Tectonic evolution of the Arctic onshore and offshore regions of the West Siberian petroleum province

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ScienceDirect Russian Geology and Geophysics 58 (2017) 343–361 www.elsevier.com/locate/rgg

Tectonic evolution of the Arctic onshore and offshore regions of the West Siberian petroleum province V.A. Kontorovich a,b,*, D.V. Ayunova a, I.A. Gubin a, A.Yu. Kalinin a, L.M. Kalinina a,b, A.E. Kontorovich a,b, N.A. Malyshev c, M.B. Skvortsov d, M.V. Solov’ev a,b, E.S. Surikova 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 c Rosneft, Sofiiskaya nab. 26/1, Moscow, 117997, Russia d All-Russian Research Geological Oil Institute, Shosse Entuziastov 36, Moscow, 105118, Russia Received 29 August 2016; accepted 1 September 2016

Abstract The paper deals with the South Kara regional depression covering the southern part of the Kara Sea and presents a comparative analysis of the geologic structure of potential exploration targets in the offshore and onshore regions of the study area. Regional tectonic processes are considered, and the main stages of the formation of large tectonic elements are reconstructed. A comparison of the anticline traps in the onshore parts of the study area and offshore Kara Sea regions is made. © 2017, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: reflector; seismic sequence; tectonics; structure; trap

Introduction The West Siberian petroleum province has been a focus of attention of many geologists and geophysicists since the late 1940s. As a result of a large exploration effort expended in this region and the discovery of more than 700 oil and gas fields, West Siberia has been a leading domestic oil and gas producing region for about one-half a century. Systematic exploration in West Siberia started in the southern regions close to the largest populated locations and continued even more northerly. At present, one of the key research challenges facing Russian petroleum geologists and geophysicists is the study of the geologic framework of the Russian Arctic, including West Siberia and Kara shelf, considering their high potential for hydrocarbons (Bochkarev et al., 2010; Kazanenkov et al., 2014; Kontorovich et al., 2013). This study aims at analyzing the geologic framework of the South Kara depression in the South Kara basin and providing a comparison of its hydrocarbon potential with that of the Arctic regions of continental West Siberia, including * Corresponding author. E-mail address: [email protected] (V.A. Kontorovich)

the Yamal Peninsula and the Gydan Peninsula. Mesozoic sediments extending over 530,000 km2 cover an area of 348,000 km2 offshore (excluding bay areas) and 182,000 km2 onshore. The overwhelming amount of seismic activity and deep drilling over the past decades resulted in numerus oil and gas discoveries both offshore and onshore. The fields discovered to date are Arkticheskoe, Antipayuta, Bovanenkovskoe, Kruzenshtern, Malyginskoe, North Tambeiskoe, Kharasaveiskoe, South Tambeiskoe, etc. in the northern part of the Yamal Peninsula and Geofizicheskoe, Gydanskoe, Salmanovskoe, Shtormovoe, etc. in the Gydan Peninisula (Ermilov et al., 2004). Three discoveries were made in the Kara Sea. Rusanovskoe and Leningradskoe gas-condensate fields that are unique by their reserves were discovered in the Soviet era in 1989 and 1990, respectively. A large oil-gas-condensate discovery known as Pobeda was made in 2013 by Rosneft by drilling of the discovery well at the Universitetskaya structure located close to the Novaya Zemlya Archipelago (Fig. 1). The principal hydrocarbon reserves of the study area occur in fields with anticlinal closures, in Jurassic and Cretaceous sandstone reservoirs.

1068-7971/$ - see front matter D 201 7 , 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.201 + 6.09.010

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Fig. 1. Schematic map of the Kara Sea–Yamal region. 1, administrative boundaries; 2, CDP reflection lines; 3, seismic lines and sections discussed in the text; 4, fields; 5, wells. Fields: 1, Pobeda; 2, Rusanovskoe; 3, Leningradskoe; 4, Malyginskoe; 5, Tasiiskoe; 6, Syadorskoe; 7, North Tambeiskoe; 8, Shtormovoe; 9, West Tambeiskoe; 10, Kharasaveiskoe; 11, South Tambeiskoe; 12, North Bovanenkovskoe; 3, Salmanovskoe; 14, Kruzenshtern; 15, East Bovanenkovskoe; 16, Verkhnetiuteiskoe; 17, West Seyakhskoe; 18, Bovanenkovskoe; 19, South Kruzenshtern; 20, Ladertoiskoe; 21, Nerstinskoe; 22, Gydanskoe; 23, Neitinskoe; 24, Baidaratskoe; 25, Geofizicheskoe; 26, Arkticheskoe; 27, Soletskoe + Khanaveiskoe; 28, East Bugornoe.

Seismic and geological characterization In the West Siberian petroleum province, the Paleozoic, Triassic, Jurassic, Neocomian (Berriasian–Lower Aptian) and Aptian–Albian–Cenomanian seismic megasequences (Kontorovich, 2009; Kontorovich et al., 2001, 2016), corresponding in their stratigraphic volume to the respective petroleum-bearing sedimentary megacomplexes (Kazarinov, 1958, 1963) have been traditionally recognized on seismic time sections. Areally extensive sequences of shales with anomalously low acoustic properties forming regional top seals of these megasequences can readily be tied to continuous and highly energetic reflectors that are be traced over much of the West Siberian basin (Table 1). The Turonian–Cenozoic megasequence forming the topmost part of the sedimentary fill comprises the Turonian–Maastrichtian and Cenozoic sedimentary complexes. Analysis of seismic time sections and deep drilling data reveal that the overall structural style of Paleozoic and Mesozoic–Cenozoic sedimentary sections in the continental part of northern West Siberia and in the southern Kara Sea is

similar. Since all seismic megasequences typical of West Siberia are widely developed on the Kara Sea shelf, this part of the study area represents the northern extension of the West Siberian basin. Figure 2 shows seismic time sections along two regional profiles, each extending 890 km across the Yamal and Gydan Peninsulas and southern Kara Sea. The Kara Sea–Yamal profile extends across the Tatrinov and West Matochkin uplifts in the Kara Sea, as well as the Kruzenshtern, Bovanenkovo, Middle Yamal, and Novoportovskoe uplifts located onshore. The Kara Sea–Gydan profile extends across the Kropotkinskoe and Central Kara structures within the South Kara Sea regional depression, as well as the Salmanovskoe and Gydanskoe fields associated with the dome-shaped structures on the Gydan Peninisula. The results of the integrated interpretation of well and seismic data were used to: 1. Create structure and isopach maps of the Mesozoic–Cenozoic seismic sequences at all stratigraphic levels. Since both

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Table 1. Stratification of reflecting marker horizons Reflector index Stratigraphic confinement (West Siberia) F

Basement top

A

Base of Triassic terrigenous deposits / top of Paleozoic platform deposits / top of Paleozoic megasequence

T

Base of Jurassic / top of Triassic / top of Triassic megasequence

B

Top of Bazhenov (Gol’chikha) Formation, Upper Jurassic, Volgian / top of Jurassic megasequence

M

Top of Koshai (Neite) member, Lower Cretaceous, lower Aptian / top of Neocomian (Berriasian–Lower Aptian) megasequence

G

Base of Kuznetsov Formation, Upper Cretaceous, top of Cenomanian / top of Aptian–Albian–Cenomanian megasequence

S

Top of Gan’kino Formation, top of Cretaceous / top of Turonian–Maastrichtian sequence

Jurassic and Cretaceous strata have the highest hydrocarbon potential in the study area, emphasis is placed on the geological structure of these stratigraphic levels (Fig. 3). 2. Create tectonic maps of the Jurassic and Cenomanian structural stages (Fig. 4) and paleotectonic maps of the top of the Jurassic (late Cenomanian), and to estimate trap presence and effectiveness based on the following parameters: trap area (S), vertical closure (A), and closed contour lines (L). Trap assessment was performed based on the classification of tectonic elements developed by the Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, using the traditional approaches discussed in the previous papers by F.G. Gurari, V.D. Nalivkin, I.I. Nesterov, M.Ya. Rudkevich, N.N. Rostovtsev and others, which allowed more detailed tectonic zonation and more precise delineation of structures of different orders (Kontorovich et al., 2001, 2004). Uplifts of III–IV orders reaching 25–2500 km2 in areas were considered as major potential anticlinal traps. Structure maps of the tops of the Jurassic, Neocomian, and Aptian–Albian–Cenomanian megasequences showing the boundaries of large depressions (megasyneclises) and depth contours of closed III–IV order positive structures regraded as potential anticlinal traps are presented in Fig. 3. Maps, present-day and paleoseismic sections provide the basis for tectonic analysis and structural interpretation and were used to illuminate the history of regional tectonic processes and predict the conditions of formation of anticlinal traps in the study area.

