Biofacies of Upper Jurassic and Lower Cretaceous sediments of central West Siberia

Paleontological study of Upper Jurassic and Lower Cretaceous sediments recovered by boreholes in the Agan-Vakh and Nadym-Vengapur interfluves clarified environments of their deposition. As is shown, influx of siliciclastic material to central areas of the West Siberian sea basin varied through time. Taxonomic composition and ecological structure of nektonic and benthic fossil assemblages are analyzed and considered in terms of environmental factors such as hydrodynamics, aeration, temperature, and salinity of seawater.     

Callovian and Upper Jurassic foraminiferal and ammonite biostratigraphy of the Shaim petroleum region (West Siberia)

The results of biostratigraphic analysis of foraminiferal assemblages were used to identify a nearly complete succesion of biostratigraphic units in the Callovian-Upper Jurassic section of the Shaim region. The finds of the ammonites from different parts of the Callovian-Volgian sections helped to correlate the identified foraminiferal biostratigraphic units to the General Stratigraphic Scale. Analysis of variations in the taxonomic composition and structure of foraminiferal assemblages allowed refinement of paleontological characteristics of the foraminiferal biostratigraphic units for different facies conditions in the southwest of the West Siberian Basin.     

Thermal conductivity of sediments in high-latitude West Siberia

The thermal conductivity of rocks (λ) used previously to estimate heat flow in high-latitude Siberia was assumed to be λ = 1.8–2.0 W/(m?K), according to published evidence, but was almost never measured specially. We measured the thermal conductivity of core samples from boreholes drilled in the northeastern West Siberian Plate and in the Yenisei–Khatanga basin in the 1990s, using two advanced instruments: a comparator and a scanner of thermal conductivity. Altogether there were 305 samples of air-dry sediments (λ_d) from the 1100–4200 m interval of 23 holes, out of which 77 samples were then moistened (λ_w). The average thermal conductivities of rocks in dry and wet conditions were found to be λ_d = 1.9 ± 0.2 and λ_w = 2.6 ± 0.1 W/(m?K), respectively. Thus, the true thermal conductivity of sediments, which are saturated with water in this northern area, must be about 20–30% higher than that assumed in the previous heat flow determinations, and the latter are thus underestimated correspondingly. The updates to the thermal conductivity values imply major revision to the existing ideas of the thermal regime in the Siberian high latitudes. Such a serious revision of geothermal data will obviously require a large amount of additional experimental work.     

Hydrogeological conditions of the Pre-Yenisei petroleum subprovince

The study presents results of a hydrogeological and hydrogeological research conducted on the Mesozoic–Cenozoic sedimentary cover and Precambrian–Paleozoic platform deposits of the Pre-Yenisei petroleum subprovince. The hydrogeological structure of the study area is found to be of a transition type from the West Siberian to Tunguska artesian basins, with its own set of pertinent parameters, such as groundwater depths, rock permeabilities, water chemistry and gas compositions, gas saturation, vertical zoning, etc. The upper part of the sedimentary section is known to be flushed with the infiltration waters to depths of 2–2.5 km. The deeper intervals contain the older sedimentary waters with the moderate metamorphic grade. The results of the study provide implications for the petroleum potential of the area of interest.     

Problems of phytostratigraphy and the correlation of the Lower Jurassic continental sediments in West Siberia and Kuznetsk and Kansk-Achinsk basins

Paleofloral and palynological records of Lower Jurassic sediments in West Siberia, Kuznetsk (Kuzbass), and Kansk-Achinsk basins and their correlation are discussed. In a number of recent papers dedicated to the Jurassic stratigraphy of Siberia this problem is ambiguously treated. The reference palynological scale has been developed for the Jurassic West Siberian sediments and an uninterrupted succession of floral assemblages associated with it and with regional stratigraphic units has been recognized. On this basis the scheme of the correlation between the Lower Jurassic sediments of the Kansk-Achinsk and Kuznetsk basins and West Siberia permitting a better age estimate of coal-bearing deposits, is proposed.     

The Meso-Cenozoic tectonics and petroleum potential of West Siberia

The relationship between the petroleum potential of the West Siberian province and the Mesozoic to Cenozoic tectonic processes is analyzed. The studies were based on structural and isopach maps of seismogeologic megacomplexes compiled from generalized geological and geophysical data on the province at the Trofimuk Institute of Petroleum Geology and Geophysics as well as on the results of interpretation of regional seismic CDP (common depth point) profiles. The main stages of formation of structures of different ranks and faults have been established. It is shown that the petroleum potential of the province was determined mainly by its structure and tectonic processes at the Cenozoic stage of evolution. At that time, the Koltogory–Urengoi megatrench formed, which became the main zone of hydrocarbon generation, as well as large positive structures—petroleum accumulation zones. Also, disjunctions originated, which served as channels for hydrocarbon migration from the oil source rocks of the Bazhenovo Formation to the main Neocomian and Aptian–Albian–Cenomanian petroleum reservoirs of the province.     

