Abstract: With the wide application of oil-based drilling fluids in the Xihu Sag, the log response characteristics of reservoirs have changed. This increases the difficulty in identifying complex fluids, rendering fluid assessment methods based on water-based drilling fluids unapplicable. Therefore, this study developed a complex fluid identification method suitable for oil-based drilling fluids based on differences in the gas logging composition, the geochemical characteristic parameters of light hydrocarbons, and spectral morphologies between condensate gas and light oil reservoirs. The results are as follows: (1) Based on the data processing and the selection of optimal sensitive parameters, Pearson correlation analysis was conducted on 10 gas logging-derived parameters. Then, chart boards were developed using sensitive parameters Hc and Hb, allowing for qualitative distinguishment between condensate gas and light oil; (2) Through the fitting of Hc, the parameter with the highest correlation with the gas/oil ratio, to the pump sampling and stratigraphic test data, this study established a quantitative calculation model for the gas/oil ratio, yielding a correlation coefficient exceeding 0.98. Therefore, the model can be used to quantitatively predict the gas/oil ratio while drilling; (3) Significant differences can be observed in the morphological characteristics of geochemical light hydrocarbon spectra between condensate gas and light oil reservoirs. Specifically, the condensate gas reservoirs exhibited high nC1 to nC4 contents, an absence of normal alkanes beyond nC5, and incomplete and low peaks of iso-alkane and aromatic hydrocarbons due to small detection ranges. In contrast, the light oil reservoirs displayed complete n-alkanes components from nC1 to nC9, along with higher peaks of iso-alkanes and aromatics hydrocarbons. (4) Based on the differences in light hydrocarbon ratios between condensate gas and light oil reservoirs, gas index Ig and oil index Io were selected to establish chart boards, which can effectively distinguish both fluids. The combination of gas logging and geochemical light hydrocarbon analysis can effectively overcome the challenge of fluid identification under the condition of oil-based drilling fluids in the Xihu Sag, deserving wide application.