Lower Vendian deposits in south-western slope of Nepsky arch (Eastern Siberia): lithology and depositional settings

In one of the large East Siberian oil and gas field situated in the south-western part of the Nepsky Arch, exploratory drilling with total coring of the main oil and gas bearing intervals was carried out, including the coring of the basal part of sedimentary cover composed of the Nepsky Fm Vendian sediments. The core was subjected to laboratory studies with the purpose of detailed description of the lightly saline Lower Vendian Nizhnenepsky reservoir intervals, and they are the subject of research in this work. This paper mainly presents the results of the laboratory studies of the Nizhnenepsky rock lithology. A sequence of the cycle members composing the basal, middle, and upper series of the Nizhnenepsky Member is identified. Each rock group has different lithological and sedimentological characteristics, reservoir properties, degree of salinization and silting. Fine- and medium grained mesomictic sandstone from the middle section of the member are the most saline and carbonated, and fine-grained arkosic sandstone of the upper section are lesser saline and carbonated. A number of relatively approximate wells in the center of the study area are similar in terms of high content of mainly sulphate group of evaporites and carbonate minerals in the rocks of the middle section of the Nizhnenepsky Member. Among the carbonate minerals, dolomite that entirely composes individual layers is dominating. It is also present as cement in sandstone lenses and interlayers, in certain samples its content exceeds 90 %. In the well drilled in the marginal western segment of the study area, the section pattern in the middle interval is markedly different. Clastic halite-saline deposits were encountered throughout the section of the target interval. Basing on the integrated analysis of the available geological and geophysical data, lithologies of the rocks are described, and salt occurrence intervals are identified in the well columns. In addition, a methodological approach to predicting zones of salinization of terrigenous rocks and zones promising for exploratory drilling is proposed that can be used as early as the initial interpretation of 3D seismic data