Abstract:  The heavy oil reservoirs in  Beibu Gulf basin of the western South China sea are mainly distributed in  Weixinan sag and Wushi sag, It is mainly composed of ordinary heavy oil and extra heavy oil. This type of heavy oil has a wide burial depth unit and can be discovered from shallow to middledeep layers, with good physical properties and characteristics of medium high porosity, permeability, and high resistivity. However, conventional gas logging often only includes total hydrocarbons and methane, the gas measurement value is very low and there is no abnormal display, no fluorescence or oil bearing indication is found in most areas during cutting logging, which brings great difficulties to formation fluid identification. Therefore, classification and logging characteristics analysis are conducted for different types of heavy oil in this area. On this basis, a fast identification method for heavy oil is proposed by combining logging curves while drilling with the oil content of the wall core as the main means, supplemented by gas logging maps and threedimensional quantitative fluorescence recognition of the wall core. By utilizing the monitoring of fluid properties in underground formations and the results of rock pyrolysis analysis during the sampling process, it is pointed out that the typical hydrocarbon composition of heavy oil itself creates its unique gas logging characteristics. Meanwhile, the analysis suggests that the strong hydrophilicity of rock wettability and the special adhesion between asphaltene heavy oil and sandstone skeleton particles are the main reasons for the lack of fluorescence and oil content display in some heavy oil reservoir cuttings. It is also pointed out that the threedimensional quantitative fluorescence of the wall core and sampling fluid can truly reflect the properties of the formation fluid.

Key words: heavy oil reservoir, heavy hydrocarbon component, wettability, bitumen adhesion, fluorescence logging, downhole fluid analysis, Beibu Gulf basin