Distribution Patterns of the Main Structural-Group Parameters of Crude Oils from the Timan–Pechora Oil-and-Gas Basin According to 13C NMR Data

Abstract

Using ¹³C NMR spectroscopy, a representative set of crude oils and gas condensates (75 samples from 51 oil fields) from the Timan–Pechora oil and gas basin (OGB) was investigated. The structural-group composition of crude oils, and specifically the total concentrations of aromatic and n-alkyl moieties, were identified for the first time over the entire basin. Distribution density plots were presented for all the composition parameters measured. It was demonstrated that, because all the parameters exhibit polymodal distribution and substantially deviate from normal, a nonparametric statistical analysis is needed for the data processing. In particular, medians and median confidence intervals, rather than means or root-mean-square deviations, should be relied upon. When compared to the crude oil composition in the previously explored OGBs, the Timan–Pechora crudes were found to be closest to the West Siberian OGB. A considerable difference in aromaticity was observed compared to the East Siberian and North Caucasus oils. An equally large difference between the Timan–Pechora and North Caucasus oils was found for the n-alkyl composition characteristics. The average length of n-alkyl moieties in the Timan–Pechora OGB markedly exceeds that in the East Siberian crudes. When compared to the Volga–Urals basin, the Timan–Pechora oils exhibit a relatively small (although statistically significant) difference in aromaticity and a similar content of n-alkyl moieties. As was the case in an ¹H NMR investigation of the same oils that was carried out previously, the Timan–Pechora OGB was classified into four major tectonic groups: the Varandei–Adzva zone, Khoreivei depression, and Kos’yu-Rogovskaya depression; the Izhma–Pechora syneclise and Upper Pechora depression; the Pechora–Kozhva aulacogen; and the East Timan megaswell. Furthermore, the crude oils from Triassic and Permian reservoirs were considered separately from those from Devonian reservoirs. For the majority of composition parameters, only small (although statistically significant) differences were found between most crude oil groups: significance levels in the range of 0.01–0.05 prevailed over those between 0.001 and 0.01. Among Devonian deposits, crude oils from the Pechora–Kozhva aulacogen stand out for their lower aromaticity.