M. B. Smirnov

Distribution and interrelation of the main structural-group composition parameters of Western Siberia crude oils according to NMR data

The distribution and the degree of correlation of 14 main and most easily measured characteristics of the structural-group composition of Western Siberia crude oils have been studied. It has been shown that the distribution pattern of the parameters greatly differs from the normal distribution and substantially varies, with the distributions of the quantities that reflect the concentration and composition of aromatic components being the most complex. The trends of variation in composition of the oils in basic stratigraphic plays have been revealed. The necessity of calculating Spearman’s instead of Pearson’s pair correlation coefficients for determining the degree of interrelation between the parameters has been shown. It has been demonstrated that the conclusions made earlier on the basis of Pearson’s linear correlation coefficients about interrelations between the composition parameters and their correlation with the mode of occurrence are not quite certain; all of them need to be verified. Strong interrelation of many composition parameters has been proved, and the necessity of studying in detail the feasibility of their joint use for comparative analysis of oil samples has been suggested. A strong negative correlation between the parameters reflecting the concentration of the aromatic and n-alkyl components has been revealed.

Relations between the main structural-group composition parameters of Western Siberia crude oils according to NMR data

Equations to describe the relationships between 37 pairs of fairly well correlated basic and most easily NMR-measurable characteristics of the structural-group composition of Western Siberia crude oils have been derived. Different variants of graphing the regression lines have been considered. A method for processing initial data has been proposed that suits the standard situation for petroleum geochemistry when the scatter of data points around the trend line is determined by differences between the samples, rather than measurement errors. The characteristics of the deviations of values for the composition parameters from their trend lines, by which a reasonable sequence of the extension of the sets of measured parameters can be built in a comparative study of the composition according to the research objectives, are reported. It has been demonstrated that the given description of the composition can help to identify objects that stand out and are unique in this sense and provides new criteria for grouping fluids of this kind. It has been shown that the parameter Car defining aromaticity of the samples from Western Siberia can be calculated with good accuracy from the value of the rapidly and inexpensively measurable quantity Har. A correlation between the aromaticity of the samples and the amount of n-alkyl structures in them has been revealed.

Codetermination of the composition of monomethyl-substituted and isoprenane alkyl fragments of saturated petroleum hydrocarbons by the 13C NMR technique

About 100 13C NMR signals of the main n-alkyl-and isoalkyl fragments of molecules (monomethyl-substituted and isoprenane moieties containing up to 15 carbon atoms) in petroleum fractions of saturated hydrocarbons of different compositions were assigned, thus allowing for the detailed characterization of the quantitative composition of these fractions including the ratio between the diastereomers of isoprenane fragments. Two groups of compounds with dimethyl-substituted alkyl fragments (each of them can make up ∼1% of the total saturated hydrocarbons) were found for the first time in crude oils. A nonequilibrium diastereomer ratio for the isoprenane fragments was found in a sample of hydrothermal oil from the Uzon volcano caldera: the configuration of the chiral centers of dominant isomers is the same as that of the C-7 and C-11 atoms of natural phytol. It was assumed that the analysis of the diastereomer ratio of the 2,6,10Me3-fragment in specimens at relatively early stages of catagenesis (black shale hydrocarbons, hydrothermal oils) can be a convenient means of assessing their maturity. A method is described for taking into account the influence of the recording temperature and the analyte concentration in a solution on the position of the signals in the 13C NMR spectra with an accuracy sufficient for the assignment of the signals at any composition of the saturated hydrocarbon fraction.

A 13C NMR study of crude oils: Identification of compounds with nonisoprenane k,(k + 3)-dimethylalkyl moieties in oils and bitumenoids from the southern part of Anadyr basin

A group of saturated nonisoprenane dimethyl-branched, alkyl-substituted hydrocarbons of the R-CCCCCC(C)CCC(C)CCCCC-R′ type has been found for the first time in crude oils and dispersed organic matter (DOM). Their concentration in the saturated hydrocarbon fractions of crude oils and DOM from the southern part of the Anadyr basin is 1–2 wt % and is well above that of their structural isomers with the closest structure in the same samples. Some ideas about possible sources of these hydrocarbons have been advanced. In the methodological aspect, the simplicity and efficiency of 13C NMR spectroscopy as applied to searching for previously unidentified alkyl structures in crude oils and DOM have been demonstrated, using as spectral parameters coefficients that determine the dependence of chemical shift on the recording temperature and the concentration of the fraction in the solution.

Distribution patterns of main structural-group parameters of crude oils from the Volga–Urals oil and gas basin according to NMR data

The character of the distribution of 16 main NMR-measurable parameters of the structural-group composition of Volga–Urals crude oils has been assessed. It has been shown that the distributions quite significantly differ from the normal distribution; they are in general asymmetrical and essentially different. Total aromaticity and the amount of n-alkyl structures in oils of the major stratigraphic plays are on average the same, although there are differences between the northern, central, and southern parts of the oil-and-gas basin. Regarding the concentration of unsaturated hydrocarbons, there are differences between both the plays and territories. Similarities and dissimilarities between the West Siberian and Volga–Urals basins in distributions have been revealed. Some features observed for Western Siberia oils have to be other in the Volga–Urals basin. Upper Carboniferous and Lower Permian carbonate reservoirs in the northeast of the basin have been found to contain oils with a uniquely high concentration of aromatics that evidently differ in composition from those normally present in crude oils. The advantages of interval plots of probability density over histograms for comparing the distributions have been demonstrated. Using the particular material, it has been shown that the Student t-test conventionally used to compare average values for oil groups leads to false conclusions. To determine systematic shifts between distributions, it is necessary to use the Mann–Whitney test and compare the medians of distributions.

