Probable distribution of Upper Jurassic and Lower Cretaceous deposits on the Laptev Sea shelf and their petroleum resource potential

The analysis of Upper Jurassic and Lower Cretaceous marine sections developed in surrounding structures of the Laptev Sea revealed that all of them are composed of terrigenous rocks, which enclose abundant concretions cemented by calcareous material. The Upper Jurassic portion of the section is the most variable in thickness and stratigraphic range of sediments usually including hiatuses. Its Lower Cretaceous part represented by the Boreal Berriasian (=Ryazanian) and lower Valanginian stages is most complete. The Upper Jurassic and Lower Cretaceous sections are usually composed of fine-grained rocks (clays and mudstones) in the west and coarser cemented varieties (siltstones and sandstones) with rare mudstone intercalations in the east. Practically all the investigated Upper Jurassic and Lower Cretaceous sections include readily recognizable age and facies analogs of the Bazhenovo Formation and Achimov sandstones, which are petroliferous in West Siberia. There are grounds to assume the occurrence of these formations also on the Laptev Sea shelf, which is confirmed by seismic records. Conditions favorable for the formation of potential hydrocarbon reservoirs could exist in the western part of the paleobasin along the Nordvik Peninsula coast and northeastern Tamyr Peninsula margin. Paleotectonic reconstructions presented in this work are well consistent with stratigraphic conclusions.     

Early postglacial environmental conditions in the northeastern Laptev Sea region

A comparison of the first results of a comprehensive micropaleontological analysis (pollen, spores, organic-walled microfossils, diatoms, ostracods) and radiocarbon ages (AMS¹⁴C) from sediment core recovered in the northeastern outer shelf of the Laptev Sea (51 m water depth) revealed a temporal coincidence between terrestrial and marine environmental changes that occurred between 11.2–10.3 cal ka. This interval provided evidence for a landscape transition from grass tundra to shrub tundra and the development of a freshwater estuarine basin with the strong influence of riverine discharge and a minor advection of North Atlantic waters. The establishment of a warmer and wetter climate promoted the expansion of shrub tundra habitats. The interval of 9.5–7.5 cal ka recorded a transition from a shrub tundra environment to forest-tundra vegetation. This interval also revealed a series of short-term temperature fluctuations, when summer temperatures were 3–4°C higher than today. The active advection of North Atlantic waters and the increase in salinity were also recorded by this interval.     

Environmental conditions of the Laptev Sea region in the late postglacial time

The comparison between the first results of comprehensive micropaleontological analysis (pollen, spores, foraminifera, and ostracods) and those of radiocarbon dating (AMS¹⁴C) for the sediments of the eastern inner shelf of the Laptev Sea (the core collected from depth of 37 m) indicates that considerable changes in natural conditions in the sea and on land coincide in time and refer to the time period of 1500–1700 years B.P. This period is characterized by changes in microfossils: appearance of thermophilic pollen and planktonic foraminifera and increase in total number of benthic foraminifera and ostracods. Intense warming and humidification of the climate reconstructed for this 200-year period promoted the expansion of large-shrub tundra. Summer air temperatures were lower than that in the peak mid-Holocene climatic optimum by 2°–3°C, but 1°C higher than the present-day temperature. An estuary freshwater basin developed: it was strongly affected by river discharge, but North Atlantic waters also intensely penetrated here in short-term intervals. In general, the studied microfossil complex reflects the relatively stable environmental conditions and decrease in seawater salinity in the eastern part of the Laptev Sea shelf during the last 2300 years.     

Picophytoplankton of the Laptev Sea in Autumn

Doklady Earth Sciences - The abundance and biomass of picophytoplankton, the total concentration of chlorophyll a, and the contribution of the picofraction to chlorophyll a were studied in the...     

