Home – Home – Jornals – Russian Geology and Geophysics 2014 number 5-6

The study provides an overview of key trends underlying the long-term and medium-term scenarios in the world energy outlook and prospects of fossil fuel resource depletion in the 21st century.

Almost all gas-oil and oil-gas fields in the Lena-Tunguska province (Siberian Platform) were discovered in the Baikit, Katanga, and Nepa-Botuobiya petroliferous regions, which form an E-W-striking band. The latter, named “the main petroliferous belt,” includes one giant and 11 large fields (by recoverable oil reserves). New large oil pools are predicted within the main belt. The prediction is based on the analysis of geological data and the quantitative estimation of hydrocarbon resources. Promising areas with large fields in the Baikit, Katanga, and Nepa-Botuobiya petroleum regions are substantiated.

The results of more than 40 years long authors’ investigations in the field of the freshwater (river input) and marine (ocean waters) hydrospheres are summarized. The latest estimations of the global average concentrations of many chemical elements in river water and suspended matter and in ocean water and suspended matter are presented. It is shown that particulate suspended forms of many elements are predominant in river waters (“rivers are the kingdom of suspended forms of elements”), while their dissolved forms prevail in ocean waters (“ocean is the kingdom of dissolved forms of elements”). Sedimentary and biogeochemical processes of the river material transformation in the river-sea mixing zone (the so-called “marginal filter of the ocean”) were studied thoroughly. It was shown that radical quantitative and qualitative changes of dissolved and particulate suspended substances take place in this zone, resulting in the governed transformation of dissolved forms into suspended particulate forms and their following deposition on the bottom. The first data on the losses of 35 chemical elements in the river-sea mixing zone are presented. These data prove that the concentrations of dissolved elements in river and ocean waters are in regular and close relationship with their losses in the river-sea mixing zone and with the types of element distribution in ocean water column (conservative, biogenic, and lithogenic). This indicates the existence of a geochemical system in the entire (freshwater and marine) hydrosphere, which calls for deep and detailed investigations.

For paleogeographic reconstruction of the West Siberian basin during the Cretaceous we used a set of paleogeographic maps, which were compiled for the main epochs of the Cretaceous period. The paleogeographic maps presented in this study suggest progradational filling of the deep basin with avalanche-type sedimentation during Volgian-Barremian regression. The paleorelief and provenance of terrigenous sediments were reconstructed.

An integrated lithofacies analysis of the Middle-Upper Bathonian petroleum horizon J₂ in the northeastern part of the latitudinal Ob’ region was carried out. The new data were used to supplement and refine the available data. The recognized lithofacies and their associations clearly mark off the boundaries of reconstructed sedimentation conditions. A petrographical study of the silt-sandy rocks of the horizon was carried out with a detailed calculation of their grain size and petrographic compositions. Grain size coefficients were analyzed with the use of genetic, dynamic, and dynamogenetic diagrams. The new data permitted a considerable refinement of the reconstructed sedimentation conditions of the horizon. It has been found that the transgressive change of sedimentation conditions is reflected in regular changes in some characteristics of the studied sediments: a decrease in the total number and thickness of coal interbeds, an increase in the degree of sediment bioturbation, a change of ichnofossils, and an increase in the amount of pyrite. More detailed paleogeographical schemes have been constructed for the formation of the lower (continental), middle (transitional), and upper (littoral marine) parts of the horizon. They provide a better understanding of the regularities of development of the Middle–Late Bathonian sedimentary basin in the study area.

Based on results of deep drilling and the CDP-2D seismic profiling, the relations between the Early and Middle Cambrian carbonaceous rocks and the underlying, overlying, and synchronously formed deposits in the Siberian Platform are analyzed, and the lithologo-paleogeographic, paleotectonic, and tectonic conditions of their formation are considered. It is shown that these carbonaceous rocks are intimately related to the Cambrian organogenic structures, up to their mutual transitions.

