Home – Home – Jornals – Russian Geology and Geophysics 2011 number 8

The Arctic shelf of Russia bears huge oil and gas resources. Twenty-seven oil and gas fields (including ten sea-coastal) were discovered in the Barents (with the Pechora) and Kara Seas. Today, however, none of the offshore fields is developed, and the overall degree of the geological and geophysical study of the shelf is still low. A complex of actions is proposed to accelerate the exploration and development of oil and gas resources of the Russian Arctic seas.

Oil shows from the thermal springs of the Uzon volcano caldera have been studied by gas chromatography-mass spectrometry methods. Based on the composition and distribution of biomarker molecules, their genetic identity with the organic matter of Pliocene-Quaternary deposits has been established. It has been shown that the Uzon caldera is a unique natural laboratory of the present-day oil formation from the organic matter of Pliocene-Quaternary sediments. It has been stated that attempts to consider the compounds forming these oil shows as a product of hydrothermal abiogenic synthesis are absolutely unfounded.

The paper deals with the geochemistry of lanostane homologue molecules, the age and localization of organic matter (OM) enriched in lanostane biomarkers, and organisms synthesizing lanosterol and other lanostane compounds.
Steranes of the OM bitumens of the Lower Cambrian Sinyaya Formation on the northern slope of the Aldan syneclise were studied by chromato-mass-spectrometry. Lanostanes C₃₀ have been found for the first time in the organic fossil of the Cambrian sedimentary basins, and the presence of norlanostanes C₂₉ is assumed.
The sedimentation and diagenesis conditions and the degree of maturity of OM containing lanostane and its derivates are considered. Lanostanes are proposed to use as biomarkers of the petroleum-generating rocks of the Sinyaya Formation for elucidating the source of naphthides in the eastern Siberian Platform.

The current geological and geophysical exploration maturity and the structure of resource base have been presented for the West Siberian Basin. The patterns of oil and gas content distribution, both in areal extent and vertical, have been considered throughout the entire cover and its constituent oil-gas plays. Structural constructions for four major stratigraphic levels have been analyzed: the bottom of the Mesozoic-Cenozoic cover, the Jurassic and Lower Aptian top, and the bottom of Turonian deposits. The distribution patterns of oil and gas accumulations in each oil-gas play are controlled by a specific combination of parameters such as the distribution of reservoirs and seals, the presence of oil-and-gas source rocks, the level of organic matter maturity, facies and tectonic conditions, thermobaric and hydrogeochemical subsoil environments, etc.

Modern views on the structure of the main productive Cenomanian complexes in the north of the Siberian Platform are presented. Main zones of oil and gas accumulation are determined. An appraisal of the predicted resources is given, and methods for their development are recommended.

The western part of the Yenisei-Khatanga trough is one of the least explored regions, which are thought to possess excellent hydrocarbon resource potential. This region is geographically located in the southern part of the Taimyr Peninsula, within the Krasnoyarsk Territory.
Utilizing regional seismic data, structure and thickness maps for seismic sequences allowed the reconstruction of the structural framework and tectonic evolution of the region. The study reveals a number of tectonic processes in the region that took place in the Paleozoic, Mesozoic, and Cenozoic and establishes the main evolutionary stages of a series of large structures of different ranks.
The study also provides some insights into the structural features and depositional environments of Paleozoic, Triassic-Jurassic, Neocomian, Aptian-Albian-Cenomanian, and Turonian-Cenozoic sedimentary complexes. Based on the relationship between tectonic activity and hydrocarbon generation, the conclusion was made about a high hydrocarbon resource potential of the Neocomian clinoform complex in the Yenisei-Khatanga regional trough.

We show the present state of the set of parallel zonal scales for the Siberian Jurassic, based on various fossil groups, and the principles of their construction. We discuss the significance of Siberian biostratigraphic scales for the Boreal zonal standard of the Jurassic units. The stratotype region for this standard must have a typical Boreal (Arctic rather than mixed) fauna. A possible candidate is Siberia (and the Arctic biochorema), which is located in the center of the Panboreal Superrealm, where the set of interrelated scales for various fossil groups is the most complete. The set of parallel zonal scales for the Siberian Jurassic is efficient for the subdivision and correlation of Jurassic units in various Arctic regions (Barents Sea shelf, northeastern Russia, Arctic Alaska, Arctic Canada).