Integrated electromagnetic and geochemical surveys for petroleum exploration in West Siberia

Electromagnetic soundings with controlled and natural sources (TEM and MT, respectively) integrated with IP and geochemical surveys have been tested for petroleum exploration in West Siberia. The TEM method, with loop sizes smaller than the depth to the target, provide high resolution, sufficient penetration depth, and data locality. The MT method sounds deeper earth and can place constraints on the Paleozoic basement structure and its electrical properties. The petroleum implications of IP and geochemical data are associated with secondary alteration (mineralization) of rocks over oil traps.     

Palynostratigraphy of the Lower-Middle Jurassic deposits,borehole section Vostok 4 (Southeast of West Siberia)

This work presents the results of palynological investigation of the Vostok 4 borehole section situated in the Azharma region of the Ob-Irtysh facies area, where drilling has penetrated through the Urmanskaya, Ilansk, Peshkovskaya, and Tyumen formations. Six palynoassemblages are distinguished in the section based on the analyzed distribution of spores, pollen of terrestrial plants, and microphytoplankton. According to their succession, the section is subdivided into biostratigraphic units ranked as beds with palynomorphs. Comparative analysis and correlation with the Siberian palynostratigraphic standard scale of the Lower-Middle Jurassic, where succession of palynoassemblages is coordinated with ammonite zones, substantiate the Pliensbachian-Bajocian age of the deposits studied. Diverse microphytoplankton (dinoflagellate cysts, acritarchs, prasinophytes, zignematacean and colonial algae) occurs throughout the section. The facies environment of sedimentation is reconstructed based on quantitative proportions of microphytoplankton taxa.     

岩相古地理研究与油气勘探

古地理学已具有200余年的悠久历史。它的发生、发展与沉积学和地层学的发展密切相关。古地理学的理论发展和相关的技术进步在勘探开发沉积矿产资源等方面发挥了重要作用,开拓了在河流、三角洲及滑塌浊积扇、深水重力流沉积、滩坝、礁以及碳酸盐岩建隆中找油的新领域。中国岩相古地理的研究为推动油气资源勘探开发起到了不可替代的作用,已在不同地区和地质层位中找到了极为丰富的油气资源。中国含油气盆地油气勘探表明,岩相古地理控制了油气成藏的基本要素组合和油气资源的分布,地层和岩性圈闭的发育和分布受控于古地理条件的改变。中国原油地质储量的13％、55.3％和12.6％分别分布在河流、三角洲和水下扇的沉积物中。讨论了未来岩相古地理研究应采取的指导思想、工作方法和面临的重要科学问题,指出多信息、多尺度、多元化和数字化的古地理研究必将在未来沉积矿产勘探开发和综合利用中发挥积极的重要作用。     

Formation conditions of the Middle Jurassic sandy reservoirs of the Tazovskii deposit,West Siberian petroleum basin

The exploration prospects of the deep Middle Jurassic rocks of the Tazovskii deposit are related to the specific lithological-mineralogical features of reservoirs, which are the major reserve of the region. One of the topical exploration problems is the determination of the formation conditions, which will allow the determination of the prospects of the possible petroleum potential, both of this deposit and the northern part of the West Siberian petroleum basin.     

The Cambrian sediments of the Cis-Yenisei basin (West Siberia): stratigraphy and correlation

A scheme is proposed for the stratigraphic division and correlation of the Cambrian sediments of the Upper Proterozoic-Paleozoic Cis-Yenisei basin. It is based on data from the drilling of parametric wells (Lemok-1, Averinskaya-150, Tyiskaya-1, Vostok-1, Vostok-3, Vostok-4, and others). Two structure-facies zones are recognized in the study area: Kas zone (Lemok-1, Averinskaya-150, Tyiskaya-1, Vostok-4, and Eloguiskaya-1 wells), in which the sedimentary complexes accumulated in a salt subbasin, and Ket’ zone (Vostok-1 and Vostok-3 wells) with open-sea-basin sedimentation. The boundary between the structure-facies zones passes along the reconstructed zone of a barrier reef stretching in the N-S direction. The Vostok-4 well is localized in the western Kas structure-facies zone, at the salt subbasin/barrier reef boundary. Local stratigraphic units (formations) are described and compared with the adjacent Turukhan-Irkut-Olekma facies region of the Siberian Platform.     

