Integrated Petrophysical Workflow and Recovery Factor Index for Unconventional Resource Assessment of the Lower Napo Shale, Oriente Basin, Ecuador

Abstract The progressive decline of conventional hydrocarbon reserves and the increasing importance of unconventional resources have intensified interest in shale oil assessment worldwide. Ecuador’s Oriente Basin, historically recognized as the country's principal conventional oil-producing province, contains several organic-rich intervals with potential for unconventional hydrocarbon development. This study presents an integrated geological and petrophysical evaluation of the Lower Napo Shale within the central structural domain of the Oriente Basin with the objective of identifying prospective unconventional fairways, estimating shale oil resources, and reducing uncertainty associated with recovery-factor selection. The study area focuses on the Sacha–Shushufindi corridor, which represents the principal hydrocarbon generation kitchen of the basin and is characterized by favorable thermal maturity conditions and the occurrence of Type II kerogen. More than 27 wells were evaluated using wireline logs processed with Interactive Petrophysics (IP) software, integrating stratigraphic interpretation, thermal maturity analysis, geochemical analogs, and quantitative petrophysical characterization. Geological screening included gamma-ray response, resistivity, total organic carbon (TOC), vitrinite reflectance (Ro), shale thickness, porosity, and hydrocarbon saturation. Results indicate that the Lower Napo Shale exhibits gamma-ray values between 150 and 190 API, suggesting TOC values greater than 2%, while resistivity values exceeding 15 ohm-m indicate favorable hydrocarbon retention. Thermal maturity analysis demonstrates that the central domain lies within the oil generation window, with Ro values ranging from 0.7% to 1.0%, burial temperatures near 128°C, and heat-flow values exceeding 58 mW/m². Detailed petrophysical evaluation of four representative wells from the Sacha Field yielded average effective porosity of 15.9%, hydrocarbon saturation of 54.3%, and net shale thickness of approximately 60 ft. The prospective area was delineated over approximately 4,000 km² within the central domain of the basin. Volumetric calculations yielded an estimated Original Oil in Place (OOIP) of 33.89 × 10⁹ bbl. Applying the Petroleum Resources Management System (PRMS) methodology and recovery factors derived from analog shale developments, technically recoverable prospective resources were estimated at 2.37 × 10⁹ bbl under the P50 scenario. To address uncertainty associated with recovery-factor selection in the absence of representative PVT data and production history, a Recovery Factor Index (RFI) was developed integrating geological quality, petrophysical properties, drilling efficiency, and hydraulic-fracturing effectiveness. Application of the proposed methodology resulted in a Recovery Factor Risk Index (RFRI) of 0.845, corresponding to an estimated recovery factor of 8.45% and technically recoverable resources of 2.86 × 10⁹ bbl. The results indicate that the Lower Napo Shale constitutes one of the most promising unconventional oil opportunities identified in Ecuador. Furthermore, the proposed RFI framework provides a transparent and reproducible methodology for estimating recovery efficiency in frontier unconventional plays where production history and representative fluid-property data are unavailable. The approach therefore represents a practical contribution to unconventional resource assessment and offers a complementary tool for reducing uncertainty during the early stages of shale resource evaluation