Considerable heavy oils were discovered on the North Steep Slope (NSS) of the Dongying Depression in the Bohai Bay Basin. Oils from the NSS and adjacent Lijing Sag were collected for detail qualitative and quantitative investigation. It was observed that most of the oils from the NSS were subjected to biodegradation or water-washing, which result in saturated hydrocarbons and aromatics of the oils that are depleted in certain degree and absence of 25-norhopanes in most of the oils. It was observed that the oils with different degrees of deformation were mixed commonly in NSS based on composition and relative distribution of biomarkers, suggesting probably at least two main periods of oil charging in the area. The results also show that biomarkers quantification absolutely is useful for identifying the level of biodegradation and providing information of resistant ability of the biomarkers in the oils against microbial removal. According to detail geological and hydrological investigations combined with geochemistry, we suggest that chemically, biologically and physically such actions as biodegradation, water-washing and oxidation are the main genetic mechanisms resulting in the formation of heavy oils, which make most light components of crude oil escaped while heavy components concentrated. It is concluded that special geological backgrounds such as weathering crust of burial hills on the NSS, high-porosity and high-permeability coarse sands and conglomerates of orderly distributed fan reservoirs, shallow buried oil pools, poor cap rock property and lateral plugging conditions, and development of complex faults and several unconformities are the most essential factors responsible for the formation of heavy oils on the NSS. Coexistence of heavy oils and relative normal oils on the NSS indicates that there is still a good prospect for the exploration of normal oils.