Dispersed organic matter from Silurian shales of the Barrandian Basin, Czech Republic: optical properties, chemical composition and thermal maturity

Abstract

Samples of lower Silurian (Liten and Kopanina formations) dark shales from the Barrandian basin, Czech Republic, were examined using reflected light microscopy, Fourier-Transform infrared spectroscopy (FT-IR) and gas chromatography-mass spectrometry (GC-MS). The samples contained uniform assemblage of organic material that included dominant graptolites, chitinozoa, bitumens and minor anthracite to meta-anthracite rank particle of unknown origin. Two types of graptolite morphologies were recognized: a nongranular blocky type and a granular type. Bitumen is also common in these rocks and forms discrete, oval bodies resembling droplets or angular particles with granular internal fabrics. The thermal maturity of the sediments was examined by measuring random reflectance of organic particles. The variations in graptolite reflectance in between three localities in the western part of the basin were relatively minor, suggesting a similar level of diagenetic transformation. The values range from 0.78% to 1.53% R r. The reflectance of graptolites was correlated with the reflectance of chitinozoa and bitumen, and also indirectly with illite crystallinity data, which indicates thermal maturation levels in the high-temperature part of the oil window to the onset of wet gas/condensate zone. Mature petroleum that commonly impregnates veins crosscutting the sequence further supports a maturation level within the oil window. Graptolite and bitumen reflectance values markedly elevated above the regional diagenetic background were found in several centimeter-wide contact zones immediately adjacent to basalt sills that locally penetrate the Silurian strata. As shown by computer modeling, heating in the range of 600–800 °C, which lasted only several years, was sufficient to promote an increase in reflectance (up to 2–2.5% R r) in the contact samples. With increasing thermal stress, the graptolite periderm undergoes progressive optical and structural changes being gradually converted into a highly condensed aromatic residuum structurally and chemically similar to highly matured kerogen. On a basin-wide scale, however, the overall impact of basalt intrusions on organic maturity of enclosing shales was minimal.