The Lower Fangliao Basin has been regarded as one of gas hydrate-bearing provinces in offshore southwestern Taiwan. Multi-channel seismic data hints major control of mud diapirism to gas hydrate and associated free gas preservation in the area. Based on distribution of seismic amplitude characters and reflection strength with respect to depth of bottom-simulating reflectors (BSRs), the hydrocarbon prospects can be divided into gas hydrate zones above BSRs, free gas zones above BSRs, and free gas zones below BSRs. Deciphering the in-place gas resources provides an estimate of scale and potential of gas hydrate as future energy resource. Its accuracy level of estimation depends on the amount of available data, which is limited in the early stage of exploration. Aided by 3D seismic interpretation, goodie extraction from seismic attribute can serve as proxy for geometry delineation of prospective areas. On the basis of Monte-Carlo simulation, the prospects appear to hold around 2,035 Bcf of total gas volume over an area of 60 square km. This finding shed lights on first-order estimation of subsurface methane resources in the Lower Fangliao Basin as an example of gas hydrate prospect in the upper accretionary wedge offshore southwestern Taiwan mountain belt.