The biostratigraphy and sedimentary environments of the Lower Cretaceous section,Malaya Kheta structural-facies region,West Siberia

The biostratigraphic study of the Low Cretaceous sediments of the Middle Messoyakha swell (Malaya Kheta region) distinguished 12 biostratons based on biofossil data and thus confirmed Boreal Berriassian, Valanginian, and Lower Hauterivian strata. The genetic types of the sediments were determined by lithofacies and biofacies analyses. In the deposit, the Bazhenovo and Kulomzino horizons (Berriassian to basal Valanginian) consist of relatively deep-water marine, shallow-water marine, and coastal-continental facies that displace one another along the strike. The Tara and Ust’-Balyk horizons (middle Lower Valanginian to Lower Hauterivian) are composed of shallow-water marine facies. Paleontological data suggest a low-hilled island covered with conifer-gingko forests with fern understory in the place of the Middle Messoyakha swell in the Berriassian Age.     

Earliest Cretaceous (late Berriasian) glendonites from Northeast Siberia revise the timing of initiation of transient Early Cretaceous cooling in the high latitudes

The studies of past climatic changes form the basis for predicting our future anthropogenic world and are among the most prominent topics in current Earth sciences. Although the Cretaceous is generally considered as a greenhouse period in Earth's history, a number of significant cooling events based on an array of climatic proxies have been identified. Here we present the first data on Berriasian (Ryazanian) glendonite findings from the paleontologically well dated Lower Cretaceous succession of northeastern Siberia. Based on well calibrated Buchia and ammonite biostratigraphy, the stratigraphic interval across which the glendonites occur is restricted to the late Berriasian. Stable carbon isotope (δ¹³C) values of the studied glendonites clearly suggest the precipitation of ikaite from marine water without any significant contamination from biogenic methane. Our results, when integrated with other available paleoclimatic proxies from elsewhere in the high latitudes, suggest a revision of the initiation of Early Cretaceous cooling in the high latitudes from the Valanginian to the late Berriasian. All known occurrences of Lower Cretaceous glendonites in both the northern and southern hemispheres are reviewed.     

The effect of Meso-Cenozoic tectonic processes on the formation of Upper Jurassic and Cretaceous hydrocarbon pools in the north of the Aleksandrov arch (West Siberia)

The effect of tectonic processes on the petroleum potential of the Upper Jurassic and Cretaceous sediments is estimated by the example of the deposits in the north of the Aleksandrov arch. The formation history of the structures bearing Upper Jurassic and Cretaceous hydrocarbon (HC) pools is discussed.The results obtained lead to the conclusion that anticlinal traps complicated by faults cutting the Meso-Cenozoic sedimentary cover are the most promising for the formation of large HC pools in Cretaceous sand reservoirs. These traps serve as channels for HC migration from the oil-producing rocks of the Bazhenovo Formation into the overlying reservoirs. In the Upper Jurassic sediments, anticlinal traps free from Cenozoic faults are the most promising for HC accumulation. These conclusions are confirmed by a number of examples.     

Paleomagnetism and magnetostratigraphy of Upper Cretaceous and Cretaceous-Paleogene boundary intervals,southern Kulunda basin (West Siberia)

The study presents new paleomagnetic data on the Upper Cretaceous and Cretaceous-Paleogene boundary intervals of the southern Kulunda basin (Alei area), which were obtained from core samples collected from a 305-m-thick section penetrated in two wells. The paleomagnetic sections of each well were compiled and correlated based on the characteristic remanent magnetization (ChRM). Paleomagnetic, geological, stratigraphic, and paleontological data were used to compile the Upper Cretaceous and Cretaceous-Paleogene magnetostratigraphic section of the southern Kulunda basin. The magnetostratigraphic section consists of five magnetozones, one normal polarity zone, and four reversed polarity zones spanning the Upper Cretaceous and Lower Paleogene. The lower part of the Gan’kino Horizon, showing normal polarity, forms a single normal polarity magnetozone N. The upper part of the Gan’kino Horizon comprises two reversed polarity magnetozones (R₁km and R₂mt). The Talitsa and Lyulinvor Formations of Lower Paleogene age correspond to two reversed polarity magnetozones (R₁zl and R₂i). The compiled Upper Cretaceous and Lower Paleogene magnetostratigraphic section was correlated with the geomagnetic polarity time scale. Two options were considered for correlating the lower normal polarity part of the section with geomagnetic polarity time scale of Gradstein.     

