Yu. V. Erokhin

Inorganic Geochemistry of the Oil of West Siberia: First ICP-MS Data

The evolution in innovation technologies and analytical devices has raised geochemistry to a higher level. As is known, modern petroleum geochemistry is mainly composed of organic geochemistry. The trace element composition of oils was studied usually in combustion products, whereas works on direct determination of the trace element composition of hydrocarbons are scanty and generally related to a very small number of elements and/or heavy fractions [1]. The rapid progress in the method of mass spectrometry with inductively coupled plasma (ICP-MS) has made it possible to study the trace element composition of not only rocks, but also complex organic compounds, for instance, oil and its derivatives. A series of works has recently been reported on the geochemistry of asphaltenes and bitumens, i.e., solid components of oil [2, 3]. We analyzed for the first time the trace element composition of crude oil from West Siberia by the ICP-MS method. More than 50 trace, rare earth, and other elements were determined in oils from the Shaim and Srednii Ob fields (see [4‐6] for geological data). The analyses were conducted using an ELEMENT2 highresolution mass spectrometer following the technique developed at the Laboratory of Radiogeology (Zavaritskii Institute of Geology and Geochemistry) [7, 8]. The measurement results are shown in the table. Oils from West Siberia are characterized by extremely low contents of the majority of elements. The PM-normalized trace element contents are equal to ~0.1 u for the most depleted ultramafic rocks and ~0.001 for oils. It should be noted that the PM-normalized contents of trace elements in Triassic basalts of The table shows that crude oils of the Shaim and Srednii Ob fields have relatively high contents (>1 g/t) of major (Mg, Al, Fe, Na, Ti) and transition elements (Cr, V, Ni, Cu, Zn). In the Ni‐Cu‐Cr diagram [3] for the resinous‐asphaltene fractions, these oils belong to the chromium type. Their data points fall at the continuation of the trend outlined by the authors of [3] for the West Siberian province. The contents of other trace elements in these crude oils are lower (<1 g/t), but often higher than in some rocks. The Cs, Rb, Sr, and Zr contents in the oils are comparable with those in ultramafic rocks. The U content in oils is similar to that in the basaltoids and significantly higher than that in ultramafic rocks, chondrites, and intermediate rocks [9]. Such a high U content in oils is presumably related to the reducing conditions and, correspondingly, to the presence of a geochemical barrier. These conditions could also promote the high Pb content (up to 0.3 ppm). The Ag content is also relatively high (up to 0.1 ppm), while the Au content is low (<0.002 ppm).

Nature and Age of Metamorphic Rocks from the Basement of the West Siberian Megabasin (according to U-Pb Isotopic Dates)

The SHRIMP-II zircon U-Pb dates for metamorphic rocks from the West Siberian basement are determined for the first time. It is established that the major protolith of the metamorphic strata from the Shaimsk-Kuznetsovsk meganticlinorium is composed of sedimentary Late- and Middle-Devonian rocks (395–398 Ma). It is likely that the greywackes, whose strata were mainly formed under erosion of ophiolitic rocks, served as a substrate for the metamorphic rocks. The metamorphic transformations of the rocks occurred under conditions of greenschist and occasionally lower amphibolite facies of metamorphism during the Late Carboniferous-Early Permian period.

Chemical Microprobe Th-U-Pb Age Dating of Monazite and Uraninite Grains from Granites of the Yamal Crystalline Basement

994 Nowadays, the study of Arctic geology is of partic� ular importance, especially in connection with the oil and gas potential of this territory. Accordingly, study the basement of sedimentary basins in the Arctic is one of the priorities. The Yamal is the main gas province of our country and one of the few regions where the crys� talline basement is exposed for visual study. It is also important that gas condensate inflows at the fields on the Yamal Peninsula (Novoportovskoe, etc.) origi� nated from the complexes of the Paleozoic folded basement. By now, on the Yamal Peninsula the basement rocks have been exposed in about 100 wells drilled. Intrusive complexes were found at the Bovanenk� ovskaya, Novoportovskaya, Verkhnerechenskaya, and

The Kunashak meteorite: New data on mineralogy

Add info about the mineral composition of the Kunashak meteorite that fell in the Chelyabinsk region in 1949. It was found that the cosmic substance is composed of olivine (chrysolite), orthopyroxene (bronzite), clinopyroxene (augite), plagioclase (albite), maskelynite, chromite, magnetite, wustite, ilmenite, metals iron and nickel (kamacite and taenite), sulphides (troilite and pentlandite), chlorapatite and merrillite. This augite, ilmenite, pentlandite and chlorapatite identified in the Kunashak meteorite for the first time. For all minerals presents data on the chemical composition. Himself meteorite is an ordinary chondrite stone and belongs to petrological type L5-L6.

