INTRODUCTION

The continental shelf of the Eastern Arctic and the adjacent deep-water areas have been in the focus of researchers for the past two decades. During these years, the national programs of the geological study of the Arctic deep-water basin and the works on assessment of the oil-and-gas-bearing capacity of the region have dramatically intensified. The volume of seismic materials and the study of the geological structure of the Arctic islands have increased significantly, expanding our understanding of the sedimentary cover structure and the tectonic development of the Arctic Basin [1–8].

The systematic stratigraphic drilling in the Russian part of the Arctic shelf was initiated by NK Rosneft [9]. Since 2020, wells have been drilled within the license areas (LA) in the northern part of the Kara Sea (North Kara LA, 2020), in the Laptev Sea (Anisinsky–New SIberian LA, 2021), and in the Chukchi Sea (North Wrangel-1 LA, 2022).

In August–September 2022, joint drilling was carried out on the Arctic continental shelf in the northwestern part of the East Siberian Sea at the initiative of Rosnedra, by VSEGEI and Rosgeologiya according to the long-term stratigraphic drilling program of NK Rosneft and with its assistance (Fig. 1). These works made it possible to obtain for the first time direct data on the composition and age of sediments composing the uncovered geological section of the Mesozoic–Cenozoic cover of the De Long Rise, and to make the age correlation with the seismic boundaries and complexes.

Fig. 1.
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The scheme of the arrangement of the seismic profiles and stratigraphic wells: DL-1—VSEGEI, 2022; PJSC Rosneft, 2021; ACEX, 2006. Yellow lines indicate the boundaries of geomorphological structures.

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GEOLOGICAL BACKGROUND DATA ON THE REGION

The area of the De Long Rise, where shallow drilling was conducted, is located in the northwestern part of the East Siberian Sea shelf and is adjacent to the marginal part of the deep-water Podvodnikov Basin, which belongs to the Amerasian Basin of the Arctic Ocean. Seismic exploration here is relatively well-developed, because many institutions including the Laboratory of Regional Geodynamics (LARGE), Dal’morneftegeofizika (DMNG), the State Research Navigation-Hydrographic Institute (JSC GNINGI), Marine Arctic Geological Expedition (JSC MAGE), etc., have worked in this region since the late 1980s. By now, numerous time-related, deep seismic-geological, and geological–geophysical sections have been constructed (Fig. 1). Data on the topography, thickness, and velocity characteristics of the sedimentary cover have been obtained. As a result of long-term study, the New Siberian Islands and the shelf of the East Siberian Sea are covered by geological surveys on scales of 1 : 1 000 000 and 1 : 200 000.

Multidisciplinary scientific studies were also carried out here, which resulted in the creation of numerous models of the geological development of the region, differing generally in the interpretation of the origin of tectonic processes, their succession, and time ranges. Most researchers agree that the key tectonic event that determined the structural plan of the region and specific features of the sedimentary cover, was the Early Cretaceous (Pre-Aptian) orogeny, which was manifested here due to the closure of the South Anyui Ocean, followed by continental rifting ([10–16], etc.).

STRATIGRAPHIC DRILLING PROJECT IN THE EAST SIBERIAN SEA

The main tasks of the stratigraphic drilling on the continental shelf in the East Siberian Sea included direct verification of the stratigraphic correlation of seismic profiles, characteristics of geophysical data, and the preparation of additional geological materials for defending the Application of Russia to the United Nation Commission on the Limits of the Continental Shelf. It was necessary to uncover the maximum age interval of the section, to correlate reliably the seismic horizons on the profiles, and to obtain direct data on the material composition of shelf deposits.

Analysis of the seismic and geological data resulted in selecting a point on the continental shelf in the northwestern part of the East Siberian Sea DL-1 (77.75336073° N and 146.6321214° E) located directly on the OGT seismic profile DL11-01 (performed by OJSC MAGE during the expedition of 2011–2012 on board R/V Geologist Dmitry Nalivkin) (Fig. 2).

