stratotype sections of the above formations enabled us to identify time equivalents of the Bazhenov Formation. The geophysical parameters measured in each well encountered the stratotype sections of the above formations were taken as a reference for distinguishing the Bazhenov Horizon. For stratotype wells with an incomplete suite of conventional logs, a section encountered in a well with a complete suite of conventional logs drilled within the same second-order tectonic element was taken as the stratotype. The boundaries of the Bazhenov Horizon were

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Fig. 2. Facies-stratigraphic zonation of the Bazhenov Horizon, West Siberian sedimentary basin. 1–5, boundaries: 1, state, 2, administrative, 3, Mesozoic deposits, 4, Upper Jurassic deposits, 5, formations containing rocks of the Bazhenov Horizon (a), ibid, inferred (b); 6–8, wells in which: 6, the stratotype of formation containing the Bazhenov Horizon was identified, 7, type sections of the Bazhenov Horizon were identified, 8, the presence of the Bazhenov Horizon was confirmed. The numbers denote wells: 1, Rassokhinskaya 1; 2, Deryabinskaya 6; 3, Paiyakhskaya 4; 4, Malokhetskaya 10; 5, Tukolando-Vadinskaya 320; 6, Khalmerpayutinskaya 2099; 7, Tagulskaya 8; 8, Lykhminskaya 172; 9, Em-Egovskaya 4; 10, Danilovskaya 10009; 11, Lazarevskaya 10120; 12, Severo-Salymskaya 1183; 13, Vostok 4; 14, Nyarginskaya 1; 15, Yarskaya 1; 16, Archinskaya 47; 17, Chulymskaya 1; 18, Nikolskaya 1; 19, Tatarskaya 1; 20, Yushno-Chulymskaya 1.

redefined based on the results of an integrated analysis of paleontological, well-log, and geochemical data. The second step involved the interregional correlation between the type sections of the Bazhenov Horizon within the neighboring formations (Fig. 2). For this purpose, several wells

located between the stratotype sections, with sufficient paleontological, lithological and geophysical data were chosen to clearly establish the interval of the section correlatable with the Bazhenov Horizon. The wells where only geophysical logs are available were analyzed simultaneously. The sections

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were correlated along profiles oriented approximately EW and NS across first-order positive and negative tectonic structures. The underlying and overlying sequences were delineated by picking the intervals extended 100 m above the top of the Bazhenov Horizon and 50 m below the base of the Vasyugan Horizon.

Correlation of sections of the Bazhenov Horizon Below, we present the correlation results for the sections of the Bazhenov Horizon across the entire Western Siberian sedimentary basin. The description is given from the center of the basin outward by individual facies regions. Central regions. In the central part of the West Siberian sedimentary basin, the interval of the Bazhenov Horizon is equated with the hypostratotype of the Bazhenov Formation defined in the Salymskaya 170 well (Braduchan et al., 1986). It is referred by many researchers as the Salym type of the Bazhenov section. A characteristic feature of the Salym-type section is the high oil saturation. In this study, the section penetrated in the Severo-Salymskaya 1183 well was taken as a standard for the Salym-type section of the Bazhenov Formation (Fig. 2). A comprehensive analysis of recent lithological, paleontological and geophysical data allowed a more detailed description of the section types of the Bazhenov Formation, which have been proposed previously in Braduchan et al. (1986). The northeastern and northern margins of the the Bazhenov Formation were recognized as a specific, low-resistivity type of the section. Variations in apparent resistivity and gamma-ray activity values across the section of the Bazhenov Formation reflect vertical and lateral lithological heterogeneities in carbonaceous, siliceous, argillaceous, and carbonate parts of the section (Kontorovich et al., 2016). The Salym-type section of the Bazhenov Formation can be divided into three parts based on its lithology. The lower, middle, and upper parts are composed of silicites, kerogenbearing silicites, and kerogen-bearing carbonate silicites and mixtites, respectively (Kontorovich et al., 2016). At the same time, a more detailed subdivision allows the recognition of six members (Panchenko and Nemova et al., 2015; Panchenko et al., 2016). The following trend has been detected in the Bazhenov Formation on flanks of the Khantei hemianteclise, east of the Salym-type section toward the Nizhnevartovsk arch. The section has a relatively constant thickness of ~30 m and exhibits a strong variation of resistivity values, as indicated by interbedding low- (up to 20 Ohm⋅m) and high-resistivity (up to 200 Ohm⋅m) layers. The thickness of low-resistivity (up to 20 Ohm⋅m) members is not greater than 5 m. In the same direction, the maximum resistivities are reported from the lower part of the section. As noted above, a similar trend is observed in the southern parts of the formation. A characteristic pattern on the gamma-ray logs in the same direction is different from that measured in the southern

