The results of the research of sandstones closed porosity


Keywords: gas inclusions, microdeformations, catagenesis, closed porosity, sandstone

Abstract

The article presents the results of the study of gas inclusions in quartz grains of Donets Basin sandstones. It describes the main genetic types of inclusions, their conditions of formation, shape, size, as well as features of the transformation under the conditions of catagenesis of rocks.The presence and distinctive features of the primary and secondary inclusions were established by studying of their homogenization temperatures. Thanks to these studies, it was found that the homogenization temperature of the primary inclusions is 1.5–2 higher than for secondary ones. Also, characteristic of the primary inclusions is that they are mainly gas-liquid. In size, primary inclusions are 1.5–2 times larger than secondary ones, and on average they are 2–3 µm. Secondary inclusionsessentially consist of gas. A characteristic feature of secondary inclusions in quartz grains of the sandstones of the Donetsk Basin is that they decorate the microdeformations of these grains, thereby forming numerous Boehm stripes. The gas inclusions of Boehm strips are indicators of paleotemperature, and the Boehm strips themselves carry information about paleopressure. To calculate the volume of gas inclusions, a methodwas proposed.This method is easy to use and does not require significant financial expenses. For its implementation, standard petrographic thin sections are used, which are examined using an optical microscope with a total magnification of 1000-1200 times. Using the proposed method, the volumes of gas inclusions in quartz grains of sandstones of different substages of catagenesis were established. Considering that gas inclusions are part of closed porosity, it is proposed to conditionally subdivide closed porosityinto cement closed porosity (volume of closed pores in the cementing substance of the rock) and grain closed porosity (volume of closed pores in detrital grains of the rock). It is established that the indicator of grain closed porosity is 2-3 times higher than the indicator of cement closed porosity. The largest volumes of gas inclusions are established for the middle substage of catagenesis. Considering that rock and gas outbursts occur only at the middle substage of catagenesis, this confirms the theory about the additional effect of gas inclusions on the progress of gas-dynamic phenomena in mine opening.Also, duringthe determination of absolute porosity of rocks in laboratory conditions, it is very important to grind the rock to micron-sized fraction. This will allow opening a part of inclusions in the clastic grains of rocks, which will substantially complement the indexes of absolute porosity.

Author Biographies

V. A. Baranov
Institute of Geotechnical Mechanics named by N. Poljakov of National Academy of Sciences of Ukraine
Y. V. Antipovich
Institute of Geotechnical Mechanics named by N. Poljakov of National Academy of Sciences of Ukraine

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Published
2019-01-08