After more than half a century of exploration and development, significant progress and breakthroughs have been made in oil and gas/unconventional oil and gas (shale oil/natural gas hydrates) extraction in offshore basins in China, and these basins have become an important area for increasing oil and gas reserves and production in China, as well as a new growth pole for fossil energy resources. Based on previous research and a large number of oil and gas exploration and development achievements in recent years, this article analyzes and summarizes the structural types, sedimentary fill characteristics, oil and gas genesis types, and hydrocarbon supply relationships of the main basins in the Bohai Sea, East China Sea, and northern South China Sea near China. It further elucidates the genesis and reservoir combination types of oil and gas/unconventional oil and gas in the main basins, pointing out that they generally have the characteristics of “terrestrial marine storage, lower generation upper storage, self-generation self storage, and new generation ancient storage” and “one source, multiple flows”. On this basis, it further reveals the differences in oil and gas accumulation and enrichment in different basins and zones. Exploration and research show that the Bohai Basin, Beibu Gulf Basin in the South China Sea, and the shallow water area in the northern region of the Pearl River Mouth Basin belong to “oil type basins”, producing mainly oil. The East China Sea Basin, Yinggehai Basin in the northern region of the South China Sea, Qiongdongnan Basin, and deep-water area in the southern part of the Pearl River Mouth Basin are “coal-type gas basins”, forming mainly coal-type gas fields or oil and gas fields. The differences in oil and gas accumulation and enrichment in China’s offshore basins are mainly controlled by the “basin source heat transport” mechanism. Among these basins, conventional oil and gas distribution is mainly enriched in hydrocarbon-rich depressions and their periphery. The quality of source rocks and the type of reservoir combination determine the characteristics of the oil and gas resource production and distribution enrichment in the basin. Unconventional shale oil and gas are mainly enriched in the sedimentary fill of rich hydrocarbon generating depressions with thick burial depths and are closely related to the distribution scale and hydrocarbon generation potential of high-quality shale source rocks. Although the distribution and enrichment of natural gas hydrates follow the source control theory, similar to conventional oil and gas, natural gas is mainly transported and accumulated in a water-soluble phase, controlled by gas sources and high-pressure and low-temperature stable zones, and produced in the form of solid mineral deposits. At the end of the article, it is emphasized that conventional and unconventional oil and gas are vertically distributed and enriched in the deep-water basins of the northern South China Sea, with the significant characteristics of shallow hydrate and shallow biogenic gas/pyrolysis gas coexisting with deep high maturity oil and gas and complex enrichment. This provides an important basis for decision-making, a reference for evaluation and prediction of oil and gas and unconventional oil and gas enrichment areas in the future, and the implementation of oil and gas exploration and development strategies such as simultaneous development of oil and gas and exploration and co-production of both normal and unconventional oil and gas.