Three-Dimensional Identification of In Situ and Migrated Organic Matter in Shale Based on Micro-CT

Abstract

Accurate differentiation of in situ organic matter (IOM) from migrated organic matter (MOM) and their three-dimensional characterization are fundamental for understanding hydrocarbon generation, migration, and accumulation in shale reservoirs. However, conventional micro-computed tomography (micro-CT) cannot distinguish organic matter subtypes due to insufficient resolution. This study establishes a novel workflow integrating micro-CT with high-resolution scanning electron microscopy (SEM) through image registration. A cubic sample with parallel opposite faces facilitated ion milling and SEM-CT registration. Under SEM supervision, IOM, MOM, and pore–fracture space were interpreted in the micro-CT volume. Applied to the Chang 7 Member shale in the Ordos Basin, results reveal IOM occupies the largest volume fraction (6.96%), followed by MOM (0.94%), while pore–fracture space accounts for only 0.13%. MOM exhibits four morphological types interconnected by throat-like bridges at the ~3.7 μm scale. MOM and pore–fracture components share similar thickness distributions peaking at ~3.7 μm, reflecting their common inheritance from the pore–throat system, whereas IOM is controlled by sedimentary and compactional processes, favoring finer fractions (<3.5 μm). MOM volume far exceeds current pore space, indicating early hydrocarbons predominantly filled connected networks. This workflow overcomes conventional micro-CT limitations, providing a more objective approach for micro-component identification in shale micro-CT data.