Organic and inorganic geochemical study of the Lower Permian Walchia shale of the Intrasudetic Basin (SW Poland)

Abstract

The Lower Permian Walchia shale of the Intrasudetic Basin is lacustrine sediment enriched in organic matter and base metals. Rock-Eval geochemical analysis was conducted on 110 drill-core rock samples of the Walchia shale in order to characterize the quantity, genetic type and maturity of dispersed organic matter. Concentrations of U, Th, Cu, Ni, V, Pb, Zn, Co, Mo, Cr, P, S and Fe were determined and microscopic studies to recognize mineral composition were performed on selected samples. Organic matter of the Walchia shale is immature and dominated by terrestrial type III kerogen with an admixture of planktonic type II kerogen. The Walchia shale is generally poor source rock, but some parts of the profile exhibit good and even excellent hydrocarbon potential. The average concentrations of analysed metals are higher than the average reported for black shales. The concentrations of base metals do not correlate with TOC contents, suggesting that their presence is associated with inorganic features of the Walchia shale or they could have been related primarily to organic matter which was subsequently oxidized and altered. Uranium may be partly associated with phosphates. Results of microscopic investigations revealed the lack of detectable radioactive minerals, abundance of framboidal pyrite, and prevalent galena, sphalerite and chalcopyrite.