Structural and tectonic characterization Tectonically, the study area comprises the South Kara megasyneclise and the northern part of the Antipayuta–Tadebeyakha megasyneclise, as well as the Yamal–Gydan megasaddle between them and megamonoclines at their margins. The boundaries of the South Kara megasyneclise were delineated by contour lines in the structural relief of seismic markers. For example, the Yamal–Kara megasyneclise is enclosed by the –3600 m, –2600 m, and 1100 m depth contours at the tops of the Jurassic, Neocomian, and Cenomanian, respectively. The South Kara megasyneclise gradually broadens out upward to the northeast and south. It has an area of 83,130 km2 on the top of the Jurassic and 157,100 and

180,740 km2 on the top of the Neocomian and Cenomanian, respectively. In contrast, the Antipayuta–Tadebeyakha megasyneclise decreases in size with shallowing depth, gradually expanding southward toward the center of the the West Siberian basin. Variations in the geometry of the megasyneclise lead to changes in the boundaries of the Yamal–Gydan megasaddle. For example, because of the southward expansion of the South Kara megasyneclise, the Malygina, Pyasedai, Preobrazhenka and other uplifts delineated in the northern part of the Yamal–Kara megasaddle on the top of the Jurassic are localized in the southern part of the South Kara megasyneclise at the top of the Cenomanian (Figs. 3, 4). At the same time, the South Kara megasaddle also tends to expand southward, embracing Geofizicheskoe uplift and Tadebeyakha megadepression at the top of the Cenomanian. Yamal–Gydan megasaddle and northern part of the Antipayuta–Tadebeyakha megasyneclise In the continental part of the study area (geographically covering the area of Yamal and Gydan), the Nurma megaswell located in the western part of the Yamal–Gydan megasaddle is the largest closed positive structure (Figs. 3, 4). On the top of the Cenomanian, the Nurma megaswell is identified along a depth contour of –880 m and has an area of 10,845 km2 and a vertical closure of 390 m. The structure is complicated by two III-order positive tectonic elements (Bovanenkovo mesouplift and Arkticheskii mesoswell). The Bovanenkovo mesouplift located in the northern part of the Nurma megaswell has an area of 6250 km2 and a vertical closure of 355 m and is complicated by the Bovanenkovo dome-shaped uplift, Kruzenshtern and Kharasavei local structures. The Arkticheskii mesoswell located in the northern part of the Nurma megaswell has an area of 2650 km2 and vertical closure of 315 m. It is complicated by the Neite dome-shaped uplift and Arkticheskii local uplift. Therefore, five III–IV-order structures (Kharasavei, Kruzenshtern, Bovanenkovo, Neite, and Arkticheskii) are mapped at the top of the Cenomanian within the Nurma megaswell, all five being host to significant hydrocarbon fields of the same name. The Nurma megaswell is reliably traced at the top of the Jurassic where it becomes twice as small having an area of

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Fig. 2. Seismic sections along regional composite seismic profiles Kara Sea–Yamal (A), Kara Sea–Gydan (B). Red vertical lines indicate faults.

4375 km2. The megaswell is delineated by the –2950 m depth contour and has a vertical closure of 595 m, being complicated by the Bovanenkovo dome-shaped mesouplift and Arkticheskii swell. At the top of the Jurassic, the Bovanenkovo mesouplift that embraces the Bovanenkovo, South Kruzenshtern, Kruzenshtern, and East Bovanenkovo local uplifts has an area of 2385 km2 and a vertical closure of 545 m. The Arkticheskii swell located in the southern part of the Nurma megaswell is not complicated by any local structure and has an area of 1195 km2 and a vertical closure of 455 m. Two dome-shaped uplifts (Kharasavei and Arkticheskii) are mapped on the top of the Jurassic to the north and south of the Nurma megaswell. Several relatively large positive structures hosting hydrocarbon accumulations are mapped to the northeast of the Nurma megaswell, in the central and western parts of the Yamal–Gydan megasaddle. The North Tambei mesoswell and

South Tambei dome-shaped mesouplift are delineated at the top of the Jurassic in the central part of the megasaddle. The North Tambei mesoswell, trending northeastward, is complicated by the West Tambei and North Tambei dome-shaped uplifts. The structure is identified along the –3500 m depth contour and has an area of 3515 km2 and a vertical closure of 237 m. The South Tambei uplift with a general isometric shape is traced along the –3500 m depth contour and has an area of 2539 km2 and a vertical closure of 223 m. At the top of the Cenomanian, the North Tambei and South Tambei uplifts are traced along the depth contour of 970 m a single closed II-order positive structure, Tambei dome-shaped mesouplift, with an area of 3435 km2 and a vertical closure of 55 m. Just north of these structures, the Upper Tambei mesotrough, trending to the northeast, is bound to the north by the Malygina swell. This northeastward-trending swell follows the –3530 m depth contour at the top of the Jurassic

Fig. 3. Structure maps of the top of the Jurassic (A), Neocomian (B), and Aptian–Albian–Cenomanian (C) megasequences. 1, megasequence boundaries; 2, megasyneclise boundaries; 3, III–IV-order uplifts. I, South Kara megasyneclise; II, Antipayuta–Tadebeyakha megasyneclise; III, Yamal–Gydan megasaddle.