Geology and petroleum potential of the Fairway Basin in the Tasman Sea

The Fairway Basin is a large, generally north – south-trending, sediment-filled structure in water 1500 – 3000 m deep, on the eastern slope of the Lord Howe Rise in the Tasman Sea, and is partly within Australian jurisdiction. It was poorly known until a few years ago, when seismic profiling and piston coring cruises were carried out. The basin, about 1100 km long and 120 – 200 km wide, can be divided into three segments—north, central and south—that trend northwest, north and north-northwest, respectively. All three segments probably formed by thinning of continental crust during breakup of Lord Howe Rise and surrounding aseismic continental ridges in the Late Cretaceous and Paleocene. Normal faulting, large inputs of terrigenous sediment and subsidence to bathyal marine depths occurred during that time. A period of compression, perhaps related to overthrusting on New Caledonia, occurred in the Eocene, leading to uplift (and in parts, erosion) of northern Lord Howe Rise, and reversal of faulting in the basin. By the Oligocene, the area was again in bathyal depths, and pelagic ooze and some turbidites accumulated. The basinal sequence is generally 2000 – 4000 m thick, with 1200 – 3200 m of Cretaceous to Eocene sediment concentrated in depocentres, capped by 500 – 800 m of Oligocene and younger sediment. In the depocentres, numerous sedimentary diapirs pierce sedimentary sequences. The sedimentary diapirs appear to be fed by Cretaceous muds deposited during rifting. Often, these diapirs are overlain by faults extending to the seafloor, and hummocky bathymetry is possibly caused by fluid escape. The overall geology suggests that the Fairway Basin may be a large frontier hydrocarbon province. Seismic profiles display a bottom-simulating reflector above many depocentres, 500 – 700 m below the seafloor. The bottom-simulating reflector has positive polarity, which could result from a diagenetic phase transformation, a thin gas hydrate layer with a sharp top, or from the sharp base of a gas layer (probably beneath gas hydrates). Standard piston cores taken above diapirs and apparent fluid-escape features have recovered little gas. Other than drilling, the next steps in assessing petroleum potential are to clearly document fluid-escape structures, and to sample any fluids emitted for hydrocarbons.     

Alkanes in Quaternary deposits of the Laptev Sea

The distribution and genesis of n-alkanes in sediments from the 38-m sequence obtained during core boring in the Ivashkina lagoon were studied. Sediments were formed in the Holocene as a result of thermokarst and penetration of seawater. The sequence mostly includes permafrost rocks partially molten in the upper horizons and covered by Quaternary deposits, which are mostly the products of thermoabrasion.     

Initial geologic hydrocarbon resources of the Laptev Sea shelf

Formation of the passive continental margin of the Laptev Sea (Laptev Plate), which was part of the Siberian Platform till the Late Cretaceous, was related to the Late Mesozoic–Cenozoic rifting of the Arctic geodepression. The regime of the passive continental margin still continues. The maximum thickness of the deposits of this age seems to exceed 6 km in the northeastern part of the shelf. The hydrocarbon resources of the Late Precambrian–Cenozoic deposits forming the Laptev Plate cover are evaluated. Based on the concept of the similar evolution of the Laptev Plate and Vilyui syneclise, the geochemical characteristics of dispersed organic matter of the coeval deposits of the Vilyui syneclise are used.     

Source potential of the upper Jurassic rocks of the Barents Sea petroleum basin

A preliminary assessment of the source potential of the Jurassic section was obtained using organic geochemical data on samples collected from outcrops on Franz Josef Land and Svalbard archipelagoes as well as boreholes in the Barents Sea basin. The presence of organic-rich shale units with good source potential was reported for the first time within the studied section of Early to Middle Jurassic age, along with well-documented Upper Jurassic source rocks. The study provides an assessment of regional variations in the kerogen type, hydrocarbon generation potential, and maturation of organic matter from Jurassic sediments.     

