Petrographic, palynological and geochemical recognition of dispersed organic matter in the black Anthracosia Shales (Sudetes, south-west Poland)

Abstract

We describe the organic petrography, palynology and Rock-Eval pyrolysis values of lacustrine black shales termed the Anthracosia Shales (Upper Carboniferous/Lower Permian) in the Intrasudetic Basin (Sudetes, SW Poland). Samples were taken from cores of two boreholes: Rybnica Leśna PIG 1 and Ścinawka Średnia PIG 1. Maceral composition, miospore assemblage composition, palynofacies and geochemical characteristics of dispersed organic matter in the Anthracosia Shales were used to determine conditions of the environment and to evaluate their petroleum potential. Data from both organic petrography and palynology analyses enabled recognition of three broadly distinct organic associations in these shales: bituminous, humic, and intermediate, while Rock-Eval pyrolysis revealed the presence of bituminous and humic kerogen types I and III. Type I corresponds to the bituminous association, with amorphous organic matter (AOM) dominant in the palynofacies, and type III corresponds to the humic association with phytoclasts prevailing in the palynofacies. The thermal maturity of the organic matter is determined by the values of: (1) vitrinite reflectance VRo = 0.53–0.73%, (2) palynomorph 3–4 colour index, and (3) Tmax = 443–447°C, which indicate oil window maturity. Some of the TOC results (1.6–2.9 wt.%) indicate that the Anthracosia Shales are good and very good petroleum source rocks, though the thickness of this interval is low (4–5 m). Shales with TOC values <0.5 wt.% prevail, and may be classifed as poor source rocks.