An area-based model for estimating vitrinite reflectance of micro-sized particles

The maturity of organic matter is a primary control on the generation and distribution of shale oil and gas resources. However, accurate determination of vitrinite reflectance (Ro) in organic-rich shales remains challenging, particularly when vitrinite particles are smaller than the photometric aperture, so that the aperture is not fully occupied by vitrinite and strong background interference arises. In this study, we propose an area-based normalization approach for estimating the true reflectance of micro-vitrinite. Four sets of artificially matured coal samples with different reflectance levels were analyzed: three were used to construct the model and the fourth to validate it. The results reveal a consistent sigmoidal relationship characterized by a "slow-rapid-slow" growth pattern. After normalization, a Weibull model was fitted to the data, yielding a single curve applicable to all samples. Validation against the independent sample shows that the true reflectance can be estimated from as few as two apparent reflectance measurements made at contrasting areal proportions: of the 28 pairwise estimates, two that deviate by more than 1% from the measured value are excluded, and 18 of the remaining 26 (69%) reproduce the true reflectance to within 0.3%, the largest deviation among the retained estimates being 0.7%. Provided that the background reflectance is stable and the vitrinite areas can be determined reliably, this approach offers a practical, quantitative method for correcting background effects in micro-vitrinite reflectance measurements and for evaluating organic matter maturity in fine-grained sedimentary rocks.