On the basis of the integrated interpretation of seismic-prospecting, GIS, and deep-drilling data in the southern parts of the Kaimysovy arch and Nyurol'ka megadepression, structural and isopach maps of seismogeologic megacomplexes are drawn. The tectonic evolution of the area in the Mesozoic and Cenozoic is analyzed. Ruptures have been recognized and traced throughout the study area, and the main stages of formation of faults and structures of different ranks have been established.
The conclusion has been drawn that, in terms of tectonics, the petroleum potential of the area was affected mostly by the Early Neocomian, when large tectonic elements (petroleum generation and accumulation zones and local uplifts (hydrocarbon traps)) formed in the Upper Jurassic beds. In the Kaimysovy petroliferous region, the structure formation was accompanied by rupturing; the ruptures broke through the Jurassic strata, thus determining the structural-tectonic position of most of the hydrocarbon pools in the area.

The evolution of views of the stratigraphy of Berriasian-Lower Aptian deposits of West Siberia is described.
In connection with a clinoform model of the section structure, suggestions are made for its stratification; according to the new recommendations to the stratigraphic scheme, paleogeographic reconstructions were carried out for the four stratigraphic subdivisions identified within the Berriasian-Early Aptian play.

In this study we report new Ar-Ar ages for basalt (261.0 ± 15, 249.4 ± 3.0, 247.0 ± 3.1 Ma) and rhyolite (246.9 ± 2.6 Ma) samples collected from the Triassic volcanosedimentary sequence, which is one of the remarkably complete and structurally complex sections in the southwest of the West Siberian geosyneclise (WSG) (East Kurganskaya-42 (EK-42) and East Kurganskaya-43 (EK-43) wells), largely comprising basalts and conformable layers of rhyolites, acid tuffs, limestones, and silty mudstones. It was shown that basalts, which constitute most of the section, are geochemically similar to medium-K tholeiitic and calc-alkaline basalts in the southern extension of the Koltogory-Urengoi rift (Nikol'skaya-1 well). Rhyodacites and rhyolites from the section of interest belong to the potassic to potassic-sodic series of normal alkalinity. The trace element compositions of the rhyolites and host basalts suggest fractionation from primary magmas during formation of felsic rocks. No low-K sheet-flow basalts have been encountered in the sections penetrated by the three wells (EK-42, EK-43, and Nikol'skaya-1), as opposed to sections in the northern part of the Koltogory-Urengoi rift (TSD-6 well) and the Siberian flood basalt province.
Despite their small thickness, the sedimentary units provide evidence for different depositional settings during the evolution of a volcanosedimentary basin. Similar to the Koltogory-Urengoi rift, deposition of the Presnogor'kovsk graben fill indicates intermittent marine conditions with accumulation of hyaloclastites, and fine, calcareous, silt- and clay-rich carbonaceous oozes associated with a background sedimentation, which contain abundant marine fauna, pyrite, and rare distal turbidites with glauconites. The geochemical features of these sedimentary rocks were largely defined by a bimodal syndepositional volcanism. During the Early Triassic, a system of rifts and grabens that was developed within the continental plateau over much of the WSG allowed the intermittent ingress of sea water from the north (in present coordinates) far southward and southwestward up to the latitude of Omsk and Kurgan. As a result of extensive erosion of the continental plateau, the packages of low-K sheet-flow basalts were removed from the upper part of the sections of deep wells, which became dominated by the eroded roots of the West Siberian traps (subalkaline and alkaline rift basalts).

The sedimentary cover of the Timan-Pechora petroleum basin records several phases of tectonic activity in the area: rifting and incipient ocean opening; a passive-margin setting with intracontinental rifts and aulacogen, an intrashelf depression and inversion swells; collisional orogeny with the development of a foredeep; vertical isostatic movements associated with the development of a late syneclise and renewed orogenic movements. The deposition of source and reservoir facies occurred during the divergent phases of the tectogenesis (Ordovician-Tournaisian). The formations that accumulated during a convergent tectonic regime (Visean-Triassic) comprise mostly reservoir rocks. Because of the thermal maturity of organic matter, the sedimentary rocks in the area span the broad range of oil and gas generation zones. The identified hydrocarbon kitchen areas were developed in different geodynamic settings: in the east of a passive margin, within rift-related troughs, and in the Ural foredeep. The first two were more liquid-prone fairways, with predominant oil and condensate generation, whereas the latter was largely a gas- and gas-condensate-generative source. The change of structural settings, the extension of oil and gas kitchens, variation of the regional dip, and recurrent faulting caused both the formation of new play fairways and the destruction of existing ones. The new oil-prone fairways that formed synchronously with the onset of intense oil generation might have later evolved into oil- and gas-prone fairways or even oil-, gas-, and condensate-prone fairways at the expense of the gas component of petroleum systems, which is thought to increase as a source rock generating mostly oil became capable of generating oil, gas, and condensate. In addition, tectonically active zones may provide the most favorable conditions for the formation of gas and gas-condensate accumulations owing to a decrease in formation pressure and expulsion of a free gas phase or owing to its updip migration from great depths. Analysis of hydrocarbon generation and accumulation conditions provided insights into the evolution of a petroleum system of each play and transformation of a sedimentary basin into a petroleum basin. The study revealed the areal distribution patterns for oil and gas plays. The long-lived oil-prone plays are confined to a series of paleouplifts within the Izhma-Pechora and Khoreiver tectonically stable blocks. Oil-, gas-, and condensate-prone plays are associated with a tectonically active setting (Pechora-Kolva aulacogen and Timan block), whereas mostly gas- and condensate-prone plays are found within the Ural foredeep.

