An innovative non-electrical logging technique for gas saturation estimation in shaly sandstone reservoirs influenced by the excavation effect

Fluid saturation is a fundamental reservoir property, essential for accurate reservoir characterization and the optimization of exploration and development planning. Various saturation models derived from the Archie equation have consistently been the fundamental approach for saturation interpretation in electrical logging. However, due to differences in reservoir quality, the single fixed rock-electro parameters derived from experiments cannot adequately represent the conductive mechanisms of different reservoir types. This limitation results in saturation predictions that fail to accurately reflect the reservoir's actual gas-bearing properties. Although the “excavation effect” is often applied to identify fluid types in gas-bearing shaly sandstone reservoirs, it has seldom been used for quantitative saturation evaluation. This study is based on the interactive analysis of saturation, the excavation effect, and density-neutron separation degree (separation degree of density and neutron logging curves). It systematically analyzes the response mechanism between density-neutron separation degree and saturation under varying shale content and porosity conditions through numerical simulations. A new predictive method for gas reservoir saturation is developed by formulating a linear model that integrates density–neutron separation, shale volume, and porosity. When applied to the H Formation in the Y Basin, this approach demonstrated superior accuracy. The new DC-based model achieved a relative error of only 1.17 %, which is significantly more accurate than the Archie model (14.81 %) and the Indonesian model (5.63 %). The results validate the accuracy and robustness of the proposed method for gas saturation evaluation, offering practical insights for characterizing heterogeneous shaly sandstone reservoirs.