A Сomparative Analysis of Oils from Carbonate Sediments of the Upper Usolskaya Subformation and the Vendian–Cambrian Pay Horizons in the Central Part of the Nepsko-Botuobinskaya Anteclise

Abstract

The study of the prospects in the central part of the Nepsko-Botuobinskaya Anteclise is focused on the subsalt oil and gas bearing horizons of Group B (Osinsky, Upper Ust-Kutsky, Lower Ust-Kutsky, and Preobrazhensky) and Group V (terrigenous formations of Vendian age). However, higher, in the Lower–Middle Cambrian carbonate-halogen deposits, there are also promising horizons of Group A (Kelorsky, Atovsky, Khristoforovsky, and Balykhtinsky) whose sufficient oil and gas content was confirmed in the neighboring Angara-Lena stage and in the north of the Nepsko-Botuobinskaya Anteclise and its adjacent structures. This paper examines the results of geochemistry tests of oil samples from the Osinsky horizon (layer B1) and the nameless dolomite layer located between the salts in the lower part of the Upper Usolskaya Subformation, which was previously considered gas-bearing at best. The tests conducted by the method of chromatography-mass spectrometry and the comparison of geochemical parameters of oils from different reservoirs show the similarity of hydrocarbon fluids of the Upper Usolskaya Subformation and of underlying pay horizons, which makes it possible to attribute them to a single oil and gas bearing system.