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Fig. 4. tectonic maps of the Jurassic (A) and Aptian-Cenomanian (B) megasequences in the Arctic regions of the West Siberian petroleum basin. 1, basin boundaries; 2, shoreline; positive structures: 3, I-order, 4, II-order, 5, III-order, 6, IV-order; negative structures: 7, superorder, 8, 0-order, 9, I-order, 10, II-order, 11, III-order; intermediate structures: 12, megasaddles, 13, monoclises of the Plate interior, 14, monoclises of the Transitional zone and Outer belt. TECTONIC ELEMENTS OF JURASSIC MEGASEQUENCE: Superorder structures: A, South Kara megasyneclise; B, Antipayuta–Tadebeyakha megasyneclise; C, Yenisei-Khatanga megathrough. 0-order structures: I, South Kara syneclise; II, Yamal–Gydan megasaddle; III, Northern megasaddle. I-order positive strcutures: I, Kropotkinskii megasalient; II, Rogozinskii megasalient; III, North Taimyr megasalient; IV, Neupokoeva megasalient; V, Pai-Khoi megasalient; VI, North Gydan megasalient; VII, Nurma megaswell; VIII, Geofizicheskii megaswell. I-order negative structures: I, Siberian Threshold; II, East Kara megadepression; III, West Kara megadepression; IV, North Gydan megadepression; V, West Nurma inclined megatrough; VI, Tadebeyakha megadepression; VII, Yaptiksale megadepression; VIII, Belov megadepression. II-order positive structures: 1, North Tambei mesoswell; 2, North Gydan dome-shaped mesouplift; 3, South Tambei dome-shaped mesouplift; 4, Bovanenkovo dome-shaped mesouplift. II-order negative structures: 1, North Siberian Threshold mesodepression; 2, South Siberian Threshold mesodepression; 3, West Rogozinskii mesoincision; 4, Osevaya mesodepression; 5, Upper Tambei mesotrough; 6, Drovyanoi mesotrough; 7, Nyabyyakha mesodepression; 8, Murtyyakha mesotrough; 9, Upper Saveiyakha mesodepression; 10, Parisentovskaya mesodepression; 11, South Geofizicheskaya mesodepression; 12, Sydai mesodepression; 13, South Belov mesodepression. TECTONIC ELEMENTS OF THE APTIAN–ALBIAN–CENOMANIAN MEGASEQUENCE: Superorder structures: A, South Kara megasyneclise; B, Yamal–Gydan megasaddle; C, Yenisei–Khatanga megatrench. 0-order structures: South Kara syneclise. I-order positive structures: I, Leningradskoe–Rusanovskoe megasalient; II, Pyaseidai–Malygina megasalient; III, Pai-Khoi megasalient; IV, Nurma megaswell. I-order negative structures: I, Novaya Zemlya megadepression; II, West Nurma megatrough; III, Tadebeyakha megadepression; IV, Yaptiksale megatrough. II-order positive structrues: 1, North Yamal mesosalient; 2, Tambei dome-shaped mesouplift; 3, Bovanenkovo dome-shaped mesouplift; 4, Neite dome-shaped mesouplift. II-order negative structures: 1, Northern mesodepression; 2, Southern mesodepression; 3, South Kara mesodepression; 4, Upper Tambei mesotrough; 5, Murtyyakha mesodepression; 6, South Geofizicheskaya mesodepression. UPLIFTS of III–IV orders: 1, Vlas’ev; 2, Nansen; 3, Rogozinskoe; 4, Universitetskoe; 5, Kop’ev; 6, Vikulov; 7, West Shchitovoe; 8, Shchitovoe; 9, Tatarinov; 10, North Taimyr; 11, Matysevicha; 12, Kropotkinskoe; 13, West Kropotkinskoe; 14, Taimyr; 15, Vilkitskogo; 16, South Kropotkinskoe; 17, Central Kara; 18, Rozhdestvenskoe; 19, Beloe; 20, Rusanovskoe; 21, Esipov; 22, Yarnatuyakha; 23, North Skuratov; 24, Shubertovskoe; 25, Polyarnoe; 26, Mininskoe; 27, South Rusanovskoe-1; 28, Skuratov; 29, North Leningradskoe; 30, South Rusanovskoe-2; 31, Neupokoeva; 32, North Obruchev; 33, West Matochkin; 34, Nyarma; 35, Sidyananguevayakha; 36, Khalyanga; 37, Drovyanoe; 38, Obruchev; 39, Leningradskoe; 40, West Malygina; 41, Preobrazhenka; 42, Tarma; 43, Malygina; 44, South Preobrazhenka; 45, Shmidta; 46, Tasiiskoe; 47, Pyasedai; 48, North Kharasavei; 49, Mokhovoe; 50, Shtormovoe; 51, North Tambei; 52, West Tambei; 53, Kharasavei; 54, East Zelenyi Mys; 55, West Khariusnoe; 56, Khortyyakha; 57, Malo-Tambei; 58, East Kharasavei; 59, Ernikovoe; 60, Pukhutsyayakha; 61, South Yavai; 62, Amderma; 63, South Sharapov; 64, Sabettayakha; 65, South Tambei; 66a, North Kruzenshtern; 66b, Central Kruzenshtern; 66c, Kruzenshtern; 66d, South Kruzenshtern; 67, West Kharasavei; 68, North Baidaratskoe; 69, West Seyakha; 70, Utrennee; 71, Tomboito; 72, Khondeyakha; 73, Verkhnetiutei; 74, Khus’yakha; 75, West Kruzenshtern; 76, Khanebchetoi; 77a, Bovanenkovo; 77b, North Bovanenkovo; 78, North Seyakha; 79, Seryakha; 80, Gol’tsovoe; 81, Malo-Gydan; 82, Gydan; 83, Verkhneyasaveyakha; 84, Ostromysovskoe; 85, Neite; 86, Nilivoiyakha; 87a, Nadokhoyakha; 87b, East Nadokhoyakha; 88, Seyakha; 89, Ventoi; 90, East Neite; 91, Novolunnoe; 92, Bystritsa; 93, Amposyakha; 94, Nerutoyachskoe; 95, Baidaratskoe; 96, Salpadayakha; 97, Yasavei; 98, Central Geofizicheskoe; 99, East Yungiyakha; 100, Arkticheskoe; 101, Yungiyakha; 102, Laduket; 103, South Gydan; 104, Khanavei; 105, Trekhbugornoe; 106, Nerosedayakha; 107, Pagodskoe; 108, Tyngevapaetayakha.

and has an area of 1450 km2 a vertical closure of 142 m. At the top of the Cenomanian, the Malygina swell shifts to the northeast without any change in its overall configuration. This swell is delineated by the 1100 m depth contour and has an area of 1405 km2 and a vertical closure of 67 m. At the top of the Aptian–Albian–Cenomanian megasequence, the swell extends outside the Yamal–Kara megasaddle and forms part of the Pyasedai–Malygina megasalient, a semiclosed I-order positive structure, which is located in the southern part of the South Kara megasyneclise. The Pekseda dome-shaped uplift, with an area of 1331 km2 and a vertical closure of 95 m is mapped at the top of the Jurassic, southeast of the Tambei group structures and disappears at the top of the Aptian–Albian–Cenomanian megasequence. A similar situation is observed for the Shtormovoe and North Gydan uplifts located to the northeast. At the top of the Jurassic, the North Gydan mesouplift, Shtormovoe dome-shaped uplift, and the more northerly located Preobrazhenka and South Preobrazhenka uplifts are mapped collectively as the North Gydan megasalient, a semiclosed I-order tectonic element. Unlike the North Gydan

and Shtormovoe structures, the Preobrazhenka and South Preobrazhenka uplifts are readily identified at the top of the Cenomanian where they appear as a single semiclosed II-order positive structure, North Gydan mesosalient, complicating the southern part of the South Kara megasyneclise. At the top of the Jurassic, the North Gydan megasalient is bound to the east by the Drovyanoi mesotrough, to the west by the North Gydan megadepression, a I-order negative structure, having an area of 6213 km2 and a vertical closure of 214 m. Farther to the north, the Neupokoeva megasalient, an E–W-trending semiclosed I-order positive structure, is complicated by a dome-shaped uplift of the same name and the Khalyanga local dome-shaped uplift. Both the Neupokoeva megasalient and North Gydan megadepression are not mapped at the top of the Cenomanian. The Geofizicheskoe prospect, with an oil-gas-condensate discovery of the same name, is located in the southern part of the study area, which corresponds to the northern part of the Antipayuta–Tadebeyakha megasyneclise on the top of the Jurassic and southern part of the Yamal–Gydan megasaddle on the top of the Cenomanian. The Geofizicheskii megaswell,