Stable carbon isotope variations in primarily offshore Proterozoic carbonates of the Eselekh, Neleger, and Sietachan Formations in the Kharaulakh Range of northern East Siberia provide important information on the depositional history of the Riphean complexes and allow an age estimate to be made for potentially petroliferous Precambrian strata in the northeast of the Siberian Platform. The results of petrographic, geochemical, and isotopic studies of the measured samples demonstrate that the carbonates recrystallized without substantial postdepositional alteration of the carbon isotope system and that the acquired δ¹³С values are accurate and can be used for the purposes of chemostratigraphy. The Riphean strata of the Kharaulakh Range are characterized by mostly high (5.5–8.6‰) δ¹³С values. Based on the carbon isotope data, the studied section could not be correlated with Mesoproterozoic strata of the Anabar and Olenek uplifts but occupies a rather higher stratigraphic position. It can be correlated with the Baikal Group of the West Pre–Baikal Area and the Dal’nyaya Taiga Group of the Patom upland; specifically, the negative shift in the uppermost Sietachan Formation possibly corresponds to the Zhuya negative excursion. Comparison with the model curve of carbon isotope evolution in the Precambrian ocean suggests that the age of the studied section does not exceed 820 Ma. Most likely, the studied strata are younger than 635 Ma (i.e., postdate the Marino glaciation) but older than the Gaskiers glaciation (580 Ma).

Consolidated crust in the North Barents basin with sediments 16–18 km thick is attenuated approximately by two times. The normal faults in the basin basement ensure only 10-15% stretching, which caused the deposition of 2–3 km sediments during the early evolution of the basin. The overlying 16 km of sediments have accumulated since the Late Devonian. Judging by the undisturbed reflectors to a depth of 8 s, crustal subsidence was not accompanied by any significant stretching throughout that time. Dramatic subsidence under such conditions required considerable contraction of lithospheric rocks. The contraction was mainly due to high-grade metamorphism in mafic rocks in the lower crust. The metamorphism was favored by increasing pressure and temperature in the lower crust with the accumulation of a thick layer of sediments. According to gravity data, the Moho in the basin is underlain by large masses of high–velocity eclogites, which are denser than mantle peridotites. The same is typical of some other ultradeep basins: North Caspian, South Caspian, North Chukchi, and Gulf of Mexico basins. From Late Devonian to Late Jurassic, several episodes of rapid crustal subsidence took place in the North Barents basin, which is typical of large petroleum basins. The subsidence was due to metamorphism in the lower crust, when it was infiltrated by mantle–source fluids in several episodes. The metamorphic contraction in the lower crust gave rise to deep–water basins with sediments with a high content of unoxidized organic matter. Along with numerous structural and nonstructural traps in the cover of the North Barents basin, this is strong evidence that the North Barents basin is a large hydrocarbon basin.

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.

We consider the formation history of high-amplitude anticlinal trap structures for unique Cenomanian gas pools using the example of the Medvezh’e field. Analysis shows that the Turonian-Cenozoic was the most crucial stage of formation of the world’s largest gas–bearing province in northern West Siberia. This stage was marked by (1) the formation of large anticlinal traps — petroleum–promising objects; (2) the highest intensity of gas formation, which caused the filling of these traps; and (3) the formation of the Kuznetsov seal, which shields the giant Cenomanian gas pools.

We discuss the current understanding of the Vendian regional stages in the northeast of the Nepa–Botuobiya Anteclise, establish a new correlation of the Vendian sections across different facies, and propose stratotypes for the introduced stratigraphic units. A new stratigraphic chart is constructed for the Vendian terrigenous strata in the northeast of the Nepa–Botuobiya Anteclise and the adjacent territories.

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.

Application of stochastic simulation to study of the lateral migration of primary hydrocarbon pools near the roof of a collector is considered. It is shown that this approach can be used for point and interval estimations of oil accumulation in a trap and the formation of residual oil saturation.

