Nanoquartz in Late Cretaceous Deposits in the Lower Berezovskaya Subformation

The Lower Berezovskaya subformation in the Upper Cretaceous is a complex reservoir with unconventional reservoir rocks located in the north of Western Siberia. The clay-siliceous deposits in the Lower Berezovskaya subformation are represented by various types of silicites that have unique petrophysical and mineralogical characteristics that cause significant difficulties in the development of associated productive layers. The aim of the research is to study their mineral composition, the parameters of the void space structure, and the conditions of formation, as well as to determine the sources of silica and perform a detailed study on the structure of nanoquartz. To study the mineral composition, the structure of the void space, and the genesis, special methodological techniques were applied, and a comprehensive analysis of the results using a wide range of laboratory studies, including optical micro- and stereoscopy, scanning electron microscopy, X-ray diffraction analysis, and microtomography, was performed. Silicites in the Lower Berezovskaya subformation are caused by the complex structure of the void space, which includes a wide range of genetic types of voids ranging from micron and submicron dimensions to fractions of a millimeter (voids confined to the burrowing organisms, intraform voids, interform voids, lenticular voids, cellular voids, and voids confined to microstylolite seams). The most widespread type of void is the interaggregate (lenticular and cellular) void, which is formed by clay (montmorillonite) scales, on the surface of which numerous alpha-quartz nanocrystals ranging in size from 0.05 to 0.5 microns are noted. The content of such quartz reaches up to 80% of the total volume of the rock in individual samples. The source of siliceous material for nanoquartz crystals was most likely volcanic processes, since the revealed mineral paragenesis of montmorillonite, cristobalite, and zeolites indicates the active transformation of ash material that entered the basin from volcanic formations.

Automated summary

The aim of the research is to study the complex reservoir rocks in the Lower Berezovskaya subformation in the north of Western Siberia. The study reveals that the clay-siliceous deposits in this subformation are made up of various types of silicites, presenting unique petrophysical and mineralogical characteristics that pose difficulties in the development of productive layers. Through comprehensive laboratory studies, it is found that the silicites are caused by a complex void space structure consisting of different genetic types of voids ranging in size from micron to submicron dimensions.