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a large, E–W-trending positive structure, with an area of 6100 km2 and a vertical closure of 610 m is identified within the same area at the top of the Jurassic. This megaswell is complicated by a dome-shaped uplift of the same name and the Trekhbugornyi local uplift. Geofizicheskoe dome-shaped uplift complicated by local highs is mapped at the top of the Cenomanian and has an area of 853 km2 and a vertical closure of 60 m. South Kara regional depression The South Kara regional depression is a large structure, which includes a megasyneclise of the same name, bound to the west, north, and east by the Pai-Khoi, Novaya Zemlya, and Taimyr megamonoclises, respectively. The South Kara regional depression geographically covers the offshore Kara Sea regions located south of the Novaya Zemlya Archipelago and Taimyr Peninsula. To the west, north, and east, the depression is bounded by its bordering structures, such as the Yugor Peninsula, Vaigach Island, the Novaya Zemlya Archipelago, the Siberian Threshold, and the Taimyr Peninsula. In the present-day relief of the top of the Jurassic, the absolute depths greater than 4500 m are mapped in the southern part of the Yamal–Kara regional depression close to continent (Figs. 3, 4). Here, the South Kara megasyneclise, the largest closed depression, is delineated by the –3550 m depth contour, with an area of 83,130 km2 and a vertical closure of 900 m. The steep southern flank of the South Kara regional depression is complicated by a series of local structures at the top of the Jurassic. For example, in the megamonoclines that are present in the north, west, and east, the surfaces of all Mesozoic regional markers are deflected upward toward adjoining structures. The northern, western and eastern flanks become more gentle at the top of the Jurassic and are complicated by terraces with which a series of III–IV-order positive structures are associated. The South Kara megasyneclise is complicated by two large closed depressions that are delineated by the –3830 m depth contour. The first depression located in the southwestern part of the megasyneclise corresponds n its size to the 0-order structures and is recognized as the South Kara syneclise. This depression has an area of 34,375 km2 and is complicated by the West Kara megadepression. The East Kara megadepression having an area of 6165 km2 is mapped in the eastern part of the megasyneclise. In the northeastern part of the South Kara regional depression, the Siberian Threshold megadepression is identified along a depth contour of –3530 m and has an area of 6165 km2 and a vertical closure of 282 m. The Siberian Threshold megadepression striking to the northwest is complicated by two II-order negative structures, each delineated by the –3520 and –3550 m depth contours and having areas of 2305 and 8385 km2 and vertical closures of 260 and 420 m. On the top of the Jurassic, the South Kara megasyneclise and the Siberian Threshold megadepression are separated by a large intermediate structure, the Northern megasaddle, which

has an area of 32,800 km2. The Northern megasaddle is complicated by three semiclosed I-order positive structures (Kropotkinskoe, Rogozinskoe, and North Taimyr megasalients), which in turn host a number of smaller structures of III–IV orders. Three III-order structures (North Skuratov and Pyasedai dome-shaped uplifts and Nyarma swell) and twelve local IV-order uplifts are mapped directly within the South Kara megasyneclise, whereas the remaining closed positive structures are identified further north in the Northern megasaddle and within terraces opening toward the Novaya Zemlya Archipelago and Siberian Threshold. For example, the Universitetskoe uplift located close to Novaya Zemlya is host to the Pobeda field. It was noted that two large discoveries in the central part of the South Kara regional depression (Leningradskoe and Rusanovskoe fields) are controlled by anticlinal traps with Cretaceous reservoirs. At the top of the Jurassic, the Rusanovskoe dome-shaped uplift located north of the South Kara syneclise is delineated along the –3420 m depth contour and has an area of 339 km2 and a vertical closure of 65 m. However, this uplift disappears on the top of the Jurassic farther south, within the Leningradskaya prospect and the field corresponds in plan to the northern monocline-shaped flank of the depression. Variations in structural geometry of the South Kara regional depression are observed at the top of the Cenomanian where the South Kara megasyneclise doubles in size, reaching an area of 180,740 km2, and is delineated by the –980 m depth contour with a vertical closure of 610 m. In the south, the South Kara megasyneclise adjoins the Nurma megaswell and Tambei mesoswell and embraces a series of positive structures (Malygina swell, Preobrazhenka, and South Preobrazhenka dome-shaped uplifts, etc.), which were mapped within the Yamal–Gydan megasaddle in the structural relief on top of the Jurassic. Similar to the situation observed at the top of the Jurassic, the South Kara megasyneclise broadens to the northeast at the top of the Cenomanian and covers the area occupied by the Northern megasaddle and Siberian Threshold depression. At the same time, the Northern megasaddle disappears on the top of the Cenomanian, whereas the deepest part of the South Kara megasyneclise appears to be deflected to the northeast relative to the top of the Jurassic, where a syneclise of the same name complicated by the Novaya Zemlya megadepression is identified. At the top of the Cenomanian, the South Kara syneclise is traced along the –1250 m depth contour and has an area of 78,957 km2 and a vertical closure of 340 m. It is complicated by one I-order negative structure and a series of II–IV-order depressions. The Novaya Zemlya megadepression follows the –1380 m depth contour and has an area of 37,000 km2 and a vertical closure of 210 m. It is complicated by two mesodepressions identified along the –1460 m depth contour, which do not coincide in plan with the Jurassic depressions. The Northern mesodepression has an area of 7180 km2 and a vertical closure

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of 130 m while the Southern mesodepression is 5610 km2 in area with a vertical closure of 70 m. It should be noted that the deepest part of the Cenomanian depression does not coincide in plant neither with the South Kara syneclise or with the Siberian Threshold megadepression, which formed the deepest zones on the top of the Jurassic, but is mapped directly above the relatively shallow Northern megasaddle separating these depressions. Three III-order positive structures (West Kropotkinskoe, Kropotkinskoe, and Rozhdestvenskoe dome-shaped uplifts) and five local uplifts (West Shchitovoe, Shchitovoe, Matusevicha, South Kropotkinskoe, and Beloe) are delineated at the top of the Cenomanian within the Novaya Zemlya megadepression. Two more relatively large depressions are mapped in the south and west of the South Kara syneclise. The East Voronin depression with an area of 1287 km2 and a vertical closure of 65 m is traced along the –1320 m depth contour in the west. Another mesodepression (South Kara) located in the south has an area of 4325 km2 and a vertical closure of 85 m and is delineated by the –1260 m depth contour. Unlike the situation observed at the top of the Jurassic, at the top of the Cenomanian the South Kara megasyneclise shows a steeper northern flank and a gentle southern flank with numerous positive structures of III–IV orders. The southwestern flank of the depression is complicated by four III-order positive structures (Rusanovskoe, North Leningradskoe, Leningradskoe, and West Matochkin dome-shaped uplifts) and two local uplifts (South Rusanovskoe-1 and South Rusanovskoe-2). All these structures are mapped at the top of the Cenomanian as a single semiclosed I-order positive structure, Leningradskoe–Rusanovskoe megasalient, which is bound from the north, west, and east by the –1250 m depth contour and has an area of 9937 km2 and a vertical closure of 170 m. It should be noted that the Rusanonskaya prospect with the Leningradskaya prospect within it that were mapped north of the South Kara megasyneclise at the top of the Jurassic are now localized at the top of the Cenomanian southwest of the deepest part of the area. The Skuratov, North Skuratov, and Nyarma dome-shaped uplifts as well as the West Malygina and Tarma local uplifts are traced to the east of the Leningradskoe–Rusanovskoe megasalient, whereas the Pai-Khoi megasalient is mapped to the southeast. This megasalient has an area of 8403 km2 at the top of the Cenomanian and is complicated by the Amderma dome-shaped uplift and North Baidaratskaya local uplift. At the top of the Jurassic, the Pai-Khoi megasalient displays a similar structural geometry but no complicating positive structures. At the top of the Aptian–Albian–Cenomanian megasequence, the northern and northwestern flanks of the South Kara regional depression within the Novaya Zemlya megamonocline are complicated by the Vlas’ev, Universitetskoe, Vikulov, Tatarinov, and Obruchev dome-shaped uplifts, East Roz’ev, Esipov, and Minin local uplifts. The Universitetskoe and Vikulov structures are the largest ones. The Universitetskoe dome-shaped uplift extending northward is mapped