13C NMR identification in crude oils of hydrocarbons bearing ethyl and n-alkyl substituents on their main chain

Assignment of 13C NMR signals of compounds with ethyl, n-propyl, n-butyl, n-amyl, and n-C6+ alkyl substituents in different positions of carbon chains in the spectra of saturated hydrocarbon fractions of crude oils and in the spectra of crude oils, taking into account the dependence of chemical shifts on the recording temperature, the concentration of the test solution. and the composition of the object of study, has been proposed. The capabilities of the 13C NMR technique as applied to determination of this almost unexplored group of hydrocarbons in crude oils have been discussed.

A 13C NMR study of crude oils: Detection of C15+ trans-1-methyl-2-alkylcyclopentanes

The presence of C15+trans-1-methyl-2-alkylcyclopentanes in crude oil has been proven for the first time by means of the 13C NMR technique, and a procedure for their identification is described. It has been shown that these hydrocarbons represent a group of compounds bearing structurally different substituents, including the homologous series of trans-1-methyl-2-n-alkylcyclopentanes extended over a broad range of molecular masses. Their concentration in five samples of topped saturated petroleum hydrocarbons varies severalfold, with the maximal value being ∼1 wt %. The ability of hydrocarbons from this group to form complexes with urea and thiourea was examined. The importance of the analysis of the temperature and concentration dependences of the chemical shifts of signals for the determination of the structure of previously unknown compounds in oil.

Comparative 13C NMR analysis of the composition of saturated petroleum and bitumenoid hydrocarbons: Potentialities and outlook

The informativity of 13C NMR spectroscopy as applied to solving the class of problems based on the data of comparative analysis of objects has been evaluated using a material from the southern part of the Anadyr oil-and-gas basin (saturated-hydrocarbon fractions of crude oils and bitumenoids). A set of 27 parameters measured from 13C NMR spectra has been proposed to characterize the composition of the samples with different degrees of detailing the structure of all components of the fractions. Methods for the determination of the degree of closeness of the sample composition have been considered, and it has been shown that not all of them satisfy the natural requirement of transitivity. A simple comparison procedure has been proposed for the case of multiparameter description of the fractions, with the values of the parameters lying in a wide range that has the property of transitivity on the material in question. It has been shown that 13C NMR radically extends the capabilities of the comparative analysis of the composition of crude oils relative to the conventional methods (GLC, GC-MS). In contrast to the results of study of the individual composition of saturated hydrocarbons (including the presumably most informative data on the composition of cyclic biomarkers), the 13C NMR data for the given material revealed a regular change of oils from the bottom up the section and to establish that the bitumenoids from its lowest part has nothing in common with the crude oils examined. The analytical parameters derived from the 13C NMR spectral data are not, in this case, too sensitive to inevitable variations in composition associated with sampling procedures or other insignificant factors. Thus, the generally accepted view on 13C NMR as a technique useless for tackling this class of problems can be regarded as refuted.

Effect of temperature and concentration of solutions on the position of 13C NMR signals for fractions of saturated hydrocarbons forming urea adducts

An empirical equation was proposed for calculating the chemical shifts of 13C NMR signals from monomethylated alkanes and mono-n-alkyl-substituted cyclopentanes and cyclohexanes in the spectra of isoparaffin and naphthene fractions that form adducts with urea, depending on the analyte concentration in deuterochloroform C and the spectrum recording temperature Trec. Deviations of the calculated chemical shifts from the measured values did not exceed 5 × 10−3 ppm while varying Trec from 303 to 323 K and C from 200 to 600 mg/ml. It was shown that such an accuracy is sufficient for the unequivocal identification of components by means of 13C NMR technique; ignoring this dependence leads to errors in signal assignment in the spectra of petroleum fractions. The use of the chemical shift of certain signals as internal standards for determination of Trec and C was proposed. It was assumed that numerical values of the temperature and concentration coefficients that describe the dependence of the chemical shift on Trec and C would be useful for the 13C NMR detection of new groups of compounds in petroleum hydrocarbons.

Proof of formation of organic matter in upper Devonian carbonate and carbonate-siliceous sediments of the South-Tatar uplift in constant photic layer anoxia

Samples of carbonate and siliceous-carbonate deposits of Semiluk—Sargaev horizons of the N‒NE slope of the South Tatar uplift, Volga-Ural petroliferous basin, have been studied. Specific feature of the source rocks—high concentration of compounds—anoxia testifiers in the photic layer has been identified. That is, organic matter has been deposited under the conditions of the constant presence of anoxia in the photic layer of the sedimentation basin at sufficiently high thickness of the water column contaminated with hydrogen sulfide. Since by the composition of saturated hydrocarbons the studied samples are typical for deposits of the Semiluk horizon of the central part of the Volga-Ural basin, we can assume a fairly wide prevalence of such conditions within the South-Tatar arch. Changes in the concentrations of the components—anoxia markers—show cyclical changes in output layer contaminated with hydrogen sulfide or its bioproduction within the studied time.