regions. The gamma-ray curve exhibits a generally smoother pattern, with one peak value (up to 65 µR/h) in the middle part of the section. To the north of the Middle Ob region, the overall pattern of resistivity and gamma-ray curves from the Bazhenov Formation remains almost unchanged as compared to that of the Salym-type section, although resistivity and gamma-ray values are much lower (up to 100 Ohm⋅m and 45 µR/h, respectively). The eastern, northeastern, and northern parts of the Bazhenov Formations have low apparent resistivity values. These intervals are designated as a specific, low-resistivity type of the Bazhenov section. Rocks with resistivities of less than 20 Ohm⋅m are estimated to comprise as much as 50% of the thickness of the Bazhenov Formation in these areas. The maximum resistivities (>200 Ohm⋅m) were reported for the remaining part of the formation. Gamma-ray activity values here are slightly higher than in the overlying and underlying strata and vary between 50 and 60 µR/h. Western regions. Close to the Ural part of the West Siberian sedimentary basin, the high-carbonaceous intervals of the Bazhenov Formations grade westward into the lower units of the Tutleim and Mulym’ya Formations. The latter appears to grade into the upper unit of the Danilov Formation. The stratigraphy, lithology and geology of these strata have been described in detail in previous studies (Alekseev, 2010; Braduchan et al., 1986). The goal of this study was to develop a unified approach to delineating boundaries of the Bazhenov Horizon in the sections of the above formations and to analyze variations in the structure of the Bazhenov Horizon sections in the zones of facies change between the above-mentioned stratigraphic subdivisions. The stratotype of the Tutleim Formation was established in 1956 near the settlement of. Berezovo in the Chuelskaya 81 (3)-R well within the depth interval of 1628–1668 m (Braduchan and Lebedev, 1979; Kontorovich et al., 1975). The formation is subdivided into two units. The lower unit is regarded as the time equivalent of the Bazhenov Formation. The lower unit of the Tutleim Formation has the higher content of organic matter than the upper unit. The boundary between the lower and upper units is defined by a decrease in resistivity (up to 2–4 Ohm⋅m) and a sharp increase in the induction log values (>300 mS/m). A rich fauna is present at the base of the lower unit of the Tutleim Formation and near its boundary with the Abalak Formation (Fig. 3). Based on the organic matter content and distribution, three types of sections of the Tutleim Formation are distinguished: Chuelsk, Krasnoleninsk, and Tobolsk (Braduchan et al., 1986). The Chuelsk-type section (northern parts of the Tutleim Formation) is characterized by high gamma-ray (up to 40 µR/h) and apparent resistivity (up to 20 Ohm⋅m) values in the upper unit, which decrease further westward in the lower unit. The Chuelsk-type section of the Tutleim Formation appears to grade southward into the Krasnoleninsk type, which is characterized by higher organic matter contents in the lower unit. These rocks show high resistivity responses

Fig. 3. Correlation chart for Oxfordian—Lower Berriasian deposits in the western regions of the West Siberian sedimentary basin. 1–4, boundaries: 1, horizons, 2, formations, 3, hiatus, 4, facies change; 5–8, fossil remains: 5, ammonites, 6, bivalves, 7, belemnites, 8, foraminifers; 9, spore and pollen assemblages; 10, age. For other symbols see Fig. 1. Abbreviations for formation names: gl, Golchikha; jan, Yanovstan; sg, Sigovaya; tch, Tochinka; ab, Abalak; bg, Bazhenov; gr, Georgievka; mr, Mar’yanovka; mks, Maksimkin Yar; vs, Vasyugan; nn, Naunak; tzh, Tyazhin; bgn, Bagan; tt, Tatarka; dn, Danilov (dn1, lower unit; dn2, upper unit); tm, Tyumen; mul, Mulym’ya (mul1, lower unit; mul2, upper unit); tut, Tutleim (tut1, lower unit; tut2, upper unit).

