Preparation techniques for microscopic observation of dispersed organic matter and their effect on vitrinite reflectance

Abstract

Microscopic petrographic analysis of fine-grained sediments requires standardised, high-quality preparation of polished sections, generally conducted by water-involved grinding and polishing techniques following respective norms (ISO 7404-2, 2009; ASTM, 2021). However, vitrinite reflectance results show significant variations between different laboratories, partly caused by inconsistencies in sample preparation or poor sample preparation, e.g. due to the effect of swelling clays. This study evaluates the effect of alternative preparation techniques on the reliability and consistency of vitrinite reflectance data measured on shales containing dispersed organic matter (kerogen type III) using (1) an oil-based lubricant for traditional grinding and polishing and (2) BIB (broad-ion-beam) polishing. Different preparation techniques were applied on ten samples from two well sections (Hervest 5 and Prosper-Haniel) from the Ruhr Basin (Germany), covering the coal-bearing Pennsylvanian sedimentary successions of the Westphalian B and C with a maturity range of 0.56 to 0.78% and 0.73 to 0.92% vitrinite reflectance, followed by vitrinite reflectance measurements.Preparation under the use of oil-based lubricant led to similar reflectance values compared to traditional water-based grinding and polishing. Oil-based treatment improved the quality of two samples having the lowest thermal maturities, as indicated by increased vitrinite reflectance values by up to 0.1%. After BIB polishing, vitrinite reflectance systematically increased by up to 0.25% (relative increase of +29%), with the highest BIB-induced reflectance increase observed at a thermal maturity of 0.86% vitrinite reflectance (before BIB polishing). A practical geological approach was tested by calibrating 1D basin models using post-BIB vitrinite reflectance values. The data reveal a systematic trend in vitrinite reflectance between pre- and post-BIB polishing, suggesting that BIB-polishing might has future potential to be established as a routine for sample preparation for microscopic observation of organic material.