Ediacaran organic-rich Kalus Beds of western Ukraine and NW Moldova: mineralogy, total organic carbon content and hydrocarbon potential

The Ediacaran succession occurs at the southwestern slope of the East European Platform from the Black Sea to the Lublin-Podlasie Basin. It includes a black shale interval in Ukraine represented by the Kalus Beds which may have hydrocarbon source rock potential. Coeval organic-rich rocks are known to have sourced commercial accumulations of oil and gas in different parts of the world. In western Ukraine and NW Moldova, the thickness of the Kalus shales exceeds 90 m. These strata crop out locally near the border of the Ukrainian Shield and dip in the southwestern direction towards the Teisseyre-Tornquist zone, reaching the depths of over 4000 m. This paper presents a study of hydrocarbon source rock potential of the Kalus Beds and is based on the results of organic geochemical and mineralogical investigations. The organic-rich Kalus Beds are represented by clay-rich siltstone in which quartz (38–66%) and mica (8–19%) prevail over clay minerals. They also contain a significant amount of feldspar (4–11%) and chlorite (4–12%). The clay fraction is represented by illite, chlorite, kaolinite and illite-smectite. Mixed-layered clay minerals are distributed at burial depths of up to 150 m. Kaolinite occurs in a wider range of depths, but below 3800 m clay minerals are represented exclusively by chlorite and illite. Such a proportion of mineral components of the analysed beds and the grain size of quartz indicate their high brittleness and thus suitability for efficient hydraulic fracturing. The total organic carbon content in the Kalus shales varies between 0.1 and 0.89 wt%, exceeding 0.5 wt% in some parts of the study area, and thus these strata can be considered as a low-quality potential hydrocarbon source rock. The maturation level of the Kalus Beds clearly depends on the depth of burial. These rocks are immature till a depth of 100 m. Generation windows for oil and gas occur within depth ranges from 100 to 2200 m and from 2200 to 3800 m, respectively. At greater depths these rocks are overmature. In the main part of the study area, the organic-rich Kalus Beds have significant thickness and are matured to the level of oil and gas windows and hence might generate hydrocarbons and contribute to the development of the petroleum system in the region.