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(>250 Ohm⋅m). The resistivity curves exhibit high-amplitude variations with isolated peaks at 50 and 100 Ohm⋅m (Fig. 3). Gamma-ray values also increase considerably, reaching a peak of 80–85 µR/h. A transitional boundary between the Bazhenov Formation and the lower unit of the Tutleim Formation is reflected by a gradual decrease in apparent resistivity and natural gamma-ray values. The zone of facies change between these two formations can be identified only provisionally because the structure of the section (the number and thickness of members) in this area remained almost unchanged. The lower unit of the Tutleim Formation grades westward into the lower unit of the Mulym’ya Formation. The Mulym’ya Formation (Early Volgian–Early Hauterivian) was distinguished in 1972 by V.G. Eliseev and V.S. Bochkarev in the 13-R well drilled at the Mulym’yinskaya prospect. The formation extends approximately N–S and is bounded by zones where Volgian rocks are absent (Fig. 2). The lower unit containing a rich fauna is attributed to the Bazhenov Horizon, whereas the upper unit is assigned to the Berriasian–Early Hauterivian (Fig. 3). The rocks of the Bazhenov Horizon generally have low resistivity values (average 3–6 Ohm⋅m). The shallow resistivity curve shows isolated peaks at 20 Ohm⋅m and the number of peaks decreases westward. Phosphorite nodules and isolated grains of glauconite are found at the base of the lower unit of the Mulym’ya Formation (Braduchan et al., 1986). This part of the section is characterized by sharp peaks on the induction curve (up to 150 mS/m). Natural gamma-ray values of the lower unit of the Mulym’ya Formation decrease from west to east from 40 to 25–30 µR/h. At the West Lovinskaya, Yakhlinskaya, and Potanaiskaya prospects, resistivity values increase to 70 Ohm⋅m in the lower unit of the Mulym’ya Formation with a simultaneous increase in natural gamma-ray values (>50 µR/h). This unit shows a decrease in thickness to 30–35 m. The sections of the lower unit of the Mulym’ya Formation represent a facies transition to a high-carbonaceous lower unit of the Tutleim Formation. This transitional zone extends as a narrow strip from north to south. The lower unit of the Mulym’ya Formation appears to grade westward into the upper unit of the Danilov Formation. The Danilov Formation was first recognized in 1976 by Yu.V. Braduchan and G.S. Yasovich in the well 62 within the depth interval of 1734–1824 m (Braduchan and Yasovich, 1984). This formation extending as a narrow strip from north to south (Fig. 2) is subdivided into two units. The upper unit is attributed to the Bazhenov Horizon, whereas the lower unit is the time equivalent of the Abalak Formation. The boundary between these two units contains a rich fauna and is clearly traced on well logs, and thus can be reliably delineated on well log curves in uncored wells (Fig. 3). The upper unit of the Danilov Formation is characterized by high organic carbon contents and low concentrations of genetically associated uranium. The natural gamma-ray values of this unit do not exceed 10–12 µR/h while apparent resistivities vary from 2 to 4 Ohm⋅m.

A zone of facies change between the lower unit of the Mulym’ya Formation and the upper unit of the Danilov Formation is characterized by small-amplitude resistivity curves, with average values not higher than 3–5 Ohm⋅m. The peak resistivity values of 10–12 Ohm⋅m correspond to a few narrow intervals. The natural gamma-ray values show a general decreasing trend is observed for formation resistivity from west to east (from 20–25 to 10–12 µR/h). Southwestern regions. A transitional zone between the Bazhenov Formation and the Tobolsk-type section of the lower unit of the Tutleim Formation is recognized to the southwest of the Mansi syneclise. This zone covers an area of the Srednetobolsk inclined megatrough, southern parts of the Srednedem’yansk megadepression and Verkhnedem’yansk megaswell. Within megatroughs, the Bazhenov Horizon is composed of rocks of the Bazhenov Formation which differ from those of the central parts. In this region, the horizon is subdivided into two members, the upper high-resistivity (>500 Ohm⋅m) and lower low-resistivity (<10 Ohm⋅m). The section of the Pekmanskaya 274 well is shown as an example in Fig. 4. The high-resistivity member is composed of black, high-carbonaceous rocks, often with oil stains and the smell of petroleum; the low-resistivity member is composed of black to dark gray platy rocks, with a high pyrite content. As opposed to the central regions, sections in the southwestern regions can be divided into two parts by of the induction log conductivity readings: the upper low-conductivity (<20 mS/m) and lower medium-conductivity (~100 mS/m). The natural gamma-ray values are relatively high (>25 µR/h) throughout the Bazhenov Horizon but not greater than 45 µR/h. Further east, within the center and western flanks of the Upper Verkhnedem’yansk megaswell, the section of the Bazhenov Formation tends to take the form typical of the central regions of the basin. Further southwest, the Bazhenov Formation grades into lower unit of the Tutleim Formation (Tobolsk-type section) toward the central part of the Krasnoleninsk megamonoclise. The resistivity and gamma-ray values decrease monotonically in the same direction. The resistivity log response and the location of peaks change gradually southwestward, from the transition zone between the Bazhenov Formation and the Tobolsk-type section of the lower unit of the Tutleim Formation. The top and base of the unit are marked by a 10 m thick layer with resistivities not greater than 50 Ohm⋅m, while the remaining portion of the section shows low, but highly variable resistivities (up to 5 Ohm⋅m) (Cherkashinskaya 1 well) (Fig. 4). Further southwest, the maximum resistivity values in high-resistivity members in the middle part of the unit do not exceed 10 Ohm⋅m (Sorginskaya 1 and South Tobolskaya 1 wells). The natural gamma-ray values are higher than in the underlying and overlying strata, but do not exceed 30 µR/h. The lower unit is composed of black to dark gray carbon-rich, dense, massive argillaceous rocks. Cherts, interlayers of argillaceous limestones, isolated phosphorite nodules, pyrite are present. Organic remains are rare, black altered plant debris is present on bedding planes. Within the Tyumen megamonoclise, the lower

