Evaluation of Shale Reservoir Quality Using an Integrated Multidisciplinary Workflow

Abstract

Shale resources have become a major contributor to global energy supply by providing large volumes of unconventional oil and gas, thereby strengthening energy security. Reliable evaluation of shale porosity, permeability, movable oil saturation, and hydrocarbon generation potential is essential for effective exploration and production planning. These parameters govern reservoir quality, influence hydraulic fracturing performance, and underpin resource potential estimates, ultimately affecting project economics. At present, the crushed-rock GRI method, introduced in 1996, remains the most commonly applied standard for determining shale porosity, saturation, and matrix permeability. However, this technique is associated with well-recognized uncertainties. Fluid loss during crushing, water gain or loss caused by evaporation and adsorption, and incomplete removal of residual water and hydrocarbons during cleaning can all bias porosity and saturation measurements. In addition, the GRI method does not differentiate movable hydrocarbons from solid organic matter, leading to ambiguity in resource assessment. To address these limitations, this study develops an integrated workflow combining nuclear magnetic resonance (NMR), GRI analysis, and pyrolysis to improve the quantification of shale pore structure, permeability, fluid saturation, and solid organic content. The workflow begins with high-frequency NMR (HF-NMR) T1–T2 fluid typing on intact core plugs to directly determine in-situ fluid volumes. Gas-filled pore volume and permeability are then measured using pressure decay techniques. After bulk density determination, the plug is crushed and homogenized, and aliquots are allocated for GRI and Rock-Eval analyses. Following cleaning, pressure decay measurements are repeated on the GRI sample, and HF-NMR T1–T2 measurements are used to quantify residual fluids. The cleaned material is subsequently divided for Rock-Eval pyrolysis, X-ray diffraction (XRD), and mercury injection capillary pressure (MICP) analyses. Within this framework, fluid-filled pore volumes are derived from HF-NMR measurements on the intact plug, while pore volumes measured on cleaned crushed samples are corrected for residual fluids identified by post-cleaning NMR. Moreover, HF-NMR T1–T2 data enable clear discrimination between producible oil and immobile solid organic matter, providing an independent validation of pyrolysis-derived parameters. The workflow was applied to the Kongerdun shale oil interval in the Cangdong Depression. The oil in this formation is characterized by high wax content and exists in a solid or semi-solid state at ambient temperature. To enhance oil mobility, HF-NMR measurements were conducted at elevated temperatures. The results show that HF-NMR yields substantially higher movable oil saturation than Rock-Eval pyrolysis, while total hydrocarbon contents derived from both methods are in close agreement. This discrepancy indicates that conventional pyrolysis significantly underestimates movable oil volumes and correspondingly overestimates hydrocarbon generation potential in wax-rich shale oil systems. Overall, the proposed multidisciplinary workflow provides more reliable estimates of storage capacity, movable oil saturation, hydrocarbon generation potential, and permeability in shale reservoirs. By reducing methodological uncertainties inherent in traditional approaches, this workflow offers a more robust basis for shale reservoir evaluation and supports improved decision-making in exploration and development planning.