Estimation of the oil-and-gas potential of sedimentary depressions in the Far East and West Siberia based on gravimetry and geothermy data

The results of geodensity, geothermal, and paleotectonic modeling of some sedimentary depressions in the Far East region and central West Siberia are interpreted from a petroleum-geological standpoint. The prospects of the possibly oil-and-gas-bearing Paleogene and Upper Cretaceous rock complexes in the Lunsk, Makarovsk, and Aniva troughs, West Sakhalin uplift, and Middle Amur intermontane depression are estimated. Petroleum accumulation zones in the preplate complex are forecasted. Localization of Jurassic oil generation centers is defined at a new large oil field in the central West Siberian plate (West Siberian plate). Experience in petroleum geology modeling for structures of different tectonic patterns, lithologies, and rock ages makes it possible to recommend the adopted methodical approach and technology of gravimetric and geothermal interpretation as universal and efficient methods.     

Seismic stratigraphy,formation history and gas potential of the Nadym-Pur interfluve area (West Siberia)

The study presents a seismic and geological characterization of the Meso-Cenozoic sedimentary cover of the Nadym-Pur interfluve area and discusses the morphology of the Jurassic and Aptian-Albian-Cenomanian sedimentary complexes, formation history of structures, and geologic processes responsible for the formation of Cenomanian gas accumulations.     

The electrum of the Agatovskoe deposit (Northeast Russia) and its formation conditions

This paper discusses the largest electrum units precipitated in calcite of the final (“nonproductive”) stage of ore formation. Notably, during examination of the chemical composition of these gold grains, sphalerite and other minerals belonging to the early (“productive”) association have been found. The results of thermo- and cryometric investigations of fluid inclusions in calcite of the Agatovskoe deposit have indicated that in the composition of two-phase fluid solutions of admixtures, NaCl, KCl, and MgCl dominated. This is evidenced by chloride eutectics of the admixture solutions in the temperature interval from −35 to −39°C. The complete homogenization of the fluid admixtures takes place in a narrow temperature interval (from 206 to 213°C) and concentration of salts is 3.7–6.0 mass % in NaCl equivalent. The density of the fluid changes starting from 0.88–0.89 g/cm³. The derived homogenization temperatures correspond to the temperatures of gold deposition, since the pressure during formation of gold veins in the deposit was small.     

First findings of dinocysts in the upper Oligocene Turtas formation in Southern Tyumen oblast (West Siberia)

A new occurrence of dinocysts Pseudokomewuia in continental Cenosoic (Oligocene–Miocene) rocks of the West Siberian plain is identified. A complex of palynomorphs with dominant microphytoplankton (dinocysts) was found for the first time in the lacustrine-marsh sediments of the south of Tyumen oblast. The variable composition of the complex with Pseudokomewuia aff. laevigata He and Pseudokomewuia aff. granulata He species is traced upward through the section. The layers that correspond to the maximum abundance of Pseudokomewuia (akme) are distinguished and correlated with sections studied previously, which are located in the Barabinsk lithofacial region. Sediments with dinocysts are compared with sediments of the same age from North America, Northern Europe, and Southern China.     