Fig. 4. Correlation chart for Bathonian–Lower Berriasian deposits in the southwestern regions of the West Siberian sedimentary basin. For symbols see Fig. 3.

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unit of the Tutleim Formation grades into the upper unit of the Danilov Formation, which is composed of dark gray dense, argillaceous rocks, with interlayers of brownish varieties at the top and base and greenish varieties in the middle portion. This unit exhibit low-amplitude SP response, with nearly constant values, produced by shales, and low resistivity values (average 5 Ohm⋅m). Throughout much of the area, the upper unit of the Danilov Formation exhibits low natural gamma-ray values (<12 µR/h), which gradually increase to 25 µR/h in wells drilled in the southeastern part of the study area (Chelnokovskaya prospect), on the western flank of the Starosoldatskii swell (Fig. 4). Southern regions. In the southern regions of the West Siberian sedimentary basin, the Bazhenov Horizon is equated with the stratotype of the Bazhenov Formation. In this region, the Bazhenov member was first recognized by F.G. Gurari in 1959 at the Sargatskaya prospect. The stratotype of the formation was identified based on paleontological data, in the Bolsherechenskaya 1 well in the 1990s (Atlas..., 1990). The Bazhenov Formation penetrated in the Nikolskaya 1 well is accepted in this study as a reference section, based on the availability of a complete suite of well logs (Fig. 2). The formation, together with the overlying and underlying deposits, represents a lithologically homogeneous stratum. As was the case with the Salym-type section, the boundaries of the Bazhenov Formation were defined based on resistivity and induction logs: a sharp increase in resistivities and a decrease in electrical conductivity. The section under consideration differs considerably from sections in the central regions by shape of resistivity and gamma-ray curves from the base to the top of the formation, as well as by the absolute values of the measured parameters. The apparent resistivity of these rocks typically ranges from 30 to 80 Ohm⋅m, and the maximum values are reported from the lower part of the section. The natural gamma-ray values show only little variation across the section (average 20 µR/h), with two peaks, one peak around 35 µR/h detected in the lower and upper parts of the section, while the second peak covers a slightly broader interval. Unlike the sections in the central regions, the above sections are characterized by an opposed pattern of resistivity and gamma-ray responses, with the highest values at the base and at the top, respectively. East of the stratotype, within the Mezhovsk structural meganose and adjacent areas, the Bazhenov Formation has the following structure (Fig. 5). The lower formation boundary and the underlying Georgiev Formation were defined by a rich foraminiferal assemblage. The upper boundary was established based on well-log, lithological, and geochemical data on the Bazhenov Formation at the Rakitinskaya prospect. Within the above regions, the Bazhenov Formation is characterized by high apparent resistivity values (up to 50 Ohm⋅m) in its middle part, and high natural gamma-ray values (up to 40 µR/h) in its upper part, which reflect the distribution of carbonate and argillaceous material in the section. Further south, the Bazhenov Formation grades into the Mar’yanovka Formation toward the Barabinsk–Pikhtovka megamonoclise (Fig. 2).

