Maceral point counting for dispersed organic matter (DOM)

Abstract

Point counting is a widely used quantitative technique in microscopic analysis for estimating the volume percentage of target constituents within a sample. Its application in coal petrography has been standardized through international guidelines such as ISO 7404 and ASTM D2797/D2799, which were primarily developed for the analysis of carbonaceous materials such as coal and peat. These standards have governed sample preparation, maceral identification, and point counting procedures since the 1980s. However, modern applications of coal petrography point counting to dispersed organic matter (DOM), particularly in low-TOC samples, have revealed methodological incompatibilities. As a result, significant procedural modifications have been implemented across laboratories, but no universally adopted approach has been established.

To address these challenges, this study proposes and evaluates a point counting method for dispersed organic matter (DOM) using a 21-crosshair grid reticle. The method embeds 21 crosshairs within a 60-μm-diameter region (under 500× magnification) in the microscope system and requires counting at least 300 suitable microscopic fields where all crosshairs fall on sample particles. The use of 21-crosshair grid increases the probability of intersecting DOM in sample particles within pellets. This results in a 68 % reduction in analysis time (from 2.5 h to 1.0 h per sample) compared to when traditional coal petrography point counting is applied to DOM. Furthermore, the use of a denser grid in the 21-crosshair DOM point counting method improves measurement accuracy by more effectively capturing the areas of constituents within sample particles. By requiring the counting of at least 300 suitable microscopic fields, it generates a robust dataset of over 6300 points, achieving strong correlations (R² > 0.80) between the volume percentage of maceral assemblage and TOC in case studies. The method has been validated in applications such as hydrocarbon potential assessment, solid bitumen characterization, thermal maturity analysis, and depositional environment reconstruction. This paper bridges the gap between traditional standards and modern analytical needs by proposing a standardized, efficient, and reliable method for DOM point counting.