LITHOLOGICAL AND MINERALOGICAL SUBSTANTIATION OF THE LOWER BEREZOVSKAYA SUBFORMATION STRATIFICATION OF THE MEDVEZHYE AND VYNGAPUROVSKOE DEPOSITS

Abstract

Relevance. At present, the study of Suprasenomanian deposits, including deposits of the Lower Berezovskaya subformation of Cognac-Santonian age, composed of siliceous rocks, has moved from the category of theoretical interest to the category of practically important research. This is due to the fact that industrial gas content was confirmed at six sites (Van-Egansky – NB1 formation, Vyngapurovsky – NB4, Medvezhy – NB3–NB4, Novo-Chaselsky – NB3, Komsomolsky – NB1, Kharampursky – NB1). There are four layers of the Lower Berezovskaya subformation (the NB horizon), however, criteria for determining the boundaries of these layers have not yet been identified. In this paper, the substantiation of lithological-mineralogical and commercial-geophysical criteria for stratigraphic dissection of the section of the Nizhneberez subformation is given. The object of the study is the Berezovskaya formation, which deposits in the north of Western Siberia contain significant gas resources, considered as a reserve for «prolonging the life» of deposits during the development of highly productive Cenomanian deposits. The aim: improving the efficiency of geological exploration to identify gas deposits in siliceous rocks of xenon by clarifying the stratigraphic structure of the rocks of the Berezovskaya formation. Methods. The dissection of the sections was carried out on wells drilled in recent years, with continuous core sampling and with an expanded complex of geophysical studies of wells (GIS), including electrical, radioactive, acoustic, nuclear magnetic methods. The mineral composition was determined by studying the core by X-ray diffraction analysis (more than 300 samples), infrared spectroscopy (220 samples). The composition of chemical elements in rocks was determined by X-ray fluorescence analysis (more than 300 samples) and inductively coupled plasma mass spectrometry (70 samples). Results. It is shown that the boundary between the NB2 and NB3 layers does not have unambiguous GIS criteria. At the Medvezhye field, this boundary is determined by the abrupt change in the readings of the nuclear magnetic and acoustic logs, at the Vyngapurovskoe field such abrupt changes in the curves of these methods are not noted. The paper shows that the boundary between the B3 and B2 layers has a clear characteristic of the mineral composition of rocks and is determined by a sharp decrease in the proportion of zeolites (clinoptilolite) during the transition from NB3 to NB2. Along with a decrease in the content of clinoptilolite at the boundary between the B3 and B2 layers, there is also a sharp change in the chemical composition of rocks, in particular, a sharp decrease in the proportion of strontium is noted. This tendency to decrease the proportion of clinoptilolite and strontium in rocks can be explained by the attenuation of volcanic activity in the south-east of Western Siberia.

Keywords

• berezovskaya formation
• stratotype
• formation
• silicides
• silica
• opal
• quartz
• zeolite
• clinoptilolite
• strontium
• x-ray diffraction analysis
• infrared spectroscopy