Home – Home – Jornals – Russian Geology and Geophysics 2004 number 1

Numerous large and giant oil and gas discoveries have been reported recently from deepwater basins along oceanic passive margins, high-rank exploration plays of the past decade. These basins originated by a single mechanism as a result of continental rifting, continent breakup and spreading. Their histories differed only in the time of rifting and spreading, the two major events that controlled the stratigraphy and composition of the basin fill. Basin evolution went through successive rifting, early spreading, and developed spreading stages. The first and third stages produced high-quality source and reservoir rocks; salt seals formed at the second stage were deformed by faulting and salt diapirism at the third stage. Source rocks are either oil-producing black shales or organic-rich, often fatty, gas producers. According to the world experience, the greatest potential in Russia can be expected from the Bering and Okhotsk deepwater margins along the Koryak and Sakhalin shelves, and the Nansen basin in the Arctic margin.

A new concept of a "living ocean" is based on thousands of analyses of suspended particulate matter from all oceans, in situ studies of particle fluxes and sedimentation rates, and mass estimates of present and past (in cores) bottom fill.
The reported results show that sediment input from rivers is dramatically reworked in the living ocean by plankton and benthos. At the first stage, ~ 93% of material (world average) is captured in marginal river/sea filters; outside the marginal filters, elements, including hydrolysates, exist mostly in the dissolved form.
Abundant phytoplankton transforms the soluble elements that penetrate through the filters into suspended matter (biological pump 1), and zooplankton organisms then filter it out from water and aggregate into pellets to transport down to the sea floor (biological pump 2). The pumps 1 and 2 act as filters and chemical barriers at the same time and rework the entire World ocean water volume in six months. The third biological pump - benthos filters, detritus collectors, and mud eaters - acts as fast and terminates the biological rework of sediments, to be continued by bacteria in sea floor deposits.
Oceanic sedimentation occurs at the account of material supplied from the biosphere, as well as from the atmosphere, the hydrosphere, the cryosphere, and the Earth's interiors, and water-, wind- or ice-borne, or mantle-derived sediments predominate in different environments. Sediment material is produced, transported, and deposited under the effect of biogenic processes, the previously neglected key agent in chemical and physical cycles in the ocean. The reported results of advanced studies of particulate matter from water, air (aerosols), marine and continental ice (cryosols), hydrothermal plumes, and ocean-island volcanoes open up new avenues for revealing the true course of oceanic sedimentation.

The mapped heights of gas in traps and depths of occurrence of the traps in the northern and arctic regions of West Siberia have been analyzed. On the basis of this analysis, conclusions were made about the main factors controlling the formation and localization of the giant Urengoi-Yamburg gas-bearing zone. A similar anomalous gas-bearing zone with largest reserves in the Cenomanian deposits is predicted to exist in the Kara Sea (the south-southwestern Yamal oil- and gas-bearing area).

In the Early Triassic, the Siberian territory underwent intense continental rifting, which resulted in submergence of the Earth's surface and formation of the West Siberian rifting sedimentary basin in the Mesozoic and Cenozoic.
Tectonic events that lasted throughout these eras led to the formation of three structural stages in the sedimentary cover.
The Triassic rift structures and fold-block structures of Baikalides, Salairides, Caledonides, and Hercynides making up the basement determined the morphology of structures in the Meso-Cenozoic cover, especially of the lower-plate deposits of Middle Triassic through Bathonian age.
More than 200 hydrocarbon pools have been discovered in the Lower-Middle Jurassic deposits. Oil pools occur in the central part of the province, whereas gas and gas-condensate ones, in the northern and arctic regions of the plate. The pools become complicated down the section, from simple anticlinal to nonstructural, with lithologic screens. The development of marine, littoral, and continental sediments, favorable geochemistry of dispersed organic matter, and its catagenesis are responsible for the high hydrocarbon potential of the Lower-Middle Jurassic deposits of the West Siberian petroliferous province.

The Shchuch'ya inlier, geomorphologically highlighted against the plain topography of the southwestern Yamal Peninsula near the northern subduction of the Polar Urals, is a link between the buried structures of the basement of the West Siberian geosyneclise (WSG) and geodynamically well explored Paleozoic structures of the eastern slope of the Urals. Within the entire Uralo-Mongolian Fold Belt, this inlier displays unique denudation. In addition, there are numerous quarries along the railway and automobile road from the town of Labytnangi to northern Yamal. Such a profitable situation, supplemented with data of prospecting and core drilling, makes this area a key object for studying a complete set of environmental facies, stages of development, and conditions of the formation of island-arc complexes in the geochronological range from Ordovician to Carboniferous.
Results of the complex study of the Shchuch'ya inlier by detailed biostratigraphy, structure-tectonic mapping, lithogeodynamic and facies analysis, petrology, petrogeochemistry, and organic geochemistry permitted us to reconstruct the settings and evolution stages of sedimentogenesis and petrogenesis, to corroborate the island-arc nature of structure-formational complexes of this territory, and to reveal their most important morphological, structural, facies, and petrochemical indicator characteristics, which can be efficiently used for diagnosis of geodynamic settings and interpretation of drilling and geophysical data on other areas of the WSG. The specific features of the explored complexes are regular lateral and vertical combinations of thick carbonate massifs with volcanic, terrigenous, and siliceous rocks, the presence of slope facies (thick clastic mixtites and fine-terrigenous gradational gravitites) around organogenic reefs, spatial closeness of deep- and shallow-water facies due to contrasting bathymetric differentiation of the paleobasin. We have established microfacies types of organogenic reefs, conditions of formation of zones of gravitational, hypergenic, and tectonic desintegration of rocks as potential reservoirs, molecular composition, distribution, and catagenesis of organic matter in reefs and near-reef facies as factors contributing to their oil-producing potential. Comparative analysis of geological and geophysical data on other regions of the WSG shows that the island-arc complexes with thick organogenic bodies are rather widespread in the Paleozoic stage of the WSG.