Sedimentary formations of the Cretaceous Sakhalin Basin (Far East Asia)

The marine sedimentary formations of the Middle Albian to Maastrichtian in the Cretaceous Sakhalin Basin (CSB) were investigated. These successions of strata consist of interbedded sandy, clayey and calcareous rocks which are underlain by heterogeneous metamorphosed (up to greenschist facies) Paleozoic to Mesozoic (pre-Aptian) rocks. The studied sections display several different facies reflecting geological settings ranging from an inner shelf to a continental slope. Three depositional complexes bound by regional subaerial unconformities are recognized within the marine successions. Since the Albian, the CSB has been a rapidly subsiding marginal part of the Okhotsk Sea plate. The Naiba Valley succession, corresponding to a sublittoral zone, shows extremely high sedimentation rates up to 190 m/Ma. The stratigraphic distribution of lithofacies indicates that the CSB became shallower from the Middle Albian to the Maastrichtian.     

The geochemical patterns of rocks of the Bazhenov and Abalak formations (Western Siberia)

The composition of rocks of the Bazhenov and Abalak formations in Western Siberia is described. Correlations of the concentrations of the major and minor components in rocks with the concentrations of minor elements and organic material are shown. Study of the concentration of minor elements allows us to determine the conditions of sedimentation: redox potential, hydrogen sulfide contamination, and the source of sedimentary material. The results show that rocks of the Bazhenov Formation were formed under the conditions of low fluxes of clastic material, a reduced environment, and a periodically occurring hydrogen sulfide mode.     

Composition and formation conditions of sediments in the Doroninskoe soda lake (eastern Transbaikalia)

It has been established that the southern and eastern parts of the Doroninskoe soda lake bottom with the depth less than 4–4.5 m are mainly composed of micrograined quartz-feldspar sand. Silty-clayey kaolinite-hydromica particles predominate from the depth more than 4–4.5 m. Low maturity of clastic material and its insignificant role in the formation of soda minerals in the lake have been established. Very high concentrations of Cl and Na, as well as high concentrations of S and P, are specific characteristics of the chemical composition of sediments. Distribution of chemical elements is governed by a complex action of the mechanical, alkaline, sorption, and hydrosulfuric barriers. With increasing depth and distance from the shore, concentrations of Ti, Ce, Na, Mn, La, Ca, Co, and Cu increase by 3–3.5 times; Fe, P, V, and Zn, by 4.5–5 times; Mg, Ni, and Cr, by 6–7.5 times; and Cl and S, by 10–15 times. The sharp increase in the contents of Ti, Fe, Mn, Ce, and La is related to the formation of zircon-ilmenite placer horizons. Concentrations of Si, K, and Ba show negative correlation with the psammite component. The Pb content does not depend on the barrier factor.     

Composition of organic compounds in bottom sediments of lakes in the taiga and steppe zones in Siberia

The paper presents chromatographic–mass spectrometric data on the individual composition of organic compounds in sapropel-like deposits of lakes in the steppe, forest–steppe, and three taiga natural climatic zones (NCZ) in Khakassia and western Siberia. Data on the individual composition of n-alkanes, carboxylic acids and their esters, thiophenes, phytol, phytone, steroids, and di- and triterpenoids and on their distribution in the sediments show systematic variations in the composition of the organic matter (OM) with increasing ambient temperature and the aridity of the climate. The concentrations of all groups of the compounds in lacustrine sediments decrease southward, and simultaneously esters with long (up to С₂₀) alkyl substituents appear. The concentrations of oxygen-bearing compounds increase among the acyclic structures and triterpenoids, as also do the concentrations of structures ketonic substituents among the steroids. The dominance of the carboxylic acids and their esters by high molecular weight homologues gives way to the dominance of palmic acid and its derivatives. Among n-alkanes, the dominance of С₂₇ and С₂₉ hydrocarbons typical of terrestrial plants gives way to a higher percentage of homologues С₂₁–С₂₅ (macrophytes) and С₃₁ (herbaceous plants). Among the tocopheroles, the concentration of the oxidized form of α-tocopherol (acetate) increases.     

