Advancing Saturated Reservoir Evaluation: Gas Cap Identification and Downhole Constant Composition Expansion Experiment in Volcanic Breccia Formations via New-Generation Formation Testing

Abstract The Jiamuhe Formation in the Junggar Basin comprises volcanic breccia reservoirs where fluid typing remains petrophysically challenging. Traditional methods (NMR, triple-porosity logs) suggested oil-bearing zones with free water, while DST data misinterpreted gas cap signals as dissolved gas, obscuring gas-oil contacts (GOC). This study integrates new-generation formation testing (NGFT) with advanced petrophysical logging to address these limitations. This study systematically demonstrates three key advancements: ① A methodological framework integrating spectroscopy logging, 2D nuclear magnetic resonance (NMR) logging, array dielectric measurements, and conventional triple-porosity logs was established for comprehensive reservoir property evaluation and preliminary sweet-spot screening. The integrated analysis enables lithofacies classification and pore structure characterization. Subsequently, NGFT technology was employed to dynamically resolve fluid properties, including composition, viscosity, density, formation volume factor (FVF), and gas-oil ratio (GOR). Calibration of these downhole-measured fluid parameters significantly enhanced petrophysical interpretation accuracy by recalibrating critical parameters (e.g., Archie exponents, saturation models) used in log-based fluid typing workflows. ② The implementation of in-situ constant compositional experiments (CCE) using NGFT tools is presented. Through real-time downhole fluid analysis during controlled depressurization processes, bubble-point pressures were experimentally determined under reservoir conditions. This data-driven approach facilitated the construction of field-specific phase envelope, enabling precise identification of GOC through thermodynamic equilibrium analysis. ③ Through NGFT, we conducted a systematic dynamic evaluation involving step-rate production testing and pressure buildup analysis. This workflow enabled quantitative determination of key reservoir performance parameters: drainage radius, effective permeability, and productivity indices. The acquired dynamic data (flow rates vs. pressure responses) were analyzed using transient flow theory models to establish reservoir deliverability characteristics. These parameters provide critical empirical constraints for production prediction models, effectively reducing uncertainties associated with conventional log-derived permeability estimations. The NGFT technique successfully identified the reservoir fluid saturated oil with a gas cap. GOC positions were determined through in-situ CCE and density contrast analysis between gas and oil phases. These accurately characterized fluid properties provided critical constraints for optimizing petrophysical interpretation parameters, effectively resolving the long-standing challenges within volcanic breccia reservoirs. Furthermore, dynamic reservoir evaluation via NGFT enabled precise productivity assessment, with calculated productivity showing remarkable consistency with subsequent DST measurements. NGFT confirms a saturated oil reservoir with gas cap (GOR: 63.5-65000 m3/m3), resolving historical fluid typing controversies. Recalibrated petrophysical parameters improve water saturation calculations and further oil/gas identification. Productivity predictions align with DST results, validating the integrated approach.