Abstract: To address the problems of difficult fluid evaluation while drilling for Palaeocene reservoirs in the Southwest Zone of Bozhong Sag, strong multi-solution, and contradictory response characteristics of mud logging and well logging data, a water content analysis based on light hydrocarbon sensitive parameters, well field light hydrocarbon carbon ring dominance analysis, biodegradation analysis based on alkane series comparison, and a method for evaluating well field light hydrocarbon fluids based on mathematical algorithms such as principal component analysis and support vector machine were established, and the example applications were performed. The evaluation results show that the reservoir water content plate based on light hydrocarbon sensitive parameters and the biodegradation degree interpretation plate based on alkane series comparison are effective in identifying fluid properties in the study area; the well field light hydrocarbon carbon ring dominance analysis method can be used for comparative analysis of reservoir genesis and origin, and has obvious advantages over conventional mud logging methods in evaluating fluids in complex reservoirs with multi-stage charging; the light hydrocarbon analysis method based on mathematical algorithms such as principal component analysis and support vector machine has a high compliance rate of about 85% in evaluating the fluid properties of complex reservoirs. This technical method has been applied in the study area with good results, which provides a new idea for the evaluation of complex reservoir fluids while drilling and has a good application prospect.

Key words: evaluation of fluid while drilling, light hydrocarbon analysis, complex reservoir, southwest zone of Bozhong Sag, Paleocene