Composition and chemical microprobe dating of U-Th-Bearing minerals. Part 2. Uraninite, thorite, thorianite, coffinite, and monazite from the Urals and Siberia

To develop further chemical microprobe timing of U-Th-bearing minerals on the basis of upgraded measurement techniques and special age calculation, uraninite, thorite, thorianite, coffinite, monazite from several localities in the Urals and Siberia have been dated. The samples were taken from granitic rocks of the Pervomaisky pluton in the Central Urals; the pre-Jurassic basement of western Siberia and Yamal Peninsula; carbonatite-like dolomite from the Karabash ultramafic massif in the Southern Urals; granitic pegmatites of the Lipovsky vein field; and quartz-sulfide veins of the Pyshma-Klyuchevsky Cu-Co-Au deposit in the Central Urals. Scrutiny of the composition and chemical heterogeneity of mineral grains is a necessary stage of chemical dating aimed at the estimation of the degree of closeness of the U-Th-Pb system and unbiased screening of analytical data. The condition (Si + Ca)/(U + Th + Pb + S) ∼ 1 was used as evidence for significant secondary alteration of monazite; the negative correlation between Pb and Th or U +Th in uranitite was used for the same purpose. The positive correlation between Pb and U, along with low concentrations of Ca, Si, and Fe admixtures, implies that the stoichiometric composition of thorite is close to 100%. The reliability and accuracy of the chemical dating of minerals with high contents of radioactive elements can be enhanced by using bimineralic or multimineralic isochrons, e.g., monazite-uraninite, uraninite-coffinite, etc. The results obtained have been compared with the available isotopic ages of the studied minerals; the compared data are satisfactorily consistent.

First Data on Early Paleozoic Granitoids in the Basement of West Siberia

1193 The investigations of the West Siberian Plate over the last 80 years have resulted in compilation of ever more specific schematic maps of its basement zoning [1–8]. The feature in common shown in all these maps is the surrounding Paleozoic fold belts and their litho� tectonic zones extending into West Siberia (in accor� dance with the strikes of these structures and general patterns of potential fields). The basement of the west� ern part of the West Siberian Plate approximately up to the Khanty Mansiisk latitude is composed of the Uralides (structural zones of the eastern segment of the Urals) [3, 4]. To the east, they are replaced by the Altaides [1], where the basement of the plate is repre� sented by rock complexes of the Siberian Platform and its folded surrounding structures [2]. Another feature in common for these maps of the basement beneath the West Siberian Plate is the presence of the block of the Kazakhstanides immediately east of the Uralides that gradually pinches out in the northern direction. These main megazones (or domains) are separated by

Paleogeodynamics of Triassic Grabens System Formation in the Western Siberia

The Triassic is one of the main periods in the evolution of the Western Siberian oilandgas field [1–15], which is the most important geologicaleconomicregion of Russia. By the Triassic, Late Paleozoic compression and granitization had ended and the Paleozoic complexes in the territory of the future megafieldwere consolidated [5–7]. According to the ideas ofmost researchers dealing with Western Siberia, in theTriassic, compression within this region changed tosublatitudinal tension accompanied by the formationof rift and graben systems [11]. This idea was developed by some leading scientists [10, 14], but the problem is not yet solved. For example, based on the magnetic data interpretation, it was supposed [1] that theWestern Siberia basement contains oceanic elements.This hypothesis was not supported by most researcherswho argued that, in particular, Triassic deposits of theregion are of continental origin. The ideas about a Triassic superplume [3], which existed beneath Westernand East Siberia and was manifested mainly in theform of basaltoid magmatism, are also well known.According to [2], Triassic basalts of Western Siberiarefer to the same geodynamic type—socalled synorogenic—which is related to the main folding in theadjacent orogens. As a probable cause of grabens formation, mantle pulses were suggested [13]: they propagate in the form of a geodynamic wave that leads tothe appearance of shift structural parageneses. Thisbrief overview of ideas about the geodynamic nature ofTriassic units in the region shows that the problem,whose solution directly affects understanding the regularities of the formation and structure of WesternSiberian oilandgas megafield, is not completelystudied.Based on complex geological–geophysical studiesand mapping the large segments of the territory, quitedetailed geological maps of PreJurassic basement ofthe west Western Siberian Plate were compiled [5, 8,15]; in addition, it has been shown that the Paleozoicgeodynamic history of the region ended with a collision accompanied by tectonic piling, metamorphism,granitic intrusions, and the appearance of a newformed crust of the continental type. The new resultsallow us essentially to specify the evolution of theWestern Siberian megafield basement. It has beenshown [5–7] that anticlinorium “cores” are not Precambrian blocks (middle massif, rises of the ancientProterozoic protocrust), but Paleozoic deep igneousand metamorphic complexes (i.e., units from thelower and middle crust). They reached the surface (orupper crust level) when Early Triassic riftogenesis andextension of Western Siberia took place, i.e., in theperiod when the Western Siberian oilandgas megafield proper was forming.The Triassic units have been studied by manyresearchers [2–6, 8–15]. The most widespread arebasaltic and rhyolitebasaltic formations (Tura Seriesand its analogs), mainly filling some grabens. In theShaim oilandgas area, west Western Siberia, theseare Danilovskii and Polovinkinskii grabens, and someother structures. The spatial orientation of the twomentioned grabens suggests they formed duriungabsolutely different crustal motions (in different stressfields): the Danilovskii graben was produced by sublatitudinal tension and the Polovinkinskii graben, by sublatitudinal compression (Fig. 1). This provides anopportunity to date these tectonic processes. This wasdone by the Ar/Ar method at the Institute of Geologyand Mineralogy, Siberian Branch, Russian Academyof Sciences, under the leadership of A.V. Travin. Basedon the extracted pyroxene monofractions from our samples collected from the Symor’yakhskaya 10249 borehole (from a depth of 2138 m, Danilovskii graben), theobtained age was 249.4 ± 4.5 Ma. Pyroxenes frombasalts collected in the Kruglaya 1r borehole (from adepth of 1900 m, Polovinkinskii graben) yielded theage of 275.7 ± 10.4 Ma (Fig. 2). Below are some briefcharacteristics of the discussed geological structures.