Late Saalian and Eemian palaeoenvironmental history of the Bol''shoy Lyakhovsky Island (Laptev Sea region, Arctic Siberia)

Palaeoenvironmental records from permafrost sequences complemented by infrared stimulated luminescence (IRSL) and [Formula: See Text]Th/U dates from Bol'shoy Lyakhovsky Island (73°20'N, 141°30'E) document the environmental history in the region for at least the past 200 ka. Pollen spectra and insect fauna indicate that relatively wet grass-sedge tundra habitats dominated during an interstadial c. 200-170 ka BP. Summers were rather warm and wet, while stable isotopes reflect severe winter conditions. The pollen spectra reflect sparser grass-sedge vegetation during a Taz (Late Saalian) stage, c. 170-130 ka BP, with environmental conditions much more severe compared with the previous interstadial. Open Poaceae and Artemisia plant associations dominated vegetation at the beginning of the Kazantsevo (Eemian) c. 130 ka BP. Some shrubs (Alnus fruticosa, Salix, Betula nana) grew in more protected and wetter places as well. The climate was relatively warm during this time, resulting in the melting of Saalian ice wedges. Later, during the interglacial optimum, shrub tundra with Alnus fruticosa and Betula nana s.l. dominated vegetation. Climate was relatively wet and warm. Quantitative pollen-based climate reconstruction suggests that mean July temperatures were 4-5°C higher than the present during the optimum of the Eemian, while late Eemian records indicate significant climate deterioration.     

Triassic stratigraphy of the Eastern Laptev Sea coast and New Siberian Islands

Studies of the paleontology and stratigraphy of the Triassic strata from the Lena–Olenek interfluve area and Kotelny Island have important implications for improving the accuracy of interregional and global correlations, refining the Boreal standard and international stratigraphic scale for the Triassic System. The importance of this study is also underlined by the necessity of refining the stratigraphic basis for regional geological exploration in Arctic zone that now became the focus for building the country’s strategic resource base. Analysis of recent paleontological and stratigraphic data from key Triassic sections in the Laptev Sea coastal region provides new age constraints for the Triassic strata based on different faunal groups. The Triassic stratigraphic scheme for the region has been refined using new data on the paleontology, thickness variations, and boundaries of local stratigraphic subdivisions.     

Investigations on the petroleum generation potential of bituminous coals from the Saar region

The Saar basin contains bituminous coals (high volatile A and B). Five coals of Westphalian and Stephanian age with distinctly varying maceral compositions were analysed by Soxhlet extraction and supercritical extraction with different solvents and microscopically. From the data obtained it is concluded that the oldest coals have a potential for oil generation, whereas the younger coals do not. This conclusion is explained by varying liptinite contents and varying adsorption capacities of different vitrinite macerals for bitumen.     

The Upper Jurassic of the Laptev Sea: interregional correlations and paleoenvironments

The Late Jurassic evolution of Boreal and Arctic basins is reflected in the widespread deposition of organic-rich black shales (source rocks). In this connection, the priority should be placed on the development and refinement of zonal schemes for the Upper Jurassic of the Laptev Sea coast based on ammonites, foraminifers, ostracods, dinocysts, and spores and pollen from reference sections as the basis for stratigraphic, paleogeographic, and facies studies. The Upper Jurassic and Lower Cretaceous reference section of interest is located on the left side of the Anabar Bay of the Laptev Sea (Nordvik Peninsula, Urdyuk-Khaya Cape). An uninterrupted and continuous section from Upper Oxfordian to Lower Valanginian is exposed in coastal cliffs and consists mainly of silty clay deposits with abundant macro- and microfossils. A reliable biostratigraphic subdivision of the Upper Jurassic interval of this section was taken as the basis for the assessment of the correlation potential of different fossil groups and subsequent interregional correlations, facies analysis, and detailed paleogeographic reconstructions of the study area. The analysis of variations in the composition of macrobenthic communities and microphytoplankton and terrestrial palynomorph assemblages and the biofacies analysis allowed the reconstruction of the evolution of marine paleoenvironmental settings in the western part of the Anabar–Lena sea and in the terrestrial settings in the adjacent land area of Siberia.     

