Lithofacial characteristics of Devonian to Bashkirian black shales and the sedimentary environment of the Dniepr-Donets Basin, Ukraine

Black shales of the north-western part of the Dniepr-Donets Basin were investigated by XRD- and XRF-analyses, associating the mineralogical composition of 122 core samples from 15 wells to the Devonian to Bashkirian... [ view full abstract ]

Black shales of the north-western part of the Dniepr-Donets Basin were investigated by XRD- and XRF-analyses, associating the mineralogical composition of 122 core samples from 15 wells to the Devonian to Bashkirian tectono-stratigraphic sequences of the basin. The amount, type and percentage of clay minerals in a range of less than 20 to more than 80 vol.-%, the siliciclastic detritus, the carbonates and the accessory components reflect the chemical sedimentary processes in an environment with reducing conditions. The attempt was made to correlate the lithofacies of Visean core samples with their organic geochemical parameters (TOC, HI). Devonian shales of the north-westernmost part of the basin (indicating the synrift-phase with volcanic activity) show feldspar contents up to 11 vol.-% (plagioclase and alkali feldspar), a mean quartz-value of 12 vol.-%, and an increased amount of expandable clays in the mixed-layer minerals. The transition from the Devonian to the Tournaisian stage is marked by a significant rise of the kaolinite content, and a decrease of illite, chlorite and mixed-layer clay minerals. In the Visean stage (continental deposits and cyclic successions in fluvial, shallow marine, lagoonal environments) the percentage of TOC % can be correlated with kaolinite, pyrite/marcasite and the quartz content. In Lower Visean stage a content of pyrite in excess of 15 vol.-% indicates the high amount of dissolved sulfate and reactive iron minerals in the formation environment. In Lower Visean carbonacous samples a kaolinite content up to 90 vol.-% was measured. The mean value analyses of Upper Visean samples indicate a nearly equal amount of kaolinite- and mica-group minerals. Mixed layer minerals (illite/smectite) are dominated by illite. In C1v2 (19/20) increased values of anatase may indicate organic-inorganic rock fluid interactions in an early diagenetic stage. In the shallow non-marine environment of the Serpukhovian stage the quartz content increases up to 70 vol.-%, a decrease of kaolinite, and an increase of chlorite is observable. Despite the comparatively low data density of the Bashkirian stage a further decrease of kaolinite, an increase of chlorite, and the predominant occurrence of mica-group minerals are determined.