Methodological workflow for assessing heterogeneity in immature organic-rich carbonate reservoirs

Understanding the vertical and lateral heterogeneity of immature organic-rich carbonate reservoirs is essential for predicting reservoir quality, maintaining wellbore stability, and identifying sweet-spot intervals in unconventional systems. Although geological, geochemical, petrophysical, and mechanical techniques are extensively employed for rock characterization, they are frequently applied in isolation, which can hinder a systematic assessment of variability. This paper introduces a structured, reproducible workflow that integrates lithological description, multivariate geochemical classification, continuous petrophysical profiling, and non-destructive mechanical testing into a unified framework for evaluating heterogeneity. The methodological innovation resides not in the individual analytical techniques but in their sequential integration into a facies-guided workflow that seamlessly links continuous non-destructive measurements with targeted laboratory analyses and predictive data integration. The workflow combines lithological descriptions, rock classification using unsupervised machine learning methods such as silhouette analysis, principal component analysis (PCA), and hierarchical clustering on principal component analysis (HCPC), bulk geochemistry, petrophysical profiling, non-destructive rock mechanical testing, and integrated data interpretation. This integrated method allows for accurate characterization of these complex reservoirs, supporting a systematic assessment of their heterogeneity, quality, and petrophysical and mechanical behavior. Continuous measurements are first used to capture spatial variability along the core interval, after which representative samples from identified facies domains are selected for laboratory analyses, including mineralogy, organic geochemistry, and petrophysical measurements. The integration of continuous and discrete datasets enables the evaluation of key properties across the entire core interval and systematic identification of heterogeneous zones. Lateral heterogeneity is identified by comparing lithofacies transitions, chemofacies shifts, and coordinated changes in porosity, velocity (Vp), total organic carbon (TOC), and strength (UCS) and hardness (HLD) logs. Moreover, the proposed workflow provides a clear methodological pathway for assessing vertical and lateral heterogeneity in immature organic-rich carbonate systems and can be adapted to unconventional hydrocarbon reservoirs, CO₂ storage formations, geothermal systems, and mineral exploration studies.