Vegetation and climate history in the Laptev Sea region (Arctic Siberia) during Late Quaternary inferred from pollen records

Paleoenvironmental records from a number of permafrost sections and lacustrine cores from the Laptev Sea region dated by several methods (¹⁴C-AMS, TL, IRSL, OSL and ²³⁰Th/U) were analyzed for pollen and palynomorphs. The records reveal the environmental history for the last ca 200 kyr. For interglacial pollen spectra, quantitative temperature values were estimated using the best modern analogue method. Sparse grass-sedge vegetation indicating arctic desert environmental conditions existed prior to 200 kyr ago. Dense, wet grass-sedge tundra habitats dominated during an interstadial ca 200–190 kyr ago, reflecting warmer and wetter summers than before. Sparser vegetation communities point to much more severe stadial conditions ca 190–130 kyr ago. Open grass and Artemisia communities with shrub stands (Alnus fruticosa, Salix, Betula nana) in more protected and moister places characterized the beginning of the Last Interglacial indicate climate conditions similar to present. Shrub tundra (Alnus fruticosa and Betula nana) dominated during the middle Eemian climatic optimum, when summer temperatures were 4–5 °C higher than today. Early-Weichselian sparse grass-sedge dominated vegetation indicates climate conditions colder and dryer than in the previous interval. Middle Weichselian Interstadial records indicate moister and warmer climate conditions, for example, in the interval 40–32 kyr BP Salix was present within dense, grass-sedge dominated vegetation. Sedge-grass-Artemisia-communities indicate that climate became cooler and drier after 30 kyr BP, and cold, dry conditions characterized the Late Weichselian, ca 26–16 kyr BP, when grass-dominated communities with Caryophyllaceae, Asteraceae, Cichoriaceae, Selaginella rupestris were present. From 16 to 12 kyr BP, grass-sedge communities with Caryophyllaceae, Asteraceae, and Cichoriaceae indicate climate was significantly warmer and moister than during the previous interval. The presence of Salix and Betula reflect temperatures about 4 °C higher than present at about 12–11 kyr BP, during the Allerød interval, but shrubs were absent in the Younger Dryas interval, pointing to a deterioration of climate conditions. Alnus fruticosa, Betula nana, Poaceae, and Cyperaceae dominate early Holocene spectra. Reconstructed absolute temperature values were substantially warmer than present (up to 12 °C). Shrubs gradually disappeared from coastal areas after 7.6 kyr BP when vegetation cover became similar to modern. A comparison of proxy-based paleoenvironmental reconstructions with the simulations performed by an Earth system model of intermediate complexity (CLIMBER-2) show good accordance between the regional paleodata and model simulations, especially for the warmer intervals.     

Dating and correlation of reference seismic horizons in the Laptev Sea Basin

Four seismic horizons (L1, L2, L3, and L4) were recognized in the Laptev Sea Basin. The seismic complexes that lie between the horizons were found and dated (125-55.8, 55.8-33.9, 33.9-5.3, and 5.3-0Ma). A Paleozoic carbonate plate exists in the southwestern Laptev Sea Basin below the L1 horizon. Rifting in the Laptev Sea Basin began shortly after the Verkhoyansk orogeny ended at about 125 Ma BP. The evolution of the Laptev Sea Basin is correlated with that of the Eurasian Basin and the climatic history of the region.     

Thermotomographic model and oil-and-gas forecasting for the sedimentary cover of the Laptev Sea shelf

2D and 3D geotemperature models of the sedimentary cover within the Laptev Sea shelf have been calculated. The presence of several “thermal domes” (zones of uplifted isotherms) has been found. Based on analogous sedimentary basins with domes, which correlate to the location of hydrocarbon deposits, the forecasting has been made that the Ust-Lena graben and Omoloi trough are the most promising areas for oil-and-gas prospecting.     

High-resolution stratigraphy of the Upper Jurassic section(Laptev Sea coast)

Jurassic strata are widespread through Arctic Siberia and host oil and gas fields. However, in most cases, the geology of such vast areas still remains unexplored, and study of the Jurassic stratigraphy and reconstructions of geologic history are possible only through analysis of sediment cores. In this connection, there is a clear need for detailed studies of microfaunas (foraminifera, ostracods) and palynomorphs (dinocysts, spores, and pollen). The studied reference section of the Upper Jurassic and Lower Cretaceous is located on the left side of Anabar Bay of the Laptev Sea (Nordvik Peninsula, Cape Urdyuk-Khaya). An uninterrupted and continuous section from the Upper Oxfordian to the Lower Valanginian is exposed in coastal cliffs and consists mainly of silty clay deposits with abundant macro- and microfossils. Integrated field studies (biostratigraphy, lithostratigraphy, sedimentology) allow a more detailed characterization of the regional geologic framework. A detailed subdivision of the section is based on the systematic composition of ammonites from Upper Oxfordian and Kimmeridgian deposits. Several foraminiferal zones of the Upper Oxfordian and Lower Volgian are defined, and some of them are denfined for the first time. The distribution of ostracods in the section is analyzed for the first time. The section is also studied using palynological analysis, that results in its detailed subdivision on palynological data and recognition of two sequences of palynostratigraphic units. The integrated stratigraphy is used to establish the precise position of stage and substage boundaries. The continuity of the section is defined based on micropaleontological and palynological data.     