The meteorite Ural: New mineralogical data

New data on the mineral composition of the meteorite Ural found out near the village of Ural (Kurgan Region) in 1981 were obtained. It was established that the meteorite is composed of olivine (chrysolite), orthopyroxene (bronzite), clinopyroxene (diopside), plagioclase (albite), chromite, metals iron and nickel compounds (kamacite and taenite), sulfides (troilite and pentlandite), chlorapatite, and merrillite. Minerals such as diopside, taenite, pentlandite, chlorapatite, and merrilite were identified for the first time in the meteorite Ural. This work presents the chemical compositions of all minerals studied. The meteorite Ural is normal H5–H6 chondrite which is characterized by strong secondary changes both inside and outside, viz., development of a supergene mineral—goethite.

The first U-Pb age data of zircons from relict spreading zones in the Middle Urals

Zones of paleospreading and crustal extension inthe Urals, as well as around the world, are clearlymarked sheeted dyke complexes. In the Middle Uralssuch a complex was first described by S.N. Ivanov andhis colleagues in 1973 [1] as a relic fragment of theoceanic spreading crust. Later, complexes of paralleldikes were distinguished and described in detail alongthe Urals fold belt, from th e Polar Urals in the north toWestern Mugodzhary Mountains in the south [2–8,etc.]. However, there are still no reliable isotope agedata for these sheeted dyke complexes. As a rule, theage of formation of dikes was determined on the basisof age data of the surrounding rocks [2, 4, etc.] or wereconsidered to be the same as the age of gabbro andbasalts from the ophiolite sections studied.We have made an attempt to obtain isotope age datafor accessory zircons from dolerite dykes of the representative ophiolite complex of Mt. Azov (MiddleUrals) (Fig. 1). Within this complex, fragments of thesheeted dyke complex, extending with breaks over60 km in the eastern framework of the Revda massif,are part of the Uralian Platinum Belt [6, etc.]. Gabbros and pyroxenites of the Revda massif and surrounding basalts were intruded by dolerite dykes. Contacts between dykes and surrounding rocks are oftencomplicated by numerous tectonic faults. The sheeteddyke complex is well exposed among basaltic andandesitebasaltic pillow lavas in the upper part of Mt.Azov (Fig. 1), located 5 km west of the town ofPolevskoi. Parallel dolerite dykes of northeasternstrike and steep southeastern or northwestern dippingare mainly 0.5–2 m thick. Dykes form swarms anddikeindike structures. In total, the volume of dykesexceeds the volume of pillow lavas approximately twofold. As noted in [1, 6, 7], in the contact zone betweendolerites and pillow lavas, hardening zones with athickness of a few centimeters and nonhardened contacts with rugged edges occur. It follows that somedykes first intruded into the hot pillow lava strata; subsequent portions of the magma intruded into thealready cooled strata. The dykes are made of finegrained gabbrodolerites and porphyritic doleriteswith plagioclase and hornblende phenocrysts, as wellas less common aphyric and fine porphyritic varieties.Dolerites were metamorphosed under the conditionsof lower greenschist facies. Microscopically, they usually consist of a amphibole–sossurite aggregate withchlorite, quartz, pumpellyite, and an ore mineral.The dyke complex and pillow lavas are presented bylowK basalts and andesite–basalts of normal alkalinity [7, etc.]. Dolerites are characterized by a lowerTi content (TiO

Zircons of Granitoids of the Yamal Peninsula Basement: Age and Composition of Inclusions

The zircons in granitoids from the basement of the Verkhnerechenskii oil exploration area (Yamal Peninsula, West Siberia) were studied. The U–Pb age of zircons was evaluated as 254.0 ± 3.0 Ma. It was found that the inclusions in zircons are represented by various minerals: fluorapatite, titanite, monazite-(Ce), albite, quartz, chamosite, and calcite. Most likely, the latter two minerals were formed separately from zircon but belonged to later secondary minerals (the rock propylitization products). In general, the accessory zircons and inclusions belonged to the “granite” association and crystallized synchronously in the Upper Permian.