The influence of post-magmatic transformations on filtration and capacitive properties and distribution of radioactive elements in volcanogenic rocks

UDK: 553.98

DOI: 10.24887/0028-2448-2020-8-12-17

Key words: volcanogenic rocks, filtration and capacitive properties, hydrothermal-metasomatic processes, natural radioactive elements, spectral X-ray logging

Authors: S.V. Dobryden (Tyumen Branch of SurgutNIPIneft, Surgutneftegas PJSC, RF, Tyumen)

The article considers the influence of post-magmatic processes on the filtration and capacitive properties and distribution of natural radioactive elements in volcanogenic rocks. It is shown that the secondary transformations of volcanogenic rocks are determined by the primary texture features, mineral composition and type of thermal solutions. Volcanogenic rocks of the basic composition with a high content of dark-colored minerals are prone to calcification and сhloritization of dark-colored minerals. In acidic volcanogenic rocks with a low content of mafic minerals, these processes are less developed. Chloritization covers phenocrysts and volcanic glass of the bulk, calcification appears in the void space. Under the influence of high-temperature thermal solutions, alkaline plagioclases are albitized. The impact of low-temperature hydrothermal solutions leads to leaching of feldspars and quartz, chlorination, microclinization, calcification of phenocrysts , the bulk and void space of volcanogenic rocks. Under the influence of hydrothermal-metasomatic processes, the filtration and capacitive properties of volcanogenic rocks change. Leaching of mineral grains of rocks improves filtration and capacity parameters; albitization and silicification lead to the filling of the void space with post-magmatic minerals; pelitization-hydromiclization significantly reduces permeability, while chloritization weakly affects the change in filtration and capacitive parameters; carbonation, mixed-layer formations, and microclinization reduce the filtration-capacitive parameters to varying degrees. Volcanogenic rocks of intermediate and basic composition, as well as sedimentary deposits are characterized by low filtration and capacitive parameters. As a result of hydrothermal-metasomatic processes, a redistribution of the concentrations of uranium, thorium and potassium in the rocks of the volcanic-sedimentary sequence occurs. The radioactivity of volcanogenic rocks of acidic composition increases from albitized, silicified, carbonated differences to chloritized, mixed-layer formations, pelitized-hydromiclised and further to microclinized differences. The processes of albitization, silicification, and carbonation form a single group in terms of the content of natural radioactive elements and have the highest ratios Th / K and U / K. The values of the Th / K and U / K ratios decrease from chloritized to mixed-layer formations, pelitized-hydromicled, to microclinized rocks.
An example of a petrological separation of a volcanic-sedimentary sequence by a complex of geophysical methods using data from the spectral modification of the gamma ray is given. It is shown that the use of spectral gamma-ray logging data in the complex of logging methods allows to increase the reliability of the petrological separation of a volcanic-sedimentary sequence and the allocation of productive intervals of the volcanic-sedimentary sequence.
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