Home – Home – Jornals – Russian Geology and Geophysics 2010 number 3

We present results of a complex study of evaporite sediments from one of the small saline lakes with carbonate sedimentation in the Ol’khon area and substantiate their high significance for paleoclimatic reconstructions. The mineral composition of the bottom sediments was studied by XRD analysis, IR spectroscopy, scanning electron microscopy, elemental analysis (SR-XFA), etc. By decomposition of the complex XRD profiles of carbonate minerals into individual peaks by Pearson VII function, we identified carbonate phases in each sample and determined their proportions. A high-resolution carbonate record has been obtained for the first time for the lacustrine sediments. It bears the information about the stratigraphic distribution of Mg-calcites (a continuous series of structurally disordered low- to high-Mg calcites, up to Ca-dolomites), in which the amount and proportions of phases with different Mg contents are controlled by Mg/Ca, salinity, and total alkalinity of the lake water changing depending on the climatic cycles and lake level fluctuations. Comparison of the carbonate record for the Holocene section dated by the radiocarbon (¹⁴C) method with results of lithological, diatom, and palynological analyses, data on stable isotopes (δ¹⁸ O and δ¹³ C), and the distribution of some geochemical indicators of climatic changes permitted the reconstruction of the intricate evolution of the Lake Tsagan-Tyrm basin, which was controlled by the regional climate from the Atlantic period to the present time. The directed change in various characteristics of essentially carbonate sediments evidences that the Ol’khon regional climate has become more arid in recent 6.5 kyr. Moreover, drastic frequent changes of climate and, correspondingly, the Lake Tsagan-Tyrm water level in different periods of its existence have been revealed. The widespread saline and brackish lakes in southern East Siberia, Mongolia, and North China and the highly informative (in terms of paleoclimate and paleolimnology) carbonate sediments might help to recognize the general tendencies of paleoclimate changes and local fluctuations in Central Asia.

In 1950, D. Korzhinskii hypothesized that minerals in metamorphic reactions can be in local equilibrium. Proceeding from this idea, G. Fisher and R. Joesten later developed a realistic model of metamorphism kinetics. The model included diffusive mass transport in intergranular fluid controlled by the concentrations of components and chemical potential gradients, local-equilibrium reactions between minerals and pore fluid, mass balance of components, etc. The Korzhinskii–Fisher–Joesten  model can provide clues to the duration of metamorphic events as inferred from mineral chemistry, free energy of phases, and rock textures and  structures. The respective modeling of metamorphic reactions, with regard to spatial distribution of mineral grains, has implications for the transport and balance of chemical components. The balance of components (except volatiles) is restricted to domains of hundredth fractions of a cubic millimeter to several cubic meters. This may be the minimum size of an elementary domain in which minerals are in local equilibrium.

Many gold fields host scores of discovered gold ore occurrences with unclear prospects because of their insufficient geological exploration. Since exploration is expensive, the occurrences should be first appraised by low-cost methods. Studying the typomorphic features of minerals from the gold fields of the Upper Yana–Kolyma region, we have established a relationship between the contents of isomorphic impurity elements (IIE) and the degree of solution supersaturation (DSS). Preliminary studies show that, based on this relationship, criteria can be developed for revealing “concealed” mineralogical-geochemical zonation and predicting mineralization to depth.
The determining factor for the behavior of IIE, depending on the DSS, is their distribution coefficient ( K ) with respect to the host element. If K > 1, the content of these elements decreases with increasing DSS during ore deposition; if K < 1, it increases. When a solution contains IIE with K > 1 and K < 1, they behave according to their distribution coefficients. In other words, when the DSS increases during ore deposition, IIE with K >1 will be typical of early mineral segregations, and IIE with K < 1, of late ones. If the crystallization front migrates, this must give rise to a vertical zonation in the content of both groups of elements. The content of elements with K >1 will decrease upsection, and that of elements with K < 1 will increase (“concealed” mineralogical-geochemical zonation).
In connection with this subject, the paper reports data using native gold as an example. The behavior of Ag, Sb, Hg, Cu, Ni, and Co is discussed depending on the depth of deposit formation from the paleosurface (deep-seated → shallow); in the vertical section of individual deposits; depending on mineralization stages; in the cross-section of individual crystals with a zonal structure. The behavior of Ag, Sb, and Hg has been shown to be opposite to that of Cu, Ni, and Co in all these aspects. With respect to Au, the first group of elements has K < 1, and the second one, K > 1. Copper and antimony are of particular interest. In the vertical section of individual deposits, the Cu content of gold increases with depth and the Sb content decreases. The Cu/Sb ratio can be used to estimate the erosional truncation level of deposits and thus predict their extension to depth. Examples of well-explored deposits are given.