Fig. 2.
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(a) Fragment of the CDP seismic profile DL11-01 and location of Well DL-1 on it; (b) seismic–acoustic profile passing through the well and schematic lithological and stratigraphic column. 1, Clayey sands, 2, sandy mictites; 3, interlayering of mudstones with sandstones and siltstones; 4, sandy siltstones with interlayers of sand; 5, clays; 6, interlayering of clays and mudstones; 7, dislocated mudstones; 8, interlayering of dislocated and massive mudstones; 9, sandy interlayers; 10, sampling points for the age determination.

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Drilling was performed from the R/V Bavenite, one of the best-equipped geotechnical drilling vessels in the Russian Federation, from September 5, 2022, to September 23, 2022.

Drilling was carried out with simultaneous core sampling without casing by a 215.9-mm diameter drill head using a removable 82-mm diameter core sampler.

The presence in the section of horizons with confined thermal waters migrating along the fault zones and the occurrence of decompactions in the uncovered aquifers make the work complicated. The sea depth at the drilling point was 47.5 m. The maximum depth of the well was 472 m from the sea floor (the greatest depth from which core samples were taken was 268 m); a total of 147.5 m of core was sampled.

Geophysical research methods included three types of work: profile seismic–acoustic survey in the area of the project well, vertical seismic profiling, and thermometry of fiber-optic technologies. The methodology of applying this technology was developed by NK Rosneft professionals during the season of stratigraphic drilling in the Laptev Sea in 2021. Multichannel seismic–acoustic profiling made it possible to obtain a detailed deep seismic image of the structural-geological features of the studied area with a vertical resolution of 2.5–3 m at a depth of 500 m (Fig. 3). Vertical seismic profiling in the well was conducted at a depth of 416 m and allowed us to tie the main seismic horizons, identified by the multichannel profiling, to the core material (to determine their depth of occurrence), as well as to estimate P-wave velocities.

Fig. 3.
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The seismic–acoustic profiles passing through the point of drilling of Well DL-1 and their interpretation (profile A is shown in Fig. 2b). The cartograms demonstrate the location of the profile fragments (black rectangles), the double run time in seconds for the main unconformity surface (color gradient), and the normal fault line (red dashed line). To the east of the fault, an undeformed sequence, not uncovered by the well, is visible. This is presumed to be part of the syn-rift complex.

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Well thermometry was conducted at a depth of 246 m using the distributed temperature sensing (DTS) technology. Analysis of the obtained temperature data showed the temperature distribution in the well from –1.3°С to +5°С and the presence of three local temperature maxima in the section of the Cenozoic complex (Fig. 4). The relatively stable temperature profile is characterized by average temperatures close to +4°С in the rocks of the lower structural layer. The uncovered geological section does not contain permafrost, and the temperature anomalies can supposedly be related to water migration.

Fig. 4.
figure 4

Interpretation of the DTS-thermometry data. The purple curve designates the temperature profile immediately after drilling; the blue curve designates the temperature profile after five days.

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STRUCTURE OF THE WELL SECTION, LITHOLOGICAL COMPOSITION

As a result of drilling, the core was obtained from deposits of the upper part of the sedimentary cover of the shelf to a depth of 268 m. The intervals from 2 to 10 m and from 42 to 60 m in the upper part of the well and from 130 to 180 m in its central watered part were unsampled (Fig. 2).

Two fundamentally different sequences different in composition were distinguished in the section. The upper sequence (to a depth of 130 m) is represented by interlayering of horizontally lying dark gray unlithified clays, siltstones, and sands with numerous inclusions of gravel, pebbles, and boulders and with abundant shell detritus at some levels (Fig. 5). The unsampled interval from 130 to 183 m is probably represented by loose sandy–gravel deposits with boulders taking into consideration the character of sinking a drilling tool and the impossibility of taking a full-scale core.