The Mar’yanovka Formation was recognized by Z.T. Aleskerova and T.I. Osyko in 1957 and named after the village of Mar’yanovka of the Omsk region. The stratigraphic range and areal extent of this formation have been modified repeatedly (Nesterov, 1991; Shurygin et al., 2000). The name Mar’yanovka Formation and its stratotype need to be replaced in modern stratigraphic schemes, according to the provisions of the Stratigraphic Code of Russia (Gurari, 2004). Since this task was beyond the scope of the present study, we used the former nomenclature and analyzed the present stratotype of the Mar’yanovka Formation at the Tatarskaya prospect and the proposed stratotype in the Nyarginskaya 1 well (Shurygin et al., 2000). For the described territory of the southern regions of the West Siberian sedimentary basin, a stratotype of the formation in the borehole is taken as the reference section of the Bazhenov Horizon. The formation stratotype in Tatarskaya 1 well was accepted as a reference section of the Bazhenov Horizon in the southern regions of the West Siberian basin (Fig. 2). In the southern regions, the boundaries of the Bazhenov Horizon within the Mar’yanovka Formation were defined based on correlation with wells penetrated the Bazhenov Formation in the adjacent regions (Fig. 5). The Bazhenov Horizon is predominantly composed of silt and shale. The interval of the Bazhenov Horizon is distinguished on resistivity, caliper, and gamma-ray logs. In the south, the Mar’yanovka Formation is replaced by the Bagan Formation. A transitional zone between these formations is assumed to be located to the south of the town of Barabinsk, Novosibirsk region, although no well data are available (Fig. 2). Three areas where Jurassic strata are missing have been identified in this region in the 1970s based on the results of gravimetric, aeromagnetic and seismic surveys. A zone located northeast of Barabinsk has a similar structure, and the absence of Jurassic deposits was confirmed by the Gorbunovskaya 1 well. The Bagan Formation was established by V.A. Martynov in the Yuzhno-Chulymskaya 1 well (Fig. 2) and was named after the Bagan River (Novosibirsk Region) (Nesterov, 1991). The interval equated with the Bazhenov Horizon is composed mainly of silty–sandy rocks, with shales and carbonates in its lower part. The age was assigned based on ostracod, bivalve, and foraminiferal assemblages. The boundaries of the Bazhenov Horizon were not defined by paleontological data. The corresponding interval of the section was identified in the formation only provisionally based on lithological interpretation of well log data (Fig. 5). The Bazhenov Horizon has apparent resistivities of <10 Ohm⋅m, whereas electrical conductivity values vary widely from 30 to 200 mS/m as compared to overlying and underlying strata of the Kulomzin and Georgiev Horizons. The natural gamma-ray values exhibit little variation across the Bazhenov Horizon, averaging 7 µR/h. Southeastern regions. Within the Ust’-Tym megadepression, in a transitional zone to the Mar’yanovka Formation, the Bazhenov Formation exhibits low resistivity (average

Fig. 5. Correlation chart for Callovian–Lower Berriasian deposits in the southern regions of the West Siberian sedimentary basin. For symbols see Fig. 3.

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10 Ohm⋅m) and relatively high natural gamma-ray values (>15 µR/h) (Fig. 6). In these regions, the stratotype of the formation defined in the Nyarginskaya 1 well (Shurygin et al., 2000) was accepted as a reference section of the Bazhenov Horizon within the Mar’yanovka Formation (Fig. 2). The Mar’yanovka Formation extends from the northeastern termination of the Barabinsk–Pikhtovaya megamonoclise in the south to the southwestern part of the Yenisei megamonoclise in the north. Within the Vladimirovsky structural meganose (Vezdekhodnaya and Martovskaya prospects) in the area adjacent to the Maksimkin Yar Formation (Fig. 2), the Mar’yanovka Formation is characterized by the presence of a 20 m thick silty-sand layer at the top, which is equated with the Bazhenov Horizon, as indicated by abundant micro- and macrofaunal remains (Fig. 6). The upper boundary of the Mar’yanovka Formation is defined provisionally by a lithological change because of the absence of fossils. The interval corresponding to the Bazhenov Horizon was identified on the basis of low electrical conductivity values, as compared to the underlying rocks (Fig. 6). This is characteristic of the sections of the Bazhenov Formation (Braduchan et al., 1986). The shale-dominated sections display only little variation in the apparent resistivity values (<8 Ohm⋅m). Natural gamma-ray values of the Bazhenov Horizon within the Mar’yanovka Formation vary slightly and are almost identical to those reported from the overlying and underlying strata. They may vary between wells from 10–13 µR/h to 5–7 µR/h within a single prospect. In the southeast of the West Siberian basin and in the northwest of the Teguldet megasyneclise, the Upper Jurassic section is represented by the Maksimkin Yar Formation (Fig. 2). This formation was identified by M.A. Tolstikhina in 1957 in the Maksimkinyarskaya 1 well, near the village of Maksimkin Yar. The type section was established in the Yarskaya 1 well (Fig. 2). The formation is composed of greenish gray, fine-grained, massive calcareous sandstones containing marine bivalves. The sandstones are interbedded with siltstones, mudstones and gray, greenish gray or, rarely, reddish-brown marls. The Maksimkin Yar Formation follows above the Tyazhin Formation without a break and is underlain erosively by variegated silts and shales of the Ilek Formation (Lower Cretaceous). The upper, more shaly portion of the Maksimkin Yar Formation was assigned to the Bazhenov Horizon interval based on well correlation and the presence of a bivalve fauna (Fig. 6). In the central and northeastern part of the Teguldet megasyneclise, the Maksimkin Yar Formation is absent in Chulymskaya 1, Kaskaya 1, Vostok 4 wells, etc. (Fig. 2). Northwestern regions. In the northwest of the West Siberian basin (the Yamal Peninsula, the western part of the Yamal Peninsula), the Upper Jurassic black carbonaceous shales were designated as the Bazhenov Formation (Braduchan et al., 1986; Gurari, 2004; Kislukhin et al., 2010). In this area, the Bazhenov Formation is poorly characterized by core samples and is delineated mainly by a suite of well logs

