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

New data on the mineral composition of Lower Triassic terrigenous rocks in the northern Pechora oil- and gas-bearing basin are presented. The relevance of the study is due to the fact that the terrigenous natural reservoirs confined to this part of the section have heterogeneous structure. The purpose of the article was to identify the features of the formation and distribution of minerals that fill the void space of the reservoirs. The optical microscopic, X-ray phase, and electron microscopic study revealed diverse postsedimentary transformations of sandy reservoirs. It is shown that the most widespread are quartz regeneration, transformation of mica-group minerals, as well as the formation of calcite of various generations and authigenic clay minerals. Diagenetic changes in terrigenous rocks play a significant role in the formation of reservoir properties and promote the heterogeneity of reservoirs. The large amount of calcite in sandstone cement leads to a significant decrease in filtration and capacity parameters. The pore cement of chlorite‒smectite composition in the fine-grained sandstones also causes a decrease in the reservoir quality. The coarse- and medium-grained sandstones with a low content of cement (predominantly of the kaolinite or chlorite composition) are characterized by the higher reservoir properties. The chloritic crustification and kaolinitic pore-filling cement facilitate the formation of higher quality reservoirs.