Reflectance of Vitrinite-Like Macerals as a Thermal Maturity Index for Cambrian–Ordovician Alum Shale, Southern Scandinavia

Abstract

ABSTRACT The Middle Cambrian to Lower Ordovician Alum Shale in southern Scandinavia contains kerogen macerals resembling vitrinite. The age of this rock unit excludes terrestrially derived vitrinite as an origin for these macerals. Nevertheless, the vitrinite-like macerals respond to heating in a way similar to that of suppressed vitrinite in younger shales. This similarity is demonstrated both for the natural system, where Alum Shale maturity ranges from immature to low-grade metamorphic conditions, and for hydrous pyrolysis experiments, where an immature sample of Alum Shale was isothermally heated at temperatures ranging from 280 to 360°C for 72 hr. Reflectance values from the naturally matured Alum Shale show an increase in standard deviations (i.e., anisotropy) with ith increasing maturity and a negative relationship with atomic H/C ratios of the total kerogen. These relationships are similar to those observed in post-Ordovician vitrinites. Comparisons with other vitrinites subjected to hydrous pyrolysis indicate the vitrinite-like macerals in the Alum Shale follow the suppressed-vitrinite pathway rather than the coal-vitrinite pathway. The occurrence of vitrinite-like macerals in the Alum Shale indicates that traditional vitrinite reflectance measurements are useful in evaluating thermal maturity of pre-Silurian rocks.