351

within the plate interior. It follows the –800 m depth contour and has an area of 566 km2 and a vertical closure of 127 m at the top of the Cenomanian. At the top of the Jurassic, this uplift is delineated by the –1890 m depth contour and has a slightly larger area of 913 km2 and a vertical closure of 305 m. In contrast, the area of the Vikulov uplift increases upward in the section. At the top of the Jurassic, this uplift along the –1370 m depth contour has an area of 222 km2 and a vertical closure of 84 m. At the top of the Cenomanian, this structure is delineated along a depth contour of –530 m. Its area increases approximately threefold reaching 722 m and its vertical closure also increases to 164 m. The structural and tectonic analysis shows that a total of 113 positive structures of III–IV orders have been identified in the study area, including 68 uplifts in the continental part and 45 uplifts in the southern part of the Kara Sea. Regional geological model and regional tectonic processes Paleozoic. In the South Kara regional depression, several regionally prominent seismic horizons indicating a layered, platform character of Paleozoic deposits can be distinguished in the seismic time sections below reflector A, which are correlated to the top of the Paleozoic. The largest blocks of the Paleozoic platform deposits (up to 5–7 km thick) are mapped in the west and east of the South Kara regional depression (Fig. 5). The West Kara and East Kara macroblocks are separated by the Central Kara basement inlier composed of strongly faulted rocks represented by chaotic reflections. The somewhat similar faulted block is present at the base of the Jurassic section over much of the western part of the Yamal–Kara megasaddle, which corresponds geographically to the Yamal Peninsula. Over much of the Gydan Peninsula, the Paleozoic complex is represented by a thick sequence of weakly deformed platform deposits. The Paleozoic platform blocks and Central Kara basement inlier, bounded by major faults, are strongly faulted along local highs. Analysis of seismic data shows that most of the hypsometrically closed positive structures identified in the present-day Mesozoic reflectors appear to have formed directly above these basement highs, both onshore and offshore. Triassic and Jurassic. Analysis of seismic cross sections and isopach maps of Triassic–Jurassic rocks reveals that during these stages the South Kara megasyneclise began to subside relative to the neighboring areas and Yamal–Kara megasaddle (Figs. 2, 5, 6A). In the west of the Yamal–Gydan megasaddle, at the Bovanenkovo, Novoportovskaya and other prospects, the Paleozoic units are overlain unconformably by Jurassic deposits while the Triassic units are missing in this section. At the same time, the Triassic units, which may be as thick as 2000 m in the deepest parts of the depression, are found over much of the Yamal–Kara regional depression. At the margins of the South Kara sub-basin, the Triassic units either gradually thin or completely pinch out into Paleozoic inliers within the Novaya Zemlya and Siberian

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Fig. 5. Geoseismic sections along profiles Reg_13+707+R14 (A), Reg_18 (B).

Threshold (Figs. 3, 5). A similar situation is observed where Triassic deposits pinch out into the uplifted Yamal–Gydan macroblock of Paleozoic age (Fig. 3). Tectonic processes that were active within the Yamal–Kara regional depression in the Triassic continued to operate during the Jurassic, causing further subsidence. As a result, the Jurassic strata range in thickness from 1500–1600 m in the continental part of the study area within the Yamal–Gydan megasaddle to 2500 m in the deepest part of the South Kara regional depression. During Triassic and Jurassic times, the most intense subsidence and sediment accumulation took place in areas in the west of the South Kara depression, which coincide in plan

with the deepest part of the South Kara megadepression in the present-day relief on top of the Jurassic, and in the north where the greatest thickness of the Triassic and Jurassic sediments is mapped along the Novaya Zemlya Archipelago (Fig. 3). Analysis of geological and geophysical data from the southern and central parts of West Siberia suggests that during the early Mesozoic this area was characterized by highly contrasting relief and a pinching-out of the basal layers of the sedimentary cover onto pre-Mesozoic erosional inliers that the basal layers tend to directly onlap onto the top of uplifted basement blocks. However, a different situation is observed in the South Kara regional depression where the Jurassic and Triassic strata thin

Fig. 6. Thickness maps of Triassic–Jurassic (A), Berriasian–Lower Aptian (B), and Aptian–Albian–Cenomanian (C) deposits.

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rapidly, not due to the pinching out of basal layers, but rather due to the overall decrease in the thickness of individual sequences comprising the Triassic and Jurassic complexes. The thickness of Triassic–Jurassic complexes decreases gradually toward the uplifted basement blocks, but the basal sedimentary layers of low thickness are still present in the section. This suggests that subsidence of the South Kara subbasin was generally a continuous, gradual process during Triassic and Jurassic times. Neocomian. In the West Siberian petroleum province, the Neocomian complex has a clinoform geometry and largely represents deposition in an undercompensated center of the Volgian subsiding basin (Naumov et al., 1977; Nezhdanov, 1988). Throughout this period, both the Siberian platform and southeastern margin of the West Siberian plate remained tectonically elevated relative to the Urals and served as the main source of terrigenous material to the West Siberian sedimentary basin. Therefore, Neocomian clinoforms dip regionally to the northwest over much of West Siberia. In the South Kara regional depression, the Neocomian complexes also have a clinoform geometry. They are mapped on seismic time sections as oblique reflections prograding toward the B surface. At the same time, during the Neocomian the South Kara depression received terrigenous material from the Novaya Zemlya Archipelago in the northeast of the Siberian Threshold. The E–W-seismic sections show clinoforms dipping either westward or eastward, whereas the N–S sections displays southward-dipping oblique reflections that appear to be prograded clinoforms. Analysis of time sections across the continental part and in the offshore area reveals that the boundary at which both northward- and southward-dipping clinoforms converge is located near the shoreline. From the structural-tectonic point of view, two important processes were active in the study area during the Berriasian– Early Aptian. 1. Areas of intense subsidence migrated basinward away from the bordering structures (e.g., Novaya Zemlya and Siberian Threshold) to the Yamal–Kara megasaddle. Initially sediment supply to the South Kara regional depression was from the north indicating that both the Novaya Zemlya and Siberian Threshold have been continuously growing during the Berriasian–Hauterivian. 2. During the Neocomian, subsidence in the South Kara regional depression occurred in two areas: the western and eastern depressions, corresponding in plan to the deepest part of the present-day South Kara syneclise and the present-day Siberian Threshold megadepression, respectively. Although both depressions continued to be an area of subsidence, there was a tendency for the relative uplift of the Northern megasaddle, which separated these depressions (Fig. 6). Analysis of an isopach map of Neocomian sediments shows that the Berriasian–Lower Aptian sediments deposited in the Northern megasaddle are 400 m thinner than in the adjacent paleodepressions and 200 m thinner than in the Yamal–Gydan megasaddle, which also began to subside during this period. Note that in the present-day relief of the top of the Neocomian