Fig. 5.
figure 5

Photos of the core fragments uncovered by Well DL-1. (a) Core fragment from the depth of ~ 130 m (loose Cenozoic sediments), (b) unsorted terrigenous sediments of the upper part of the lower (Cretaceous) sequence (~183 m), (c) dislocated Lower Cretaceous mudstones (~207 m). Depth of the occurrence in meters from the sea floor surface.

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The boundary interval (183–184.5 m) between the upper and lower sequences differs in the lithological composition from the underlying rocks and is represented by unsorted terrigenous sediments, sandstones, and clays with large amounts of crushed rock, gravel, and pebbles. Pebbles of both igneous and carbonate rocks are present.

The lower sequence (184.5–268 m) is basically represented by dark gray and light gray mudstones and siltstones with separate sandstone horizons. The mudstones and siltstones are strongly fractured and poorly lithified and are possibly partially disintegrated as a result of tectonic dislocations. In the upper part of this interval, from 184.5 to 230 m (Fig. 5), fractured dark gray clays contain numerous inclusions of gravel, pebbles, and crushed terrigenous rocks, thin (1–3 mm) carbonate veinlets, and interlayers (up to 5 cm) of carbonized matter. Carbonate concretions and pebbles of silicified rocks are encountered less frequently. The core at the depth 194 m contains carbonate pebbles with large fusulinids, probably of the Carboniferous age. The rocks are often dislocated and fractured. In the lower part of the interval (230–268 m, Fig. 5), dark gray siltstones and mudstones are interlayered with strong brown mudstone-like clay with gravel grains and powder of carbonate material.

THE STRATIGRAPHIC CORRELATION OF THE WELL SECTION

The age of the section was determined on the basis of the palynological analysis of ten samples taken over the entire core interval. The upper sample, taken at a depth of 130 m (Figs. 2, 6), contains pollen complexes of gymnosperm and metasperm plants, dinocysts, acritarchs, and prasinophyte and chlorophyte algae. The age is defined as the Aquitanian of the Early Miocene on the basis of the joint distribution of the dinocysts Heteraulacacysta campanula, Distatodinium biffii, Lingulodinium sadoense, Melitasphaeridium choanophorum reductum, and Spiniferites bentorii, etc.

Fig. 6.
figure 6

Dinocysts and palynomorphs from the Cenozoic and Jurassic–Cretaceous deposits of the section of Well DL-1, ruler size is 50 µm. (a) Spiniferites bentorii (Rossig., 1964) Wall et Dale, 1970 (depth of 130 m); (b) Distatodinium biffii Brink. et al., 1992 (130-m depth); (c) Lingulodinium sadoense Matsuoka, 1983 (depth of 130 m); (d) Heteraulacysta campanula Drugg et Loebl. (e) Stereisporites incertus (Bolch., 1956) Sem., 1970 (depth of 264 m), spore; (f) Osmundacidites wellmanii Coup., 1953 (depth of 264 m), spore; (g) Cicatricosisporites australiensis (Cook., 1953) Pot, 1956 (depth of 253.5 m), spore; (h) Clavifera rudis Bolch., 1968 (183 m depth), spore; (i) Nannoceratopsis gracilis Alb., 1961 (depth of 264 m), dinocyst; (j) Gonyaulacysta jurassica (Defl., 1939) Norris et Sarj, 1965 subsp. adecta (depth of 264 m), dinocyst; (k, l) Oligosphaeridium complex (White, 1842) Davey et Will., 1966, (m) (depth 234 m); (m) Schizosporis reticulatus Cook. et Dett., 1959 (depth 208.8 m), freshwater alga; (n) Tasmanites sp. (depth 253.5 m), prasinophyte; (o) Couperisporites tabulatus Dett., 1963 (depth 253.5 m), spore; (p) Sirmiodinium grossii Alb., 1961 (depth 234 m), dinocyst.

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Other samples (Figs. 2, 6) were taken from the lower sequence of the well section. Two-bagged coniferous pollen and spores of hepatic and sphagnum mosses predominate in the samples at the 183-m and 184-m depths (from the boundary interval between the upper and lower sequences); spores of Clavifera rudis gleichenia ferns typical of the Apt-Albian deposits are present too [17].

