Lower Vendian sandstones on the southwestern slope of Nepsky Arch (Eastern Siberia): reservoir properties and depositional settings
Key words: Lower Nepa subformation; terrigenous deposits; sandstone; reservoir; salinization; porosity; computed tomography; Acoustic Impedance; paleogeographic reconstruction.
For citation: Balagurov M.D. Lower Vendian sandstones on the south-western slope of Nepsky Arch (Eastern Siberia): reservoir properties and depositional settings. Geologiya nefti i gaza. 2022;(6):57–65. DOI: 10.31087/0016-7894-2022-6-57-65. In Russ.
Acknowledgments: The studies were carried out in the laboratories of the Department of Geology and Geochemistry of Combustible Fossils, Faculty of Geology, Lomonosov Moscow State University. The author extends his appreciation for the support and assistance in the work to the friendly team of the Faculty of Geology, Lomonosov Moscow State University, and in particular to E.E. Karnyushina, A.V. Stupakova, G.A. Kalmykov, D.V. Korost, K.A. Khrenov, V.L. Kosorukov, N.I. Korobova, D.R. Gafurova, and S.V. Frolov.
3D seismic survey and drilling exploration wells with full core sampling from the intervals of the vendian terrigenous deposits were a part of geological exploration on the southwestern slope of the Nepa arch. These rocks are regionally oil and gas bearing and are the subject object of the research in this work. An advanced well log suite was obtained in the wells, and laboratory studies were carried out on core. According to the results of laboratory studies, the cyclic structure of the Lower Nepa sedimentary deposits was revealed. Cycle members were identified and characterized. Interwell correlation was carried out with a description of the intervals of evaporite and carbonate mineral associations. In the well sections in the central sector of the area, fine- and medium-grained mesomictic sandstones of the subformation were found with similar structural and textural characteristics and a high mineral content: anhydrite, dolomite, halite, and calcite. Section of the Well 3 on the west of area differs significantly. The borehole encountered inequigranular feldspar-quartz greywacke sandstones, predominantly saline with halite throughout the studied interval. The data obtained allowed judging different environments of the early Vendian sedimentation, as well as different degree of rock transformation by secondary processes: salinization and carbonatization. This article presents the results of petrophysical studies. The measured reservoir properties of Lower Nepa samples made it possible to characterize the previously identified strata with cycle members. Petrophysical modelling performed using the results of a computed tomography survey determined the nature of rock void space filling with evaporite and carbonate minerals. The most porous and permeable rocks are feldspar-quartz greywacke shallow-water and coastal-marine sandstones with good sorting and roundness. These rocks were identified in the near-top part of the middle sequence. Fine-grained arkosic sandstones of tidal sedimentation settings in the near-top part of the subformation have low reservoir quality similar to saline and carbonatized fine-medium-grained mesomictic sandstones of lagoon zones. In the case of thicknesses persistence of the latter, it is possible to identify them in well sections using logging data and over the area using 3D seismic data.