Seafloor sediment samples collected from southwestern offshore Taiwan have been analyzed for hydrocarbon gases resident in samples by using the technique of headspace gas analysis. The results reveal that the gas content is in tens to thousands ppm (vol. in wet sediments). Both microbial gas (usually called biogenic gas) dominated with methane and thermogenic gas containing C₂⁺ hydrocarbons were detected, inferring that the gases involved in the potential gas hydrate occurrence in the study area may have multiple origins. The microbial gas generated by methanogenic archaea in immature sediments is more widely distributed than thermogenic gas generated in sediments of the catagenesis stage. The presence of thermogenic gas infers an effective petroleum system, which may favor the formation of gas hydrate as well as for oil and gas exploration.