Kerogens from the Lower and Middle Cambrian deposits of the Kuonamka Formation in the northeastern Siberian Platform have been analyzed by modern methods. We have determined the pyrolytic characteristics of insoluble organic matter and the contents of C, H, S, N, O, and the stable C isotope. The type and catagenesis of organic matter have been estimated, as well as the generating potential of oil source rocks. It has been found that the composition of kerogens taken from the Molodo River outcrops was affected by supergene processes and that the degree of their alteration is related to the organic content of rocks and their structure.

We generalized experimental data on the structural group composition and macrostructural characteristics of high-molecular compounds (HMCs) — hydrocarbons, resins, and asphaltenes — from more than 180 crude oil samples extracted from different oil fields of Eurasia.
Most of oil resin molecules contain 1–3 structural blocks, each consisting of 0–3 aromatic and 0–5 naphthenic rings in different combinations, with the total number of rings in each block not exceeding 5–6. The average molecular weights of resins are no more than 100–1500 a.m.u. Asphaltenes differ from resins from the same source in the larger number of structural blocks (mainly tri- or tetracyclic aromatic) in molecule (up to five), lower degrees of substitution of aromatic cores, and smaller portions of carbon atoms in saturated fragments. The average molecular weights of asphaltenes measured by naphthalene cryoscopy do not exceed 2500 a.m.u.
Owing to intermolecular interactions, HMCs form various associates and polymolecular particles, including quasi-crystalline “batches” and multibatch aggregates. Spontaneous disintegration and formation of such particles make the oils dynamically equilibrium polydisperse systems.
We have developed the main regularities of compositional and structural changes of HMCs depending on the depth of occurrence, age, and lithologic composition of oil-bearing deposits.
Being chemically labile heteroatomic substances, petroleum HMCs are more rapidly subjected to compositional and structural transformations than hydrocarbons, i.e., are more responsive to changes in strata conditions.

Naphthene-benzenes, naphthene-naphthalenes, and naphthene-phenantrenes have been extracted with methanol from mono-, bi-, and triarene fractions of oils of different genetic types from the West Siberian, Timan-Pechora, North Caucasian, and Pannonian (Serbia) oil-and-gas-bearing basins. The hydrocarbon composition and molecular-mass distribution (MMD) of naphthene-arenes have been determined by mass spectrometry. Naphthene-benzene and naphthene-naphthalene molecules contain one to six naphthenic cycles, whereas naphthene-phenantrene molecules contain one to three haphthenic rings. The number of carbon atoms in the alkyl groups of naphthene-benzenes, naphthene-naphthalenes, and naphthene-phenantrenes reaches 38, 20, and 19, respectively. Distinctive features of group compositions and MMD of naphthene-arenes have been revealed in oils generated by different organic matter.

Specific deformation patterns in sediments associated with small-amplitude strike-slip faults in the basement have been recognized over a large territory of the West Siberian basin due to 3D seismic exploration advance. These patterns represent shear structures which turn to be a phenomenon beyond the classical views of the regional basin architecture. Many densely faulted oil and gas fields of a complex geometry in reservoir rocks of a wide stratigraphic range fall in zones of shear structures. Prospecting, test drilling, and oil enhancement activities in these fields are risky without a solid mechanic and kinematic background based on simulation of sedimentary fracture patterns which are associated with basement-involved strike-slip faults and may govern hydrocarbon migration as structure controls, fluid channels, or screens.

The harmonic electromagnetic field of a vertical magnetic dipole above an anisotropic half-space has been simulated using a forward algorithm for layered conductive media with inclined anisotropy. Inclined anisotropy has been found out to change the typical behavior of frequency and transient responses. Qualitative interpretation of FD loop-loop responses of a conducting earth with inclined anisotropy requires taking into account the receiver azimuth dependence of apparent resistivities. In the case of time-domain measurements, this dependence is absent but the apparent resistivities are higher at late times.

In the course of scientific collaboration, we were involved in discussion on the capacity of a vertical loop configuration to resolve thin high-resistivity layers, which is quite an interesting and largely debated point. We report a small forward modeling study including an algorithm based on an analytical solution by separation of variables and a respective program for computing the time-domain TEM field of a horizontal magnetic dipole. We infer that the subsurface vertical loop system shows no critical advantage in resolving thin insulating inclusions.