The structure and genesis of limestones at the boundary between the Abalak and Bazhenov formations in Central West Siberia

In the central West-Siberian basin, fractured and cavernous carbonate rocks that are often oilbearing, which are referred to as correlation layer 1 (CL1), are frequently present at the top of the Abalak formation and/or at the bottom of the Bazhenov formation. They are sporadically distributed over the profile and the area; their genesis is still not completely clear. The structural features and oil-bearing capacity of carbonate rocks have been studied, as well at the distributions of carbon and oxygen stable isotopes from bulk rocks and calcite filling fractures in the CL1 layer that was penetrated by six wells. The spherolitic microstructure of limestones together with the carbon and oxygen isotope distributions (δ¹³C =–14 to–26‰ VPDB; δ¹⁸O = 0 to–5‰ VPDB) indicate the precipitation of carbonate material due to microbial activity on the surface and/or in the upper part of sediments at high methane concentrations. The fractures and caverns in limestones are frequently oil-bearing; they contain coarse crystals of calcite, pyrite, quartz, and, more rarely, barite. This degree of mineralization and the isotope composition of calcite oxygen (up to–18‰ VPDB) indicate that calcite precipitates at elevated temperature (up to 120°C) from the hydrothermal fluids that could migrate from underlying strata.     

Vendian of the Fore-Yenisei sedimentary basin (southeastern West Siberia)

Fossiliferous Upper Vendian strata are discovered in the Upper Proterozoic to Lower Paleozoic Fore-Yenisei sedimentary basin under a thick Mesozoic-Cenozoic cover in southeastern West Siberia. Two depositional systems are recognized based on sedimentological features: (1) wave- and current-agitated shoreface-forereef-biohermal reef system (Vostok-3 Borehole section) and (2) tidal flat-evaporite basin (Averinskaya-150 Borehole section). The forereef facies yielded fossilized tubular calcareous skeletons of reef-building metazoans Cloudina riemkeae, Cloudina hartmannae and Cloudina carinata, phosphatized Namacalathus-like fossils, and a diversity of tubular phosphatized and agglutinated tubular fossils. The fossil assemblage can be interpreted as the evidence of ecological complexity of the reef system. Paleontological characteristics suggest correlation of the Vendian strata with the lowermost Purella antiqua Assemblage Zone and the boundary interval with the underlying Anabarites trisulcatus Assemblage Zone of the Siberian Platform. Therefore, at least in the late Proterozoic, the Fore-Yenisei sedimentary basin was part of a larger pericratonic depositional system on the western margin of the Siberian Craton.     

Paleomagnetism of the Late Cretaceous intrusions from the Minusa trough (southern Siberia)

The paper summarizes paleomagnetic and rock-magnetic data on the Late Cretaceous diatremes and associated dikes from the Minusa trough located within the southwestern Siberian Platform. It is shown that the stable characteristic component of magnetization is superimposed magnetization (in physical sense). It is linked to Fe-rich titanomagnetite produced by the decay and oxidation of Ti-rich titanomagnetite derived from a primary magma. This process, however, coincides in time with the intrusion cooling, which is supported by paleomagnetic tests. Correlation of magnetic polarity with ³⁹Ar/⁴⁰Ar ages suggests that the acquired stable characteristic component of magnetization corresponds to magnetic Chrons C33-C32 and characterizes the Middle Campanian magnetic field (74–82 Ma). The mean paleomagnetic pole for this span is located at 82.8° N, 188.5° E, with α₉₅ = 6.1 and, within confidence intervals, coincides with the reference data from the European part of the Eurasian plate. The excellent agreement between virtual paleomagnetic poles testifies that the intraplate motions in the Mesozoic resulting in the crust deformation of Central Asia ceased in the late Cretaceous or were so small that elude detection by the paleomagnetic method.     