and Jurassic the Northern megasaddle extends 300 m deeper than the structures within the Yamal–Gydan megasaddle. The evolution of the Northern megasaddle is clearly visible in both present-day E-W seismic sections and paleosections across the South Kara regional depression (Fig. 7). The Neocomian areas of subsidence within the South Kara regional depression have deep roots because they appear to have formed above the West Kara and East Kara Paleozoic macroblocks represented by a thick succession of platform deposits; the Northern megasaddle corresponds in plan to the Central Kara basement inlier consisting of highly faulted and folded Paleozoic rocks (Figs. 5, 7). Aptian–Albian–Cenomanian. Tectonic processes operating in the Neocomian became greatly diminished during the Aptian–Albian–Cenomanian (Fig. 6C). The South Kara megasyneclise continued to develop at this time in the structural relief on top of the Jurassic and Neocomian and expanded considerably toward the south, covering much of the Yamal–Gydan megasaddle, which remained hypsometrically below the Northern megasaddle and formed a part of the Yamal–Kara depression. Post-Cenomanian tectonic processes are documented on the present-day reflector G structure contour map (Fig. 3C). Post-Cenomanian times were marked by a transition from subsidence to uplift of the Yamal–Gydan megasaddle where a series of highly contrasting positive structures (Tambei, South Tambei uplifts, etc.) were formed. This period was also marked by the formation of the Nurma megaswell and local uplifts (Bovanenkovo, Kharasavei, Arkticheskoe, etc.). The post-Cenomanian uplift of the Yamal–Gydan megasaddle did not accommodate regional subsidence of this area that occurred in the Neocomian and Aptian–Albian–Cenomanian. As a result, the South Kara megasyneclise is considerably larger in area than the Jurassic depression in the structural relief on top of the Cenomanian and embraces some of the structures that formed within the Jurassic Yamal–Gydan megasaddle (Malygina, Preobrazhenka uplifts, etc.). By the end of the Late Cretaceous–Cenozoic, the center of subsidence within the Yamal–Kara regional depression migrated northeastward to areas underlain by the Northern megasaddle, where the Novaya Zemlya megadepression and two mesodepressions (Northern and Southern) began to develop in the structural relief on top of the Cenomanian. In post-Cenomanian times, the Leningradskoe–Rusanovskoe megasalient and a series of III–IV-order uplifts (Leningradskoe, Rusanovskoe, North Leningradskoe, etc.) began to form in the southwestern part of the South Kara regional depression. The reconstructions of regional tectonic movements show that, by analogy with the West Siberian basin, tectonic subsidence accelerated in Mesozoic–Cenozoic time through the study area. At the same time, our model does not allow for total inheritance of tectonic movements. Since areas of alternating subsidence and uplift were located in different parts of the region in different time periods, it can be concluded that the present tectonic structure of the whole region and differences in tectonic structures of individual sedimentary

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Fig. 7. Present-day seismic section (A) and paleoseismic section flattened on reflector M (Koshai Member, Lower Cretaceous, Aptian) (B) along a regional composite profile Sint_WE.

megasequences are associated with differential tectonic movements.

The evolution of structures-anticlinal traps for hydrocarbon accumulations The major oil and gas resources in the West Siberian petroleum province are associated with Jurassic and Cretaceous deposits and are primarily controlled by anticlinal traps, representing positive structures of III–IV orders, with areas not exceeding 2500 km2. A similar situation is observed in the study area where all hydrocarbon accumulations are

localized in many local dome-shaped uplifts and swells, despite the presence of I- and II-order positive structures in the relief of the Jurassic and Cretaceous horizons. Considering that the principal prospectivity in the study area consists of structures mapped at tops of Jurassic to Cretaceous stratigraphic levels, the below discussion will focus mainly on post-Jurassic tectonic activity. As noted above, the goal of this study was to create a set of tectonic maps for the tops of Jurassic and Aptian–Albian–Cenomanian megasequences, a paleotectonic map for the Turonian (Jurassic) and to estimate the areas and vertical closures of all closed III–IV-order positive structures. The results of trap evaluation suggest that the formation of Jurassic and Cenomanian

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structures was controlled by post-Jurassic and post-Cenomanian tectonic processes, respectively. The parameters of traps delineated on the paleotectonic map reflect the contribution of Berriasian–Cenomanian tectonic processes. Types of anticlinal structures The principal prospectivity of uplifts associated with different sedimentary complexes was shown to be dependent on different mechanism, along with a lithologic factor, involved in the formation of anticlinal traps in the West Siberian petroleum province. Gerasimov-type structures (type I) are structures that formed above relatively monolithic erosional inliers of basement. The uplift of these inliers continued from the Jurassic to and Early Cretaceous and ceased in the Late Cretaceous and Cenozoic. Type I uplifts that become more subtle upward in the section are mapped on top of the Jurassic and Neocomian and disappear in the structural relief of the Albian-Cenomanian horizons. Bovanenkovo-type structures (type II) are confined to erosional basement inliers that display a tendency for uplift throughout the Mesozoic–Cenozoic history. Type II uplifts are mapped in the structural relief of all Mesozoic horizons and their hydrocarbon potential is estimated to be associated with all prospective sedimentary sequences (Jurassic, Neocomian, and Aptian–Albian–Cenomanian). They are characterized by either decrease or increase in their areas and vertical closures from bottom upward. Vankor-type structures (type III) are confined to basement inliers (in some cases, intra-Paleozoic, not subcropping below the sedimentary cover), which underwent uplift during the Triassic–Early Cretaceous and in post-Cenomanian times (Kontorovich and Kontorovich, 2005). Medvezh’e-type structures (type IV) are classified as rootless uplifts, i.e., not associated with large basement inliers, which formed during post-Cenomanian times as a result of Cenozoic tectonic movements (Kontorovich et al., 2016). Type III and IV uplifts, which are readily mapped at the top of the Cenomanian, may either appear or disappear in the structural relief of the underlying horizons depending on the intensity of post-Cenomanian tectonic movements and paleostructural settings. Such uplifts are characterized by an increase in their areas and vertical closures upward the section. Onshore structures The Nurma megaswell located in the western part of the Yamal–Gydan megasaddle is confined to a large basement inlier divided into several blocks by faulting. The confinement of the Nurma megaswell to a vertically uplifted basement block is the result of the inherited tectonic development of this megaswell and associated uplifts through the Mesozoic and Cenozoic. In their present-day configuration, the Nurma megaswell and associated uplifts appear to exist in the Aptian relief of the Bazhenov Formation. In the early Aptian, the

megaswell has a vertical closure of about 200 m at the top of the Jurassic before deposition of the Neite member. As noted above, in the Aptian–Albian–Cenomanian, the whole study area forming a part of the Plate interior began to subside relative to its bordering structures. During this period the South Kara megasyneclise and Yamal–Kara megasaddle had been an area of subsidence (Fig. 6C), whereas some local structures complicating the Nurma megaswell (e.g., Arkticheskoe uplift) continued to grow. However, reactivation of the tectonic processed through the Aptian–Albian–Cenomanian is thought to have caused a considerable decrease in the vertical closure of Nurma megaswell such that during the Turonian the vertical closure of the swell was 160 m at the top of the Jurassic. The formation of the Nurma megaswell and associated uplifts (Bovanenkovo, Kruzenshtern, Arkticheskoe, etc.) was profoundly influenced by post-Cenomanian (mainly Cenozoic) tectonic processes. For example, the vertical closure of the Nurma megaswell in the present-day relief of the Jurassic is more than 3.5 times that of the Turonian (595 m vs. 160 m). Figure 8A shows a paleosection flattened on reflector G and a present-day time section across the Bovanenkovo and Kruzenshtern uplifts. Comparison of these data allows us to conclude that these structures associated with basement inliers became a major area of uplift during Jurassic and Berriasian– Early Aptian times, and especially at the end of the Late Cretaceous and Cenozoic. A series of structures located east of the Nurma megaswell exhibit different trends in vertical movements that took place in the Mesozoic and Cenozoic. Most structures (e.g., Tambei group structures, Malygina swell, Preobrazhenka, Geofizicheskoe, South Preobrazhenka and other uplifts) belonging to the Bovanenkovo type are thought to have formed by the same mechanisms as the local uplifts within the Nurma megaswell, i.e., they formed during the Jurassic and Berriasian–Cenomanian and continued to develop in the late Late Cretaceous and Cenozoic. At the same time, some larger structures (East Zelenyi Mys, Mokhovoe, Shtormovoe, South Gydan, and Novolunnoe dome-shaped uplifts) formed by the Turonian, suggesting that the major uplift has slowed or ceased in post-Cenomanian time. Fourteen local uplifts classified as Gerasimov-type structures were mapped within the Yamal–Gydan megasaddle. Eighteen Medvezh’e-type uplifts, which are present at the top of the Cenomanian and absent at the top of the Jurassic were identified in the continental part of the study area. These include two III-order structures, namely, the Khanebchetoi and Khortych dome-shaped uplifts with areas of 248 and 229 km2, and sixteen local uplifts. Kara Sea structures (South Kara regional depression) Vankor- and Bovanenkovo-type structures. As noted above, the Leningradskoe and Rusanovskoe dome-shaped uplifts, acting as traps for unique gas reserves, are mapped at the top of the Cenomanian within the Leningradskoe– Rusanovskoe megasalient, which is also complicated by the

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Fig. 8. Present-day and paleoseismic sections flattened on reflector G along profiles through the Kruzenshtern, Bovanenkovo, Leningradskoe, North Leningradskoe, Universitetskoe, and Vikulov dome-shaped uplifts.

