With advances in modern science and rapid offshore development, the development of marine unconventional hydrocarbons is placed on the agenda. However, a clear definition has not been established and is in an early formative stage. At present, because of technological limitations, the concept is confined to unconventional resources in marine sedimentary strata of modern marine settings, mainly methane gas hydrate, shallow gas, and shale gas/oil. Using the Qiongdongnan Basin in the northern South China Sea as an example, the distribution and coexistence of these resources were evaluated. In the deep-water basin, methane gas hydrate is formed within the upper ~ 200 m of seafloor sediments. Shallow gas is hosted beneath the hydrate-bearing interval. Deep gas reservoirs are mainly present in the central canyon and Mesozoic buried hills. Because their pressure regimes and reservoir properties differ, co-production is required to use multi-tubing zonal production, technologies to prevent interlayer interference, multilateral drilling, and sand-control measures. The coexistence of gas hydrate, shallow gas, and deep gas makes the Qiongdongnan Basin an ideal target for integrated three-gas development. Therefore, the Qiongdongnan offshore area is considered a favorable region for multi-gas co-production.