At a depth of 208.8 m, spores with a smooth exine dominate in the palynospectrum. Spores of Schizaea and Gleichenia ferns, as well as hepatic and sphagnum mosses, are less common. Microphytoplankton is represented by dinocysts Oligosphaeridium complex and Muderongia/Odontochitina sp.; their presumed Valanginian-Barremian age is substantiated by taxonomic similarity to the coeval palynocomplexes of Western Siberia and by their position in the section.

At a depth of 234.0 m, acritarchs Veryhachachium sp., prasinophytes Crassosphaera sp., and dinocysts are present in the sample. Among them, Sirmiodinium grossii, the last occurrence of which was recorded at the Barremian–Aptian boundary, and O. complex, which appears at the base of the Valanginian, were identified [18].

At depths of 259 m and 253.5–254.5 m, microphytoplankton is represented by freshwater algae, acritarchs, prasinophytes, and dinocysts, among which O. complex and Tubotuberella apatela occur. The general age range of the microphytoplankton corresponds to the Early Valanginian [18]. The sample from the 264-m depth contains single dinocysts Gonyaulacysta jurassica subsp. adecta and Nannoceratopsis gracilis, presented against the background of the myospore complex of the typically Middle Jurassic appearance (Stereisporites incertus, S. antiquasporites, Obtusisporis junctus, Densoisporites crassus, Lycopodiumsporites intortivallus, Neoraistrickia truncata, and Dipteridaceae gen. spp.). The general age range of the dinocysts corresponds to the Late Bajocian (Middle Jurassic) [18, 19]. In the lowest parts of the well section, at depths of 267 and 268 m, the macerate is represented by destroyed plant tissues and poorly preserved brown palynomorphs. Among them, the presence of the Early–Middle Jurassic species S. incertus was established. This species may occur singularly in both the Triassic and Early Cretaceous to the Valanginian [17].

Correlation of the drilling data with the seismic profiling data. The well location was planned according to an analysis of the regional seismic profiles. The CDP seismic profile of the DL11-01 was takes as a basis (Fig. 2a). To clarify the optimal well location, four seismic–acoustic profiles were worked out immediately before the drilling (Fig. 3). These profiles were later correlated with the well data (Fig. 2b).

The seismic–acoustic profiles clearly show an angular unconformity, which corresponds to the boundary between the upper and lower sequences of the well section. The upper sequence lies almost horizontally, while the lower sequence composes a monoclinal, that is broken by, most likely, overthrust-reverse faults. The monoclinal dips to the northwest with a dip azimuth of about 315° and a dip angle of about 5°–7°. In addition to these two seismic complexes mentioned above, a third one was distinguished; it was not drilled. This seismic complex on the given fragment of the seismic profile is limited by the fault and is represented by syn-rift deposits (Figs. 2, 3), supposedly of Aptian–Paleocene age.

Analysis of the seismic–acoustic profiles and results of the drilling shows that three stages of the development can be identified in the geological history of the area. The first stage is the formation of marine open- and shallow-shelf deposits, part of which (Middle Jurassic–Lower Cretaceous) was uncovered by Well DL-1 (the lower sequence of mudstones with horizons of sandstones and siltstones). During the orogenic stage (approximately Hauterivian–Barremian to Aptian), the sequence was affected by tectonic deformation (compression) and erosion. The second stage is the formation of rift-related troughs, filled, according to the available conceptions, by Aptian–Albian and Upper Cretaceous–Paleocene sediments (the sequence not uncovered by the well and presumably identified on the seismic profiles). The third stage is the formation of the Cenozoic plate complex of the sedimentary cover.