North Leningradskoe and West Matochkin dome-shaped uplifts. The parameters of these structures are shown in Table 2. Analysis of geological and geophysical data indicates that although all structures of the Leningradskoe–Rusanovskoe megasalient are confined to basement inliers, they are not visible on the Berriasian–Cenomanian isopach map, i.e., they have not yet appeared by the Turonian in the paleorelief at top of the Jurassic and Neocomian, but are thought to have formed entirely by post-Cenomanian tectonic movements. The Rusanovskoe and West Matochkin uplifts are delineated in the present-day structural relief of all Mesozoic reflectors. The areas of the Rusanovskoe and West Matochkin uplifts at the top of the Cenomanian are 3.2 and 4.6 times that of the Jurassic, respectively. Their vertical closures may either decrease from 65 to 42 m (Rusanovskoe uplift) or increase from 33 to 45 m (West Matochkin uplift) upward in the section.

The conclusion that these structures were formed in the relief of the Mesozoic reflectors during post-Cenomanian times is supported by the results of paleoreconstructions using restored seismic sections. Figure 8B shows the present-day seismic section and paleoseismic section flattened on reflector G (Kuznetsov Formation, Turonian), along the profile across the Leningradskoe and North Leningradskoe uplifts. In the present-day relief of the top of the Jurassic and in the Turonian paleorelief at the top of the Jurassic megasequence, both Leningradskoe and North Leningradskoe uplifts are mapped within a monocline dipping to the south toward the center of the South Kara megadepression and disappear along reflector B in both sections. In the present-day seismic section, these uplifts having even greater vertical closures upward in the section are clearly visible on reflectors M and G, which are not tilted by the regional subsidence.

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Table 2. Parameters of structures within the Leningradskoe–Rusanovskoe megasalient Uplift

Top of Jurassic

Order

S

Top of Neocomian

A

L

Order

S

A

Top of Cenomanian

L

Order

S

A

Thickness of the Berriasian– Cenomanian sediments L

Order

S

A

L

Rusanovskoe

III

339

65

3420

III

1836

96

2720

III

1107

42

1220

–

–

–

–

Leningradskoe

–

–

–

–

III

151

9

2740

III

698

56

1140

–

–

–

–

North Leningradskoe

–

–

–

–

III

474

51

2730

III

348

75

1170

–

–

–

–

West Matochkin

IV

80

33

3760

III

388

46

2690

III

371

45

1140

–

–

–

–

Note. S, Area, km2; A, vertical closure, m; L, contour line, m.

All of these structures mapped within the Leningradskoe– Rusanovskoe megasalient are confined to basement inliers, which did not show a tendency for further uplift in the Jurassic and Cretaceous and were reactivated during the Late Cretaceous and Cenozoic, suggesting that the history of their formation was essentially the same as that of the Vankor uplift (Kontorovich and Kontorovich, 2005). Four III-order positive structures (Vlas’ev, Universitetskoe, Vikulov, and Tatarinov dome-shaped uplifts) were mapped within the Novaya Zemlya portion of the South Kara regional depression. Although all of these are rooted in basement inliers and can be clearly traced along all Mesozoic horizons, only two of them (Vikulov and Universitetskoe uplifts) are seen in the Turonian paleorelief of the top of the Jurassic. The vertical closure of the Vlas’ev uplift increases fivefold from 12 m in the Turonian horizon at the top of the Jurassic to 60 m in the present-day structural relief at the top of the Jurassic, Neocomian and Cenomanian. The Tatarinov dome-shaped uplift is not delineated on the mapped on the Berriasian–Cenomanian isopach map, but in the present-day structural relief at the top of the Jurassic it has an area of 164 km2 and a vertical closure of 46 m. Its area increases upward in the section and reaches 240 km2 at the top of the Cenomanian, while its vertical closure remains unchanged. Thus, the Tatarinov and Vlas’ev uplifts assigned to the Vankor-type structures were formed in the relief of the Mesozoic horizons during post-Cenomanian. Bovanenkovo-type structures. The Universitetskoe and Vikulov uplifts are best developed in the present-day relief of the Mesozoic reflectors, but are also visible on the also on an isopach map of Berriasian–Cenomanian sediments. The Universitetskoe uplift hosting a major oil-gas-condensate field (Pobeda) is associated with a highly contrasting erosional inlier, which remained a center of uplift throughout the Mesozoic–Cenozoic history. The thickness of Triassic, Jurassic, Neocomian, Aptian–Albian–Cenomanian, and Turonian–Cenozoic deposits thins out in the vicinity of the structure reliably traced along reflector G (Fig. 8C). The paleotectonic map and the results of our paleoreconstructions suggest that the Universitetskoe uplift existed by late Cenomanian time at the top of the Jurassic and had an area of 753 km2 and a vertical closure of 120 m. However, in the present-day relief of the Mesozoic reflectors, the Universitetskoe uplift displays an upward decrease in its area from 913 km2 at the

top of the Jurassic to 566 km2 at the top of the Cenomanian and in its vertical closure from 305 to 127 m. Therefore, the Universitetskoe uplift continued to exist throughout Jurassic and Cretaceous times and appeared to retain the overall tendency for uplift during the Cenozoic, which led to the increase in the vertical closure of this uplift in the structural relief of the Jurassic and Neocomian megasequences, thus causing its appearance at the top of the Cenomanian. A similar situation is seen with the Vikulov uplift, which is one of largest structures in the South Kara regional depression (Fig. 8C). We can see from the flattened seismic section reflector G that the Vikulov uplift was in existence before deposition of the Kuznetsov Formation. At the same time, a threefold increase in the area (from 222 to 722 km2) and a twofold increase in the vertical closure (from 84 to 164 m) is identified for this uplift confined to a Paleozoic inlier from the top of the Jurassic to the top of the Cenomanian. In addition to the Universitetskoe and Vikulov uplifts, other Bovanenkovo-type structures are mapped in the Yamal–Kara regional depression. They include the North Skuratov, South Rusanovskoe, West Malygina and Tarma uplifts, which are clearly visible either in Turonian paleorelief at top of the Jurassic, or in the present-day relief of the top of the Jurassic, Neocomian, and Cenomanian. Gerasimov-type structures or uplifts that appear at the top of the Jurassic and disappear in the structural relief of the top of the Cenomanian are also recognized within the South Kara regional depression. Analysis of regional tectonic processes that appear to have been responsible for the present-day configuration of the study area suggests that reveals that the structural geometry of the Jurassic stage was largely formed during the Neocomian. This period was marked by the formation of the Northern megasaddle, which became a major area of subsidence during post-Cenomanian. All III-order positive structure localized within the Northern megasaddle (Rogozinskoe, North Taimyr, Taimyr, Central Kara domeshaped uplifts and Kop’ev swell) are assigned to the Gerasimov-type. These structures, with their vertical closures decreasing upward in the section, can be clearly identified at the top of the Jurassic and are not visible at the top of the Aptian–Albian–Cenomanian megasequence. The Rogozinskoe dome-shaped uplift and Kop’ev swell are the largest structures. The Rogozinskoe uplift is present on the