Regularities of distribution and composition of heteroatomic components in Paleozoic and Jurassic oils of southeastern West Siberia

Experimental data on the distribution and composition of sulfur and nitrogen compounds in oils of Paleozoic and Jurassic deposits of southeastern West Siberia have been summarized. Benzo-, dibenzo-, and naphthobenzothiophene, quinolines and benzoquinolines, thiopheno- and benzothiophenoquinolines, and benzo- and dibenzoquinolones are shown to be the main hetero-organic compounds in all the oils. It has been established that the distribution of these compounds depends on the formation conditions of natural hydrocarbon systems and on the degree of their catagenesis.     

Main phytostratigraphic boundaries in the Jurassic deposits of Western Siberia

The study of the large collections of plant remains gained from cores of numerous boreholes drilled in Western Siberia made it possible to determine the taxonomic composition of the Jurassic flora of this region, the stages of its evolution, and the sequence of floral assemblages, which characterize the regional stratigraphic horizons indirectly correlated via series of parallel faunal, microfaunal, spore and pollen zonal scales with a general stratigraphic scale. The compositions of floral assemblages was established in the Hettangian-lower part of the upper Pliensbachian, upper part of the upper Pliensbachian, lower Toarcian, upper Toarcian, Aalenian, Bajocian, Bathonian, and Callovian-Oxfordian sediments. Criteria were elaborated to substantiate the Triassic-Jurassic and Lower-Middle Jurassic boundaries. Lithologically and biostrati-graphically, the Middle-Upper Jurassic boundary is poorly expressed.     

Comparative characteristics of the lithological composition of the incised valley filland host sediments of the Vikulovo Formation, Kamenny area, West Siberia

Comparative characteristics of the lithological composition of estuarine incised valley fill, as well as the enclosing and overlying storm-related deltaic sediments (beds VK₁, VK₂, and VK₃), in the productive part of the Vikulovo Formation (upper Aptian), Kamenny area, West Siberia are given. It is shown that the incised valley fill-reservoirs, which are characterized by a coarser structure, inferior sorting, lower mineral maturity, and predominant kaolinitic clayey cement, demonstrate higher filtration properties.     

Geochemical characterization of organic matter from the Middle Jurassic sediments of Western Siberia on the basis of the composition of Alkylaromatic hydrocarbons

The composition and distribution of alkyl-substituted bi- and tricyclic aromatic hydrocarbons were studied in dispersed organic matter (DOM) samples from four levels of the Middle Jurassic sequences of the southeast of Western Siberia. It was shown that differences in the distribution of alkylnaphtalenes and alkylphenanthrenes were caused by catagenetic factors, which promoted the occurrence of the secondary reactions of isomerization, alkylation, and dealkylation. The data on the composition of saturated and alkyl aromatic hydrocarbons were examined by principal component factor analysis, the results of which were used to divide the samples into natural families. The grouping of DOM samples is based on the similarity of their characteristic compositional parameters. It was shown that the families include samples from different units occurring at different depths and mostly in widely separated localities. This indicates that sediments accumulated and were transformed under similar conditions at different time periods and in different parts of the petroleum-bearing regions of the southwestern part of Western Siberia.     

Neocomian paleogeography,gas hydrate cementation of sediments,and abnormal sequences of the Bazhenov Formation (West Siberia)

Abnormal zones of the Bazhenov Formation originated in the Neocomian as a result of protobazhenite reworking by submarine slide and slump waste wedgings, which eroded and deformed slope toe protobazhenite. But gas hydrate (GH) cementation might restrict the rock ability for plastic deformation. The conditions for GH thermodynamic stability in protobazhenites are inferred from reconstructions of paleogeographic and paleo-oceanic evironments. Joint analysis of Neocomian marine paleodepths and deep water paleotemperatures provides an explanation of the Bazhenov abnormal-zone extension.     

Palynofacies in upper cretaceous sediments of northern Siberia

Different palynomorph groups (spores and pollen of terrestrial plants, dinoflagellate cysts, prasinophytes, acritarchs, Zygnemataceae algae, and others) have been thoroughly studied to define major patterns in their distribution depending on the impact of different environmental factors and to establish their paleoecological characteristics. The comparative analysis of palynomorph assemblages from coeval Cenomanian-Coniacian sediments of the Ust’-Yenisei area, Berezovskaya 23k, Yuzhno-Russkaya 113, Leningradskaya-1 boreholes and Santonian-Campanian sequences of the Ust’-Yenisei, Khatanga and Polar Urals regions reveals transgressive-regressive cycles, which are best evident in coastal sections and smoothed in their marine counterparts. The biofacies and compositions of palynomorphs form the regular succession from the periphery toward central parts of the West Siberian basin. The facies successions in Santonian-Campanian sections of the eastern and western parts of the basin are inconsistent with each other, which may be explained by influence of both the West Siberian and Russian seas on sedimentation in its western areas.