DISCUSSION

The preliminary results of determining the age of the sedimentary rocks of the lower sequence, uncovered by Well DL-1, indicate their formation from the Middle Jurassic to the middle Early Cretaceous. The stratigraphic analogues of the deposits of this sequence on the New Siberian Islands are, apparently, Oxfor-dian–Valanginian turbidite sequences of the Malyi Lyakhovskii and Stolbovoi islands, dislocated to varying degrees, and the Lower–Middle Jurassic terrigenous formations of the Kotelnyi and New Siberian islands.

The well probably uncovered the section of the Zhokhov Marginal Trough of the Mesozoic Orogen, the Late Jurassic–Neocomian age of which was previously assumed on the basis of the seismic survey data and the geological data on the Novosibirsk Islands [5].

Syn-rift seismic complexes in the Novosibirsk and East Anisin rift basins are distinctly traced on the seismic profiles in the region studied. These complexes are distinguished in the northern part of the Laptev and East Siberian seas; the acoustic basement is traced below them [3, 5]. The East Anisin Basin in the north extends into the rift basin between Lomonosov Ridge and the Podvodnikov Basin (East Lomonosov Basin or terrace [3, 5]). These syn-rift seismic complexes were not involved in the intense Early Cretaceous (pre-Aptian) compression deformation. Therefore, the rift basins in this region were formed already after the epoch of deformation documented by Well DL-1. Thus, the considered rift basins are not older than the Aptian in age.

Traps of the De Long Islands with ages of about 100–125 Ma are well known next to Well DL-1 well on Bennett Island [6, 7]. The trap complexes are not deformed. The lower (deformed) sequences uncovered by the well, obviously occur stratigraphically below the known syng-rift seismic complexes and traps and, partly, possibly underlines them (Fig. 7).

Fig. 7.
figure 7

The CDP seismic profile of the DL11-01 and its interpretation on the basis of data from Well DL-1. Obviously, the gently lying sedimentary cover, including syn-rift deposits, is younger than the age of the deformed sediments uncovered by the well. The acoustic basement may be composed of both Jurassic–Neocomian rocks and older complexes.

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CONCLUSIONS

The core from the shallow well drilled on the De Long Rise in the East Siberian Sea was sampled to a depth of 268 m for the first time. Two sedimentary sequences, separated by regional angular unconformity, were distinguished in the section. The upper sequence is predominantly composed of loose diverse terrigenous sediments, whereas the lower sequence is composed of more densely cemented fine-clastic rocks. The seismic–acoustic data show that the upper sequence lies horizontally, while the lower sequence forms the monoclinal modified by tectonic deformations of compression. According to the preliminary paleontological data, the lower sequence has a Middle Jurassic–Lower Cretaceous (Neocomian) age, and the upper sequence is of Cenozoic, Neogene–Quaternary age. The weathering crust on the Aptian–Albian rocks is probably observed at the boundary of the upper and lower sequences. At the point of the drilling, beneath the sea floor, there is no permafrost layer.

The boundary of the main unconformity was correlated with a series of seismic–acoustic profiles and can be traced further throughout the entire Arctic on the regional seismic profiles.

The preliminary regional correlations of sedimentary formations, taking into consideration new drilling data, indicate the following seismic complexes in the De Long Rise area: (1) the Jurassic–Neocomian complex, gradually deformed under compression (or deformed in varying degrees); on seismic profiles, it often transits to the acoustic basement without a visible internal structure (the acoustic basement may be composed of both Jurassic–Neocomian rocks and of more ancient complexes); (2) the syn-rift complex with the main phase of rifting in the Aptian–Albian (possibly, even younger) and with manifestations of trap magmatism at the base of this complex; and (3) the post-rift horizontally lying Cenozoic complex.

Further works should be focused on the detailed age determinations of sedimentary complexes, their facies analysis, and lithological and geochemical and other laboratory analytical studies of the uncovered sediments, as well as on the correlation of all available seismic profiles, taking into consideration the results of drilling the new well. A separate task is to reconstruct the evolution of the region within the sedimentary basins of the East Siberian Sea shelf on the basis of first-time well data on the steady thermal gradient.