Abstract: The ultra-deep water area of the Pearl River Mouth Basin remains poorly explored, from which only LW21-A well in the Zhusi Depression has oil and gas discovered. Through systematic molecular geochemical testing and analysis, the types of hydrocarbon source rocks encountered were specified and the genesis of oil and gas discovered were revealed. Result shows that the Wenchang Formation and Enping Formation of the well developed marine (transgression) mudstone and brackish shallow lake (offshore lake basin) mudstone. There are two types of condensate in LW21 structural gas reservoir: marine (transgressive) and lacustrine. The main component of natural gas in LW21 structural gas reservoir is CO₂, which belongs to CO₂ gas reservoir and contains a certain amount of alkane gas. CO₂ is mainly inorganic mantle-derived genetic gas, alkane gas is mixed dry gas, mainly the mixture of oil-type gas and marine biogenic gas. This understanding is of great significance to the further evaluation and exploration decision of the Zhusi Depression zone in the ultra-deep water area of the Pearl River Mouth basin.