Silurian black shales of Western Ukraine: petrography and mineralogy

Organic-rich Silurian black shale, stretching along the western margin of the East European Platform from the Baltic Sea to the Black Sea, represent a potential target for conventional and unconventional hydrocarbon exploration. In Western Ukraine, these strata occur in the most deeply buried part of the platform and their thickness reaches 1000–1500 m. On the basis of petrographic investigations of rocks enriched with total organic carbon (TOC) (up to 2.16 wt%), their mineral composition has been established and four types of rocks have been distinguished: mudstones, clayey mudstones, limy marlstones and marlstones. In these rocks quartz (30–60%) significantly (1.5 to 2 times) prevails over the clay minerals. They contain a large amount of carbonates (calcite, dolomite): from 5–15% in mudstones and limy mudstones to 51% in marlstones. It was established that quartz grains, by their size correspond to the very fine sand (0.125–0.062 mm) and coarse silt (0.062–0.031 mm) fractions. Such a proportion of minerals in the rocks and the quartz grain size indicates their high brittleness, and consequently, the possibility of the efficient hydraulic fracturing within these strata. X-ray diffraction investigations were used in order to obtain the quantitative estimate of the mineral composition of the rocks with the highest TOC content – mudstones and limy mudstones. Additionally, it was established that the clay fraction was represented by сhlorite and illite. These minerals are present in the rocks in different proportions, depending on the depth and the area of occurrence. Illite content decreases, while chlorite content increases with depth, contributing over 30% of the clay fraction. Smectite and mixed-layered minerals are lacking in the clay fraction of the rocks, which provides favourable conditions for their eventual hydraulic fracturing.