Postsedimentary Transformations in the Lower Triassic Deposits in the North of Varandey-Adzva Zone (Pechora Oil and Gas-Bearing Basin)

Abstract

New data on the mineral composition of Lower Triassic terrigenous deposits in the north of the Pechora oil and gas bearing basin are presented. The relevance of the study is due to the fact that terrigenous natural reservoirs confined to this part of the section have a heterogeneous structure both in area and in section. The purpose of the article was to identify the features of the formation and distribution of minerals that fill the void space of reservoirs. As a result of studying by the methods of optical microscopy, X-ray phase analysis, electron microscopy, signs of mineralogenesis processes, post-sedimentary transformations of sandy reservoirs were revealed. It is shown that the most widespread are quartz regeneration, transformation of minerals of the mica group, formation of calcite of various generations, formation of authigenic clay minerals, etc. Diagenetic changes in terrigenous rocks play a significant role in the formation of reservoir properties and lead to heterogeneity of reservoirs. It is shown that a large amount of calcite in sandstone cement leads to a significant decrease in porosity and permeability parameters. Pore cement of chlorite-smectite composition in fine-grained sandstones also leads to a decrease in the quality of reservoirs. Coarse and medium-grained sandstones with a low content of cement, predominantly of kaolinite or chlorite composition, are characterized by higher reservoir properties. Crustification cement of chlorite composition, pore-filling cement of kaolinite composition contribute to the formation of reservoirs with higher rates.