A non-electrical assessment method for gas saturation of argillaceous sandstones based on the excavation effect and density-neutron log coupling mechanism

Gas saturation assessment represents a key task in log interpretation and offers a critical basis for guidance on developing schemes for the exploration and exploitation of hydrocarbon reservoirs. Gas saturation is a crucial parameter for characterizing the distribution of fluids with various properties in reservoirs, and its accuracy directly affects the reserves estimation, productivity prediction, and development outcome assessment of reservoirs. Currently, the gas saturation interpretation models based on Archie′s equation and electrical methods have seen wide application, with significant results being achieved. However, their effectiveness depends heavily on the representativeness of the collected rock samples. For argillaceous sandstone reservoirs with strong heterogeneity, it is difficult to comprehensively reflect their electroconductivity mechanism using fixed electrical parameters from individual rock samples. This limitation may lead to significant errors in prediction results, thereby reducing the accuracy and reliability of log interpretation. In this study, by analyzing the excavation effect observed in gas-bearing argillaceous sandstone reservoirs, we induce the parameter of density-neutron separation degree (DC), defined as the degree of separation between the density and neutron log curves, based on the density-neutron logging correlation. The response characteristics of the density-neutron separation degree under different gas saturation levels, shale contents, and porosities are compared through numerical simulation. Accordingly, a novel non-electrical method for the quantitative assessment of gas saturation is established based on the density-neutron separation degree. Analysis of the assessment results of gas saturation in the argillaceous sandstone reservoirs of the G Formation, T Basin indicates that the conventional models based on Archie's and Indonesian equations, which employ unified electrical parameters, are less suitable due to the strong heterogeneity of the target strata. In contrast, the proposed method based on the density-neutron separation degree shows greater adaptability and higher prediction accuracy. This method provides a reference for the gas saturation assessment of argillaceous sandstone reservoirs with strong heterogeneity.