(resistivity and gamma-ray). The positions of formation boundaries are located using all three log curves: a decrease in electric conductivity and an increase in resistivity and natural gamma-ray values (Fig. 7). Both resistivity and gamma-ray values differ considerably those recorded from the stratotype of the Bazhenov Formation. A low-resistivity portion of the Bazhenov Formation is delineated on the Yamal Peninsula and adjacent areas (Fig. 2). Some researchers suggest that the Bazhenov Formation experienced either complete or partial erosion within some prospects (Kislukhin et al., 2010). Northern and northeastern regions. To the north and northeast, the Bazhenov Formation is replaced by the upper parts of the Golchikha and Yanovstan Formations. The transitional types of sections (Bazhenov–Yanovstan and Bazhenov–Golchikha) exhibit a sharp decrease in thickness and a distinct bipartite division: the upper part, with elevated gamma-ray values (similar to those of the Bazhenov Formation), containing isolated high-resistivity interlayers; and the lower part, with low gamma-ray and resistivity values. The upper part of the formation stratigraphically corresponds to the Bazhenov Horizon, the lower part is equated with the Georgiev and Vasyugan Horizons. The stratotype of the Golchikha Formation (lowermost Berriasian–Upper Bathonian) was designated by V.I. Kislukhin in 1986 at the village of Golchikha, near the mouth of the Yenisei River, in the Deryabinskaya 5 well in the depth interval of 2937–3312 m (Kislukhin, 1986; Kukushkina and Kislukhin, 1983). The formation represents a thick (up to 880 m in places) sequence of mudstones, gray to dark gray, carbon-rich, very fine-grained to silty, with rare siltstone interlayers. It occurs within the Yenisei–Khatanga regional trough and in the areas bordering in the west on the Gydan Peninsula (Fig. 2). The formation exhibits low-amplitude SP response, with nearly constant values produced by shales, low resistivity values (Kulikov, 1989), and low natural gamma-ray values. However, a few layers showing relatively high gammaray values were identified in the upper part of the Golchikha Formation in wells drilled to the deepest parts of the Yenisei–Khatanga regional trough. To the south and southeast of the Yenisei–Khatanga regional trough, within the Messoyakha inclined ridge, the Golchikha Formation is replaced by its stratigraphic equivalents (from the bottom upwards): Tochinka, Sigovaya, and Yanovstan Formations. The Yanovstan Formation (lowermost Berriasian–uppermost Lower Volgian) was distinguished in 1965 and has a stratotype in the Turukhanskaya stratigraphic test well in the depth interval of 2032–2260 m (Belkina et al., 1965, Decisions..., 1969). The formation is up to 600 m thick and consists mostly of shales and dark gray mudstones, very fine-grained to silty, with rare interlayers of carbon-rich varieties and subordinate sandstones and siltstones. It occurs within the southeastern flank of the Yenisei–Khatanga regional trough, Messoyakha inclined ridge and extends further south, toward the Yeloguiskaya and Lekosskaya prospects. The number of sand and siltstone layers increases in the southern regions and

Fig. 6. Correlation chart for Oxfordian–Lower Berriasian deposits in the southeastern regions of the West Siberian sedimentary basin. For symbols see Fig. 3.

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Fig. 7. Correlation chart for Callovian–Lower Berriasian deposits in the northwestern regions of the West Siberian sedimentary basin. For symbols see Fig. 3.

