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

Abstract

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.