New pyroclastics occurrences in upper cretaceous deposits of Western Siberia

Considerable depletion of Mesozoic gas fields in Western Siberia increasingly stimulates the studies of the Upper Cretaceous section, where gas shows and inflows have long been known. In addition to Cenomanian Pokursky deposits, the most promising is argillaceous siltstone of the Kuznetsov formation and its analogues (Gazsalinsky package or T horizon, depth of occurrence is 800-1300 m), and also clay-siliceous sediments (opoka) of the Nizhneberezovsky member (Coniacian-Santonian packages НБ1 and НБ3) overlying the Gazsalinsky package. Persistent composition and structure, as well as wide occurrence in the West Siberian territory are typical of these deposits. Clayey rocks (siltstone and opoka) of complex genesis may represent unconventional reservoirs, which primarily responds to the features of their deposition, and secondarily - by controversial history of sediment lithogenesis. The authors briefly discuss some petrographic descriptions of Upper Cretaceous rock varieties containing ash fragments of quarts, feldspar and natural (volcanic) glass.