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toward the eastern flank of the basin (Rostovtsev and Bulynnikova, 1973). The well-log characteristics of the Yanovstan Formation in the Yenisei–Khatanga trough are almost identical to those of the Golchikha Formation (Baiborodskikh et al., 1968). South of the Messoyakha inclined megaswell, the rocks of the formation show a large variation in resistivity with a few maxima (up to 20 Ohm⋅m) due to the increased proportion of sand- and silt-sized material, while the SP curves are characterized by the presence of negative amplitude anomalies (Krasnoselkupskaya 1 and Yuzhno-Sidorovskaya 4 wells) (Nezhdanov and Garelin, 1987). The Yanovstan Formation exhibits generally low natural gamma-ray values, averaging 8–10 µR/h (Borisov et al., 2017). Using an integrated suite of data (well logs, paleontological and CDP seismic data) allows us to identify a shale sequence (Upper Kimmeridgian–Lower Berriasian) equated with the Bazhenov Horizon in the upper part of the Yanovstan and Golchikha Formations. The identified sequence shows great variations in thickness, from 460 m in the axial part of the Yenisei–Khatanga regional trough to a complete absence in the crests of the Messoyakha inclined ridge and the Tundra– Volochansk megaswell (Fig. 8). In a transitional zone between the Golchikha and Bazhenov Formations, the upper part of the formation (Bazhenov Horizon) is distinguished from the above and below strata (Ader-Payutinskaya and Tota-Yakhinskaya prospects) by one or two high resistivity and gamma-ray peaks (Fig. 8). To the south, in a transitional zone between the Yanovstan and Bazhenov Formations, the upper part of the Yanovstan Formation (Bazhenov Horizon) is characterized by generally elevated gamma-ray values, similar to those recorded in zones of the typical Bazhenov Formation, but lower resistivity values (Geologicheskaya and Khancheiskaya prospects). Both resistivity and gamma-ray values increase further south and become identical to those typical for the central regions of the Bazhenov Formation. The identified trends were used as a basis for the revision of a transitional zone from the Yanovstan and Golchikha to Bazhenov Formation. In the Gydan Peninsula, the boundary of the Golchikha Formation is displaced to the east of the Geofizicheskaya prospect and to the east of the Utrennyaya and Gydanskaya prospects, where low-resistivity equivalents of the Bazhenov Formation were identified (Fig. 2). The boundary forms a narrow westward wedge in the vicinity of Ader-Payutinskaya (Semakovskaya) wells making a sharp bend to the east and passes around the western termination of the Messoyakha inclined megaswell, where it runs southeastward, into the the Yanovstan Formation. Further south, this zone extends as a narrow strip east of Pyakyakhinskaya, Zapolyarnaya, Geologicheskaya and Khadyryakhinskaya prospects and passes around the Verkhnechaselskaya, Tarelskaya, and Kholmistaya prospects from the west, and making a sharp bend to the east in the vicinity of the Udmurtskaya prospect, it runs northward of the Lekosskaya prospect until it approaches a zone of pinch-out of Jurassic deposits south of the Yeloguiskaya stratigraphic test.

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Thickness of the Bazhenov Horizon A well-to-well log correlation was used to construct a thickness map for the Bazhenov Horizon (Fig. 9). It can be seen that the thickness of the Bazhenov Horizon varies from 15 to 25 m in the central part of the West Siberian basin, within the Bazhenov Formation and in the lower unit of the Tutleim Formation. The thickness increases locally to 40–45 m in the northern part of the West Siberian basin, west of Middle Pur megatrench, and in the Urengoi and Nadym regions as well. The thickness of the Bazhenov Horizon increases towards the periphery of the basin and then decreases again down to almost complete pinch-out at the boundary with Early Volgian–Early Berriasian marine successions. Deposits of this age appear to be locally absent on flanks of the basin. The largest zone where the Bazhenov Horizon is missing was delineated in the Berezov region in the northwest of the study area. In the southwest of the basin, the thickness of the Bazhenov Horizon increases to 45 m. This area is isometric in shape and corresponds to the areal extent of the Bazhenov, Tutleim, and Danilov Formations. Tectonically, it is confined to the central part of the N–S-trending Krasnoleninsk megamonoclise and Mansi syneclise. The Tutleim Formation extends along the western flank of the basin. The maximum thickness of its lower unit is between 40 and 45 m in the Shaimsk and Karabash regions, while in the north this unit is on average 5–10 m thick. The lower unit of the Mulym’ya Formation which has a restricted areal extent is 30–35 m thick in the central part. The formation is absent in places at the crest of the Shaim megasalient. To the north and south, the formation is bounded by the apparent absence of the Early Volgian–Early Berriasian. The upper unit of the Danilov Formation was deposited at the westernmost margin of the basin. Its average thickness is 15–20 m and may increase locally to 35–45 m in the Shaimsk and Karabash regions. In the north the West Siberian basin, the thickness of the Bazhenov Horizon varies between 15 and 20 m, increasing to 40 m at the Kharasaveiskaya prospect in the southern part of the Kara megasyneclise. In the offshore Kara Sea regions, the deposits of this age were encountered only in wells drilled at the Sverdrupskaya and Universitetskaya prospects, so only provisional thickness maps can be drawn for this region. In the south of the basin, within the Mar’yanovka Formation, the Bazhenov Horizon may reach locally 40–45 m in thickness. These areas are confined tectonically to the southern part of the Krasnoleninsk megamonoclise and North Mezhovka megamonocline. In the southeast of the basin, the Bazhenov Horizon may reach 60–65 m in thickness within the East Paidugina megadepression. In the extreme southeast of the basin, the Maksimkin Yar Formation is present only locally within the northeastern part of Teguldet megahemisyneclise. In this region, the thickness of the Bazhenov Horizon increases basinward to 35–40 m. Like the Maksimkin Yar Formation, the Bagan Formation has a restricted areal extent and is identified at the southern