V.A. Kontorovich et al. / Russian Geology and Geophysics 58 (2017) 343–361

paleotectonic map and at the top of the Jurassic and Neocomian and is absent at the top of the Cenomanian. The area of the Rogozinskoe uplift increases from 164 to 658 m from the top of the Jurassic to the top of the Neocomian while its vertical closure decreases from 164 to 107 m. The Kop’ev swell, a III-order positive structure complicated by the Flissinga and West Flissinga local uplifts, can be mapped only at the top of the Jurassic. It has an area of 897 km2 and a vertical closure of 74 m. The largest structure complicating the Kop’ev swell is the West Flissinga uplift, which is readily delineated in the Turonian paleorelief of the top of the Jurassic and at the top of the Jurassic and Neocomian. The uplift is characterized by an overall decrease in its area (from 314 to 125 m) and vertical closure (from 54 to 23 m) from the top of the Jurassic to the top of the Neocomian. Medvezh’e-type structures. The other type of structures identified within the South Kara regional depression is represented by rootless structures, which are not confined to any basement inlier. They were formed by post-Cenomanian tectonic movements and remained inactive throughout much of the Mesozoic. They include the West Kropotkinskoe, Kolchanovskoe, Rozhdestvenskoe dome-shaped uplifts and a series of local structures, which are identified only at the top of the Aptian–Albian–Cenomanian megasequence. Conclusions The results of the integrated interpretation of well and seismic data from the northern part of the West Siberian petroleum province, including the Yamal and Gydan Peninsulas and Southern Kara Sea were used to: – create structure, tectonic, and paleotectonic maps, as well as isopach maps of seismic megasequences and a series of present-day seismic and paleoseimic sections; – compare the geological structure of the main prospective sedimentary complexes, both onshore and offshore; – analyze the tectonic structure of the study area, identify tectonic elements of different orders in the structural relief of the Jurassic–Cretaceous horizons, compare the structural and tectonic framework of the Jurassic and Aptian–Albian– Cenomanian megasequences;

359

– analyze the regional tectonic processes and reconstruct the main stages of the evolution of large tectonic elements that shaped the present-day structural and tectonic features within the study area; – map III–IV-order uplifts, acting as potential hydrocarbon traps, in the structural relief on tops of the Jurassic and Cenomanian and analyze their evolution. To summarize, we provide a brief overview of anticlinal traps identified in the continental and offshore Kara Sea regions. 1. A total of 113 anticlinal traps, including 68 onshore and 45 offshore (excluding bay areas) are identified in the north of the Yamal and Gydan Peninsulas and south of the Kara Sea in the structural relief of Jurassic–Cretaceous horizons (Fig. 9, Table 3). 2. One I-order positive structure (Nurma megaswell) and four closed II-order positive structures are identified in the continental portion of the study area; no large closed positive structures are mapped in the South Kara regional depression. 3. A total of 45 closed III–IV-order positive structures are recognized in the offshore South Kara Sea region (excluding bay areas) that has an area of 180 km2; of these 32 are mapped at the top of the Jurassic and 28 at the top of the Cenomanian, and 15 uplifts are pervasive and exist at the top of the Jurassic and Cenomanian. These structures are more densely spaced (3 times) in the continental portion of the study area than in the offshore region. A total of 68 uplifts are mapped in the continental portion having a total area of 180,000 km2. These are 50 uplifts delineated at the top of the Jurassic, 49 uplifts at the top of the Cenomanian, and 31 uplifts going through the Jurassic and Cenomanian. 4. In the offshore Kara Sea region, all structures identified at the top of the Jurassic have areas between 43 and 923 km2 (minimum area of 25 km2) and vertical closures between 12 and 305 m. The uplifts mapped at the top of the Cenomanian have areas between 33 and 1107 km2 and vertical closures between 10 and 164 m. In the continental portion of the study area, the areas and vertical closures of structures lie in the range of 35–1742 km2 and 10–480 m for the top of the Jurassic and 35–1742 km2 and 10–300 m for the top of the Cenomanian. 5. In the South Kara regional depression, the areas and vertical closures of closed III–IV-order positive structures

Table 3. Parameters of anticlinal traps of III–IV orders Top

S, km2

Structures

A, m

H, m

number III order

IV order

through-going

min.

max.

av.

min.

max.

av.

min.

max.

av.

Jurassic

32

15

17

15

43

923

270

12

305

57

1370

4030

3108

Cenomanian

28

16

12

15

33

1107

304

10

164

38

530

1450

1084

Jurassic

50

21

29

31

35

1742

361

10

480

108

1340

3950

3341

Cenomanian

49

16

33

31

25

2402

312

10

300

40

600

1430

920

Offshore Kara Sea

Onshore

360

V.A. Kontorovich et al. / Russian Geology and Geophysics 58 (2017) 343–361

Fig. 9. Anticlinal traps of the extreme northern regions of the West Siberian petroleum province. 1, boundary of Jurassic–Cretaceous deposits; 2, shoreline; 3, structures mapped only in the relief of the Bazhenov Formation; 4, structures mapped only at the top of the Cenomanian; 5, structures mapped at all stratigraphic levels. Structure names are the same as in Fig. 4.

range on average from 270 km2 and 57 m at the top of the Jurassic to 304 km2 and 38 m at the top of the Cenomanian. The uplifts of the South Kara regional depression are found to be less contrasting than those mapped onshore, which have areas of 361 and 312 km2 and vertical closures of 108 and 40 m mapped at the top of the Jurassic and Cenomanian, respectively. Based on current knowledge of the South Kara regional depression, the following conclusions can be drawn: 1. Additional exploration over much of the South Kara regional depression will result in the discovery of a large number of new III–IV-order positive structures, although further investigation is necessary to evaluate their configuration and parameters. 2. The presence of large tectonic features comparable in their size to those located onshore within the Yamal–Gydan megasaddle or in the Nadym–Taz interfluve is hardly predictable in the South Kara regional depression. Such structures comparable in size to the Medvezh’e and Bovanenkovo uplifts, if any, would be discovered even at the current degree of exploration maturity. The only exception can be the southeast-

ern Badaratskaya and southwestern Yenisei parts of the Kara Sea, where the exploration maturity is very low. References Bochkarev, V.S., Brekhuntsov, A.M., Kochergin, M.O., Nesterov, I.I., Brekhuntsov, A.M., Kochergin, M.O., Nesterov, I.I., Jr., Ognev, D.A., 2010. Geologic structure of the junction zone between the Kara Sea and Gydan Peninsula and its petroleum potential. Gornye Vedomosti, No. 10, pp. 6–18. Ermilov, O.M., Karogodin, Yu.N., Kontorovich, A.E., Ter-Saakyan, Yu.G., Agalakov, S.E., Belyaev, S.Yu., Borisova, L.S., Bukreeva, G.F., Burshtein, L.M., Gordeev, V.N., Dmitruk, V.V., Zhilina, I.V., Kontorovich, V.A., Krasavchikov, V.O., Suprunenko, O.I., Chupova, I.M., Fursenko, E.A., 2004. Geologic Structure and Development of Unique Gas Accumulations in the Extreme North of West Siberia [in Russian]. Izd. SO RAN, Novosibirsk. Kazanenkov, V.A., Ershov, S.V., Ryzhkova, S.V., Borisov, E.V., Ponomareva, E.V., Popova, N.I., Shaporina, M.N., 2014. Geologic structure and petroleum potential of Jurassic and Cretaceous regional reservoirs in the Kara Sea–Yamal region and forecast of hydrocarbon reserves distribution. Geologiya Nefti i Gaza, No. 1, pp. 27–49.

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