Organic petrography and geochemistry of the Lower-Carboniferous coals from the Ekibastuz Basin, Kazakhstan

The Bogatyr Komin open-pit mine in the Ekibastuz Basin, located in north-east Kazakhstan, is one of the largest coal mines in the world. It is based on three Lower Carboniferous seams (from bottom to top: seams 3, 2, and 1), which together form a 150 m thick, uniform seam complex. At the study site on the western basin edge, the seams dip steeply. Organic petrological and geochemical investigations have been performed to determine the maturity and depositional environment of these exceptionally thick seams. Vitrinite reflectance (0.8–1.1 %Rr) classifies the coal as high-volatile bituminous A coal, which is also supported by Tmax values and maturity-related biomarker ratios. Vitrinite reflectance patterns prove pre-deformational coalification and a relatively high paleo-heat flow, likely caused by magmatic activity. High ash yields and relatively low sulfur contents indicate that the coal accumulated in low-lying mires without marine influence. The input of detrital minerals decreased during peat accumulation. Relatively high concentrations of sulfur-bearing aromatic compounds, particularly in the lower part of seam 3, may reflect volcanic activity, which is also evidenced by the presence of distinct ash layers (“kaolinitic beds”) and kaolinite in coal samples. Thick wood-forming plants were rare in the peat-forming vegetation. However, samples from seams 1 and 3 contain higher concentrations of aromatic compounds derived from lignin-bearing arborescent cordiaite-conifer-pteridosperm vegetation. Enhanced bacterial activity during deposition of the upper part of seam 2 and seam 1 is indicated by elevated hopane concentrations. The exceptional thickness of the coal complex indicates a geodynamic setting with high subsidence rates. • Bogatyr coal in the Ekibastuz Basin is classified as high volatile bituminous A. • Maturity patterns suggest pre-deformational maturation and high paleo-heat flow. • The 150-m-thick coal complex accumulated during a single cycle of high subsidence. • Peat accumulation occurred in frequently flooded low-lying mires. • Precursor paleovegetation mainly comprised aquatic macrophytes and pteridosperms.