Fig. 8. Correlation chart for Bathonian–Lower Berriasian deposits in the northern and northeastern regions of the West Siberian sedimentary basin. For symbols see Fig. 3.

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Fig. 9. Thickness map for the Bazhenov Horizon of the West Siberian sedimentary basin.

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periphery of the West Siberian basin. In this region, the Bazhenov Horizon reaches a thickness of 35 m at several prospects. The maximum thickness is reported from the Yanovstan Formation in the eastern part of the basin. The N–S-trending strip of elevated thicknesses of the Bazhenov Horizon (80– 110 m) extends along the boundary between the inner and the outer belt of tectonic elements. This area extends northward, where the thickness of the Bazhenov Horizon varies from 200 m within the Vankor–Tagul inclined megaswell to >300 m on the eastern flank of the Bolshaya Kheta megasyneclise. In the northeast of the West Siberian geosyneclise where it adjoins the Yenisei–Khatanga regional trough, the thickness of the Bazhenov Horizon increases to 100 m. The area of elevated thicknesses extends further eastward and covers almost the entire Yenisei–Khatanga regional trough. In the south, this area is bounded by zones of absence of Volgian deposits in the crestal parts of the Messoyakha inclined megaridge and Rassokha inclined megaswell. This large area can be subdivided into several local zones of maximum thickness, which are delineated by the 200 m contour line. The maximum thickness of Volgian marine deposits (>350 m) is detected at the Ozernaya and Kubalakhskaya prospects. Such a dramatic increase in the rate of sediment accumulation on the eastern and northeastern flanks of the basin during the Early Volgian–Early Berriasian can be explained by the operation of the marginal filter. According to Lisitzin (1994), a marginal filter is a narrow belt extending for hundreds of kilometers along the coasts of the continents, where mixing of fluvial water and seawater and avalanche sedimentation occur. In marginal filter zones, the riverine water in the river mouth areas is subjected to the complicated impact of various sorbents, organisms, biofiltration, and some other processes characteristic only for this area. This leads to the removal of almost all suspended particles, many metals in both dissolved and suspended forms, terrigenous organic matter, etc.

Conclusions The integrated analysis of paleontological, lithological and geochemical data on core samples of Callovian–Volgian rocks, well-log and seismic data were used to delineate the Bazhenov Horizon across the entire West Siberian sedimentary basin. A facies-stratigraphic zonation of the Bazhenov Horizon reveals a close spatial relationship between the Bazhenov Formation and its stratigraphic equivalents. A new thicknesses map for the Bazhenov Horizon, coupled with the paleontological evidence from the Bazhenov Sea, lithology, rock chemistry and organic geochemistry may become the basis for reconstructing the depositional environments, diagenetic and catagenetic processes, and evaluating the generative and hydrocarbon potential of the kerogen-bearing carbonate–argillaceous–siliceous sequence of the Bazhenov Horizon.

This study was performed as part of the Basic Research Program of the Siberian Branch, Russian Academy of Sciences (IX.131.1) “Problems of Regional Geology, Sedimentology, Organic Geochemistry and Hydrocarbon Potential of Sedimentary Basins of Siberia and the Arctic Ocean, Scientific Basis of the Methodology of Environmental Monitoring at Oil and Gas Production Facilities in the Arctic.”

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Editorial responsibility: V.A. Kashirtsev