Abstract: Since the oil and gas exploration activities were carried out in Weixinan Sag of Beibu Gulf Basin, several oilfields have been found in the main depression, while the exploration of D Sub-sag has not made a breakthrough until recent years. The research on D Sub-sag, especially the first discovery of the heavy oil reservoir in Neogene, is lack of knowledge. In view of this problem, the characteristics, genetic mechanism and main reservoir forming conditions of the heavy oil in D Sub-sag of Weixinan Sag are systematically studied and analyzed by using new well data and regional geological data, geochemical and reservoir forming methods, and a series of research results are obtained. It is found that the heavy oil in D Sub-sag is the secondary heavy oil with high density, high viscosity, high asphaltene, low sulfur and wax content. At the same time, potential hydrocarbon traps of the whole D Sub-sag in Neogene are predicted by using the reservoir forming mechanism. The research results have important theoretical guiding significance for the future shallow exploration in D Sub-sag of Weixinan Sag.