We summarize and analyze the available data on naphthide shows in continental zones of active hydrothermal and modern volcanic activity (Uzon Volcano caldera, Yellowstone National Park, and New Zealand springs) and examine their similarity and difference. The analysis demonstrated that hydrothermal naphthides formed from lipids of living matter of different nature: phytoplankton, bacterial communities, archaea, and remains of higher land plants, including spores and pollen, which might have been supplied to the sediment through eolian transportation. Hydrothermal naphthides are different in the degree of maturity, but in general they are less transformed than most of basin oils. Their group composition and distribution of n-alkanes evidence that they were subject to hypergenesis, which led to the loss of light fractions, oxidation, and biodegradation.

The type of source organic matter of West Siberian oils localized at depths of 1425 to 3950 m in Cretaceous, Jurassic, and Paleozoic deposits is determined based on the composition of bio- and geomarkers. The individual and group compositions of alkylnaphthalenes of these oils are studied. The geochemical parameters of oil transformation are calculated from the composition of methyl-, dimethyl-, and trimethylnaphthalenes. Data on the individual composition of alkylnaphthalenes are processed by the method of principal components of factor analysis. The matrix is compiled of 44 oils and 22 characteristics (contents of individual alkylnaphthalenes). The molecular-mass distributions of alkylnaphthalenes in oils of different genesis served as a basis for the separation of the latter into families according to the compositional similarity of biarenes and the degree of transformation.

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 dibenzoquinolines are shown to be the main heteroorganic 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.

The distribution of biomarkers in bitumens of source rocks of different ages in the southern Siberian Platform is studied to elucidate the correlation between oils and their sources. It is shown that the bitumens of Riphean, Vendian, and Lower-Middle Cambrian source rocks are different in some parameters reflecting the type of primary biogenic material, first of all, in the distribution of n-alkanes and acyclic isoprenoids and in the portions of steranes and triterpanes in polycyclic naphthenes. The revealed difference in the composition and distribution of biomarkers in bitumens of source rocks of different ages can be used to identify the sources of hydrocarbons in dissolved pools.

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.

Coupled geomechanic and fluid flow analysis is applied to develop theoretical and experimental background for geosteering and well logging to diagnose a state of near-wellbore formation and oil reservoir. The suggested methods of processing EM induction (VIKIZ) and Russian laterolog (RLL) resistivity responses account for effects of stress-dependent fluid flow changes. The geomechanic and fluid-flow effects on resistivity well logs are investigated for the case of asymmetric invasion and related irreversible deformation. Ways of taking into account stress-dependent effects on resistivity logs are developed on the basis of 3D modeling of geomechanic and electromagnetic fields.

We analyze the results of measurements of the thermal conductivity, porosity, and permeability of Mesozoic sedimentary rocks from wells drilled in the northeast of the West Siberian Plate and in the Cis-Yenisei basin. The thermal conductivity was measured by “Thermal-conductivity comparator”, and the porosity and permeability were determined using standard techniques. A total of 135 samples of dry rocks were studied for thermal conductivity (λ_d). Ninety-two of them were then saturated with water, and their thermal conductivity was measured again. The average thermal conductivity of dry and water-saturated (λ_w) sedimentary rocks is 1.8–2.0 W/(m·K) and 2.4–2.8 W/(m·K), respectively. Porosity factor (Kpor) was measured for 95 samples, and permeability (K_perm), for 44 samples. Comparison of the results of measurements revealed stable linear correlations between the thermal conductivity of dry samples and λ_w, Kpor, and K_perm. The established dependences are proposed for the express evaluation of the thermal (λ_w from λ_d and Kpor values) and structural (Kpor and K_perm from λ_d values) properties of sedimentary rocks, though the revealed correlations need further refinement.

Algorithms and software have been designed for petrophysical 2D forward simulation and inversion of induction logs from oil wells drilled in shaly sandstone using conductivity models for different clay types. As comparative analysis of relative amplitudes and phases of tool responses has demonstrated, it is possible to estimate reliably and accurately fluid saturation and porosity from induction logs. The applied 2D numerical inversion of both theoretical and well logs yielded quantitative estimates of petrophysical parameters of reservoirs, with clay volume fraction assumed according to the chosen conductivity model. Correction for the presence of clay improves inversion quality if clay content as well as type are taken into account.