The proved, revealed, and predicted petroleum accumulation zones in the Lena-Tunguska province are the areas where prospecting and search works are carried out.
We described the petroleum accumulation zones in the South Tunguska, Baikit, Sayan-Yenisei, Angara-Lena, Katanga, and Nepa-Botuobiyan petroliferous areas and in the Lower Angara petroliferous district. Oil and gas pools have been discovered in the Riphean, Vendian, and Upper Vendian-Lower Cambrian petroliferous complexes.

We analyze new geological and geophysical data, which confirm Trofimuk's (1960) high evaluation of the petroleum potential of the Silurian-Devonian sediments in the northwestern Siberian Platform. Priority targets are set for geological exploration aimed at testing his scientific predictions.

On the basis of available sedimentological, isotope-geochemical, and geophysical data, we have refined the intrabasinal correlation and the structure of Mesoproterozoic deposits of the Baikit anteclise. A pre-Vendian section was stripped within the Baikit anteclise, which can be correlated with the section of the Mesoproterozoic Teya and Sukhoi Pit (pre-Pogoryui) Groups of the Yenisei Ridge. Most of the overlying deposits of the Tungusik Group were destroyed during the tectonic events in the Early Neoproterozoic (~860 Ma) and pre-Vendian time.
Using the results obtained, we have refined the variations in the sedimentation environments of Mesoproterozoic deposits in the Baikit anteclise and Yenisei Ridge and constructed paleogeographic schemes for particular time intervals in the modern frame of references. The predominance of peritidal complexes and numerous signs of subaerial exposition indicate that the carbonate accumulation was periodically interrupted. In the periods (sometimes, long) when this process stopped, fine silicoclastic material was transited fr om the shelf edge into a deep basin localized southwest of the Baikit anteclise. The longest transition periods were in Early Yurubchen, Dolgokta, and Late Vingol'da time, though the main flow supplying silicoclastic material to the Yenisei Ridge passed south of the Baikit carbonate shelf.
In the Mesoproterozoic, the basin with predominantly carbonate sedimentation occupied a vast area in the western Siberian Platform, extending to the western margin of the Anabar Shield, wh ere the Mesoproterozoic deposits were similar in structure and isotope-geochronological and isotope-geochemical characteristics to the pre-Vendian deposits of the Baikit anteclise. Most part of the area with carbonate sedimentation in the Baikit anteclise was remote from the shelf edge. The Mesoproterozoic shelf was much wider than its fragments in the recent structure on the western margin of the Siberian craton.

The isotopic criteria used to predict the hydrocarbon phase composition were justified by a comparison of regional trends in the δ¹³С values for hydrocarbon liquids and gases from Riphean and Vendian-Cambrian deposits of the Lena-Tunguska petroleum province. The δ¹³С ratios for liquid hydrocarbons and gases, which are defined by kinetic isotope effects due to kerogen cracking during petroleum generation, were used as a criterion for establishing a genetic relationship between different hydrocarbon phases.
The study has revealed that oil- versus gas-prone areas exhibit significant differences in the δ¹³С values and the reverse isotope distributions in the liquids and gases. In the oil-prone areas, methane becomes progressively enriched in ¹²С as compared to oils and condensates, which is the normal trend of carbon isotopic compositions of liquid hydrocarbons and gases generated by organic matter of the same type and the same maturity level. In the gas-prone areas, the δ¹³С values for methane either overlap with those of liquids or methane may show a general trend of enrichment in ¹³С relative to liquids. This relationship suggests that gases were probably generated and migrated later than oils, which may provide explanation for predominant gas accumulation in the study areas.
The results of the isotopic studies and analysis of the geologic history of the region allowed the recognition of hydrocarbon kitchen areas that were potentially expelling late-stage gas to mostly gas-prone areas.
The proposed isotopic method, i.e., a comparison of variations in carbon isotopic compositions of hydrocarbon liquids and gases, allowed us to establish a genetic relationship and discriminate between different hydrocarbon phases on the basis of the δ¹³С ratios. This can provide additional information about the timing of liquid hydrocarbons and qases generation and help to establish the likely locations of hydrocarbon kitchen areas and the history of hydrocarbon accumulations, thus improving the reliability of predictions for hydrocarbon resource potential in the study areas and the phase composition in each accumulation.