Depositional environment of the Laptev Sea (Arctic Siberia) during the Holocene

The Holocene depositional setting of the Laptev Sea was studied using three marine sediment cores from water depths between 77 and 46 m. Based on sedimentary parameters (TOC content, δ¹³C_org, sedimentation rates) controlled by radiocarbon age models the palaeoenvironment of a strongly coupled river-shelf system was reconstructed since ˜11 ka BP. Caused by a transgressing sea after the last glaciation, all cores reveal progressive decreases in sedimentation rates. Using the sedimentary records of a core from the Khatanga-Anabar river channel in the western Laptev Sea, several phases of change are recognized: (1) an early period lasted until ˜10 ka BP characterized by an increased deposition of plant debris due to shelf erosion and fluvial runoff; (2) a transitional phase with consistently increasing marine conditions until 6 ka BP, which was marked at its beginning near 10 ka BP by the first occurrence of marine bivalves, high TOC content and an increase in δ¹³C_org; (3) a time of extremely slow deposition of sediments, commencing at ˜6 ka BP and interpreted as Holocene sea-level highstand, which caused a southward retreat of the depositional centres within the now submerged river channels on the shelf; (4) a final phase with the establishment of modern conditions after ˜2 ka BP.     

The Fault System Controlling Methane Seeps on the Shelf of the Laptev Sea

Doklady Earth Sciences - The paper presents data obtained during the 69th and 72nd expeditions of the research vessel Akademik Mstislav Keldysh (2017, 2018). A mechanism of methane discharge that...     

Ionic composition of pore water in shallow shelf deposits of the Laptev Sea

The cationic and anionic compositions of pore water in shallow deposits of Buor-Khaya Bay is studied. Significant concentration heterogeneity of the vertical ionic profile in the studied drill columns is shown. It is established that the vertical ionic profile of shelf deposits of the Laptev Sea is basically formed under the influence of thawing of underwater permafrost, hydrodynamic conditions, the water mass, and heat flows and depends on the lithological and granulometric types of the deposits. The highest concentrations of ions are registered in plant detritus represented by ground grass vegetation. A relationship between the ionic composition of pore water and cryogenic state of the sequence and its variability is demonstrated using the method of major components.     

Environmental changes in the eastern shelf of the Laptev Sea in the late-glacial time

Data on the interaction of the land-sea system have been obtained based on complex palynological analysis (pollen and spores of terrestrial plants and cysts of dinoflagellates and green algae) and the SEM analysis of sediments from boreholes drilled on the Laptev Sea’s eastern shelf and dated by the radiocarbon method. The spore and pollen ensemble revealed for the first time in late-glacial sediments of the boreholes indicates that warming which started within the time interval 15.6–10.7 ka ago was accompanied by short-term cooling events. Sedge and herb phytocoenosis predominated in the vegetation cover of East Arctic Siberia during the cooling. The climate warming favored the distribution of shrub birch tundra. The results of the analysis of organic-walled phytoplankton and the low value of the AH criterion indicate cold-water conditions with a longstanding ice sheet at sea. After the interval that ended 10.7 ka ago, thermophilic species of dinocysts appeared on the outer shelf, and the AH-criterion value increased, which suggests an active inflow of relatively warm North-Atlantic water to the shelf. Based on the comparison of the obtained results, an inference has been made about frequent changes of the climate in the region and the environment in the eastern part of the Laptev Sea during the deglaciation epoch.