Resolution Choices in Rock Classification: A Comparative Study of Core Logging and XRF Techniques

Unconventional reservoirs including shale gas, tight oil, and organic-rich mudstones have emerged as key contributors to global energy supply over the last two decades, owing to advances in horizontal drilling and hydraulic fracturing. However, the inherent complexity and fine-scale heterogeneity of these formations pose significant challenges for accurate reservoir characterization. Sampling resolution is a critical but often underappreciated factor in both core analysis and X-ray fluorescence (XRF) geochemical characterization. In this study, we systematically evaluate the effects of sampling interval by comparing core logger and XRF data at 1, 3, 5, 15, and 30 cm intervals. We analyze the impacts on facies classification, continuous property measurements, and the detection of organic-rich (TOC) zones. Our results show that, although mean property values and bulk facies proportions are relatively stable across different resolutions, local agreement in facies assignment and the ability to detect thin, TOC-rich beds decline sharply at coarser intervals. High-resolution scanning (≤3 cm) proves essential for reliable advanced interpretation, including sweet spot identification, geomechanical assessment, and completion strategy. Our findings provide both quantitative benchmarks and practical guidelines for future core characterization in resource plays, emphasizing the need for careful selection of sampling resolution to avoid misinterpretation and missed opportunities.