Geologiya i Geofizika, 2002, V 43, N 3, March.


OIL AND GAS GEOLOGY

FLUCTUATIONS OF THE SEA LEVEL AND CARBON ISOTOPE COMPOSITION OF CARBONATES IN A RIPHEAN PETROLIFEROUS BASIN ON THE WESTERN MARGIN OF THE SIBERIAN CRATON (Baikit uplift) E. M. Khabarov, V. A. Ponomarchuk, I. P. Morozova, I. V. Varaksina, and S. V. Saraev 211 The Riphean (1500-1450 to 1000-950 Ma, from 40Ar/39Ar and K/Ar age methods) section of petroliferous peritidal carbonates in the Baikit uplift in the western Siberian Platform contains five sequences, each divided into three to five subsequences, distinguished on the basis of sedimentology and isotope geochemistry. The five sequences deposited in the period from 80 to 100 Ma and reflect major fluctuations of the sea level. The stability of d13C estimates is confirmed by petrographical, geochemical, and isotope data (over 350 analyses), which show that early diagenic dolomites may retain primary d13C signal subject to only slight changes. The d13C values in the lowermost carbonates (1450-1350 Ma, from isotope data) are about zero (-0.2 to 0.5<194>). The overlying section contains long intervals with low positive d13C values of 0.5 to 2.0<194>, occasionally up to 3.0<194>, alternating with shorter intervals dominated by negative d13C values (to 2.2<194>). The negative shifts correspond to the deposition of lower Yurubchen (~1350 Ma), upper Yurubchen-Dolgokta (1270-250 Ma), Kopcher (1100-1080 Ma), and upper Yukta-lower Tokur (about 1000 Ma) subsequences. The main trends in the sea level fluctuations correlate well with d13C variations: The negative isotopic shifts coincide with the boundaries of the five major sequences; the intervals with medium positive d13C record high sea stand and correspond to the periods of carbonate shelf aggradation; low d13C values often correlate with the boundaries and the lower segments of the subsequences. Isotope and geological data from other regions indicate that the low sea stand periods correlated with the boundaries of the sedimentary subsequences in the Baikit uplift are most likely associated with global geological events and reflect eustatic fluctuations. Sedimentology, carbon and oxygen isotopes, sea level fluctuations, stratigraphy, carbonate shelf, petroliferous basin, Riphean, Siberian Platform
LITHOLOGICAL AND GEOCHEMICAL CLASSIFICATION AND A SEDIMENTATION MODEL OF THE BAZHENOV FORMATION I. D. Polyakova, L. A. Krol', G. N. Perozio, and E. A. Predtechenskaya 240 Lithological and geochemical interpretation of standard log data for the Bazhenov Formation (southeastern and central West Siberian Plate), their classification and 1:500,000 mapping show an argillaceous-siliceous-sapropelic composition and high organic matter (colloalginite) content of the highly resistive and gamma-active parts of the sections. The Bazhenov deposition was controlled, among other factors, by the bottom topography and the depth of the ocean which transgressed to the southeastern West Siberian Plate in its early and, especially, intermediate evolution stages. The distribution of rock groups reflected in depth-dependent variations in apparent resistivity made a basis for distinguishing different types and classes of sections within the formation, including those promising for oil and gas. Bazhenov Formation, silica, organic matter, apparent resistivity, gamma-activity
ADAPTIVE ESTIMATION OF OIL AND GAS RESOURCES V. F. Grishkevich 252 Three conceptual fundamentals are proposed as a base for estimation of HC resources and simultaneously for analysis and planning of prospecting surveys. The first is the qualitative dynamic approach to recognition of objects for prediction of petroleum potential (reservoirs, cap rocks, traps, trapping areas, and volumes). The second is adaptive support for the oil-geological model of the region to be estimated on the basis of logged information technologies. The third is continuous refinement of HC balance of all kinds on the basis of constant comparison of current local estimates with current results of prospecting surveys. A method of geometrical parameterization is proposed for the Neocomian West Siberian clinoforms. Some implications of the proposed approach are discussed. Estimation, resource, potential, oil, reservoir, cap rock, trap, oil trapping area, oil trapping volume, dynamic, balance, prospecting, model, technology, clinoform
PETROLOGY, GEOCHEMISTRY, AND MINERALOGY