Sedimentation Model of Rocks of the Pokur Formation: Basis for the Prediction of Filtration-Volumetric Characteristics (Upper Cretaceous,West Siberia)

The HC-rich Upper Cretaceous rocks in the West Siberian petroliferous basin comprise genetically heterogeneous sediments, resulting in the specificity of morphology, structure, and properties of natural reservoirs therein. One of the possible techniques for more reliable predictions of reservoir rock characteristics is the utilization of results of sedimentation modeling based on the complex interpretation of borehole and seismic data. The paper presents an example of the realization of such approach for refining the structure of oil-and-gas pools in the Pokur Formation of the Russko-Chasel megaswell.     

Composition and conditions of formation of gold-telluride mineralization in the Tissa-Sarkhoi gold-bearing province (East Sayan)

The structure and petrologic composition of new gold-ore provinces in southeastern East Sayan (Tissa-Sarkhoi cluster) are considered. Several morphogenetic types of gold mineralization have been established: quartz veins with beresitization zones, veinlet-disseminated ores in granitoids, and listwaenitization and sulfidation zones in effusions of the Sarkhoi Group and intrusive rocks of the Late Riphean Khorin-Gol complex. According to geochronological dates and some mineralogical and geochemical features, the gold mineralization is close in age to these Precambrian island-arc complexes. Parageneses of two stages of ore formation have been recognized: early high-temperature (250–460 °C) gold-pyrite and late low-temperature (110–280 °C) gold-telluride. The latter mineralization is widespread and is represented by tellurides of Au, Ag, Pb, Bi, and Ni — petzite, calaverite, hessite, tellurobismuthite, altaite, and melonite. Native gold associated with these tellurides is characterized by a fineness of 750–900‰. The intimate temporal and spatial relationships of the gold mineralization with island-arc volcanoplutonic complexes and the wide occurrence of its veinlet-disseminated type suggest that this is porphyry gold mineralization related to the Late Riphean-Vendian island-arc magmatism.     

Cretaceous and Tertiary of the Iullemmeden Basin in Nigeria (West Africa)

The Iullemmeden Basin (Mali-Niger-Benin-Nigeria) of tectono-epeirogenic origin was invaded several times by epicontinental transgressions during the Cretaceous and Paleocene.Three major subdivisions are recognized in the Nigerian section of the basin (the “Sokoto Basin”): (1) the lower, continental beds (= Continental Intercalaire) of Late Jurassic to Early Cretaceous age, (2) intermediate marine and brackish water deposits and (3) “Continental T Terminal” of upper Eocene-Miocene age.The geology of the Iullemmeden Basin is reviewed and updated. The relationships between the “Continental Intercalaire” of francophone authors and the Illo and Gundumi Formations are analysed and the type sections of the latter described. It is suggested that the Illo and Gundumi Formations are lateral equivalents.Detailed information on the formations of the Maastrichtian Rima Group is presented. It is concluded that there is no age difference of consequence between the Paleocene Dange, Kalambaina and Gamba Formations.The epicontinental transgressions are interpreted as coming from the north-east, east of the Hoggar massif during the Cenomanian and Turonian and west of the Hoggar during the Maastrichtian and Paleocene. The Sokoto Paleocene molluscan fauna is considered to be terminal Paleocene to Eocene and somewhat younger than the Ewekoro association of coastal Nigeria.A new interpretation of the Continental Terminal is presented. The structural history of the Sokoto Basin is reviewed and partly re-interpreted in the light of new field evidence.     

The formation of hydrochemical indicators for the pay zones in the Surgut arch oil fields

The causes of the spatial variation in the hydrochemical indicators of groundwater in several oil fields in the Surgut arch of West Siberia are considered. On the basis of statistical processing of the database, including several thousand analyses of groundwater, histograms of the distribution of mineralization and concentrations of Ca²⁺ and HCO ₃ ^? ions through the main pay horizons of the fields are constructed. The influence of deep (hydrothermal) water inflow on the formation of the chemical composition of the stratum waters of oil fields, which is exhibited by an increasing content of HCO ₃ ^? ion with depth, is substantiated.