Application of carbon/oxygen logging to fluid identification in buried hill open-hole wells

Carbon/Oxygen (C/O) logging is an effective method for calculating oil saturation and is widely applied in cased-hole wells. Currently, traditional logging methods cannot accurately identify fluids in buried hill open-hole wells. Therefore, C/O logging is significant for fluid identification in buried hill reservoirs. However, the conventional C/O logging is limited due to the low porosity and complex lithology of buried hills. To address the problem, this study introduces the regularized non-negative least squares (RNNLS) approach into C/O logging, and an improved C/O logging method is proposed. The results demonstrate that selecting appropriate regularization factors effectively suppresses noise interference, while preserving C/O logging sensitivity. The method improves the stability of C/O results, and enables fluid identification in low porosity formations. Furthermore, the effect of this C/O method is analyzed in the open-hole environment of buried hills. The impact of lithology depends on whether the carbon element is present in the rock framework. Wellbore size and shale content have a significant effect on the C/O value in carbonate formations, and the effect is negligible in sandstone. The improved C/O logging method was applied in a field well example. The effectiveness of the RNNLS approach was confirmed by comparing elemental logging data and other logging results. This study highlights the significance of the C/O logging method in improving fluid identification accuracy in buried hills.