YERUDA AND CHIRIMBA GRANITOIDS (Yenisei Range) AS INDICATORS OF LATE PROTEROZOIC COLLISIONS A. E. Vernikovskaya, V. A. Vernikovskii, E. B. Sal'nikova, V. M. Datsenko, V. P. Kovach,A. B. Kotov, A. V. Travin, and S. Z. Yakovleva 259 Petrological, geochemical, and isotope (U-Pb, Sm-Nd, Sr-Sr, and Ar-Ar) studies of the Yeruda and Chirimba granitoids show that they belong to different magmatic complexes. The Yeruda low-K calc-alkalic granitoids (878±1.5 Ma, U-Pb dating from zircon) have a geochemistry corresponding to intermediate I-S-type granites. Their primary melts may have been derived from island-arc complexes, strongly contaminated with the material of old continental crust, which were molten as a result of a collision about 880 Ma ago. The Chirimba high-K and high-F subalkalic granitoids (761±8 Ma, U-Pb dating from zircon) apparently formed by melting of Early Proterozoic (Sm-Nd model age) crustal substrate, including the lower crustal material. In geochemistry and isotope composition they are similar to A-type granites that formed in postcollisional or within-plate environments. The Chirimba granitoids are 120 Ma younger than the Yeruda rocks and thus cannot have been caused by the same event. Therefore, the region of the Yenisei Range may have experienced repeated accretionary and collisional events in Late Proterozoic time. Late Proterozoic, petrology and geochemistry of granitoids, isotope data, Yenisei Range
PLATINUM IN CARBONACEOUS SUBSTANCE IN ORES FROM THE SUKHOI LOG DEPOSIT (East Siberia) E. A. Razvozzhaeva, A. M. Spiridonov, V. D. Tsykhanskii, I. E. Vasil'eva, and S. I. Prokopchuk 273 Insoluble carbonaceous substance (ICS) encountered in the Sukhoi Log gold deposit was extracted, using special techniques, from bitumen-free shales. Of two groups of ICS concentrators one is light and buoyant in water, platinum-free, and containing a sericite-carbonaceous component (carbon, silicon, aluminum, and magnesium); the other group of ICS concentrators contains carbon and platinum in the ranges of 42.2 to 91.78 and 80 to 1500 ppm, respectively. The presence of platinum may attest to strong bonds between platinum and carbon and their compounds making the Sukhoi Log ores difficult to investigate. In experiments, carbonaceous substances increase their activity on thermal and acid treatment, which may be effectuated in nature as hydrothermal ore formation. Carbon, carbonaceous shale, ICS concentrator, platinum
TYPOMORPHISM OF PARAMAGNETIC RADICALS CO2-, CO3-, AND CO33- IN NATURAL CARBONATE-APATITES L. G. Gilinskaya, Yu. N. Zanin, and V. P. Naz'mov 286 Exemplified by the carbonate-apatites from the phosphate-bearing regolith of the Magan and Yraas Massifs of the Maimecha-Kotui Province of ultrabasic-alkaline rocks and carbonatites, the EPR spectra of <$Eroman CO sub 2 sup ->, <$Eroman CO sub 3 sup ->, and <$E roman CO sub 3 sup 3- > radicals observed in them have been shown to be typomorphic and, thus, appropriate for separating endogenetic and supergene varieties. Analysis of experimental data shows that the spectra in endogenetic and supergene apatites are complicated superpositions owing to several individual centers. Computer simulation by means of the ESR1 program permitted us to recognize them and to identify and calculate proportions (weights) of components in complex spectra. It has also been documented that the form of absorption lines in endogenetic specimens is isotropic, and in supergene ones, anisotropic, i.e., isotropic and anisotropic EPR spectra of carbonate radicals with corresponding parameters are recorded. This signature reflects dynamic characteristics of radicals (rotating or fixed in the structure) and is due to molecular bound water, which is always present in supergene apatites and absent from endogenetic high-temperature varieties as inferred from data of chemical analysis, IR spectroscopy, and nuclear magnetic resonance. Apatite, ESR, radicals, typomorphic properties, Maimecha-Kotui Province
GEOPHYSICS

FACTORS CONTROLLING ACCURACY OF ACOUSTIC LOGGING MEASUREMENTS M. G. Markov 304 The paper presents a solution to the forward acoustic logging problem for a multipole acoustic radiator located in a noncylindrical well off its axis. The modeling investigates how the noncylindrical well geometry and the displacement of the radiator from the well axis influence the parameters of the measured elastic waves. Acoustic logging, synthetic wave patterns, forward problem, acoustic multipole radiator