A new approach to detection and mapping of oil and gas traps in the Callovian-Tithonian section on the basis of combined seismic, log, and core data has been tested in a field within the Vasyugan Formation. The sand bed U¹₁ in the upper Vasyugan Formation, with variable lithology and facies and mostly combination structural-depositional traps, is the most prolific hydrocarbon reservoir in southeastern West Siberia.
The reservoir quality of U¹₁ and traps in it were predicted using special seismogeological criteria developed through interpretation of synthetic geological and geophysical data, including mathematical modeling of wave fields. The same approach to mapping and reservoir quality evaluation, adapted to different geological conditions, was successfully tested in other fields of central and southern West Siberian petroleum province.

The most probable geodynamic factors affecting the accumulation of oil and gas in the volcanosedimentary cover of the Siberian Platform are considered. Among them are transregional lineaments and depocenters of downwarping, rift and overthrust zones. The effect of various geodynamic factors must be taken into account in prediction of new petroliferous objects and substantiation of the rational ways of petroleum prospecting.

A new tectonic map of the Vendian-Silurian stage in the Lena-Tunguska petroleum province has been compiled following special principles and methodology, on the basis of the pioneer digital version of the 1:1,000,000 structural map of the Vendian-Cambrian surface for the same region. Mapping revealed the distribution and morphology of tectonic elements of different hierarchies.

Riphean source beds much contributed to the high hydrocarbon potential of Precambrian deposits on the Siberian Platform. Large amounts of hydrocarbons were accumulated in reservoir traps of the Angara-Lena and Nepa-Botuobiya petroliferous provinces. In the Angara-Lena province, gas fluids played a leading role in the formation of hydrocarbon pools. We consider a geological model, formation conditions, porosity and permeability, and regularities of occurrence of sand reservoirs in the Parfenov producing horizon of the giant Kovykta gas-condensate deposit. The hydrocarbon resources of the deposit are estimated at 2.5-3 x 10¹² m³.
The Nepa-Botuobiya province is highly promising for large hydrocarbon accumulations. Considerable amounts of hydrocarbons are concentrated there in a subsalt carbonate complex. The geologic structure of the Verkhnechonskoe deposit is considered. The estimates show that the petroleum resources of the deposit can be increased 2-2.5 times, i.e., to 400-500 mln tons.
The discovery of large and giant petroleum deposits in the Precambrian deposits of the Siberian Platform provides a base for realizing a large-scale program on the development of new centers of oil and gas extraction in East Siberia.

A base of petroleum resorves has been constructed in the Sakha Republic (Yakutia), which is sufficient not only for satisfying the domestic needs but also for transporting oil and gas beyond the republic. The recentmost data dictate the trends of prospecting to enhance the base of oil and gas extraction. There exist specific conditions complicating the process of oil and gas extraction at the Yakutian deposits, which are due to the presence of permafrost in the sedimentary cover. The main problems of development of the Yakutian petroleum complex have been discussed.

Analysis of the formation of bitumoid pools in various regions of the Siberian Platform gives insight into the stages of evolution of hydrocarbons in areas of their accumulation and destruction.

Oil and gas pools in the Vendian section in the Baikit petroleum province may be associated with structural-depositional traps. These traps have been investigated through reference seismogeological modeling based on CMP profiling and log data. Different basic models correspond to structural-lithological traps in sandstones of different facies: Fluvial sandstones form traps in valleys incised into a pre-Vendian plateau, and large traps in deltas and embayments occur in shallow-sea environments near the pinchout of the Vanavara terrigenous section. Processing of over 150 well logs and 7000 km of CMP data revealed six oil and gas traps in Vendian sandstones, including two high-potential traps in the pinchout zone.

A system lithofacies analysis refined the role of primary sedimentogene factors in the formation of reservoir properties of carbonate rocks in the O-I producing bed of the Talakan petroleum deposit. The bed bears 11 microfacies and 3 mesofacies - algal-microbial organogenic buildings, accumulative bodies of peloid sediments, and small, periodically drying basins of elevated and high salinity made up of chemogenic carbonate-sulfate sediments. The best reservoir properties are observed for rocks of peloid microfacies.

The transitional region from Baltic Shield to Barents Sea petroliferous shelf, made up chiefly of Late Proterozoic sediments, is tectonically zoned. Three subparallel structural zones correspond to different parts of the large Timan rift trough of Riphean-Vendian age framing the Russian Platform in the northeast. The forecast of hydrocarbon deposits is different for each of the zones. It is important that the thrusts be of restricted occurrence and complicated structural traps of hydrocarbons be possible to exist, especially in the central zone and on its northeastern periphery. The correct forecast and prospecting require a considerable additional volume of geological and geophysical data obtained by the promising method of CMP-wave reflection over the entire transitional field, with the reported approach to their interpretation taken into account. Also, two parametric boreholes are required to be drilled northeast and southwest of the Trollfjord-Komagelv-Rybachii zone of deep disturbances.