Gamma ray log facies of nine wells were used to reflect the vertical profile of grain size and were combined with well sample data to deduce the paleoenvironment of the Erchungchi "A" Member in the Hsinyin and Pachanchi areas. Four log facies were recognized in the studied intervals: a thick funnel-shaped facies representing a prograding delta; a thin funnelshaped facies representing a crevasse splay; a boxcar-shaped facies representing a distributary channel; a bell-shaped facies representing a fluvial or deltaic channel. The paleoenvironment of the Erchungchi "A" Member in the Hsinyin area is mainly an imbricated delta system whose thickest lobe is located in Well S-1. The delta was first deposited in the headstreams of the submarine channels, followed by mudstones as cap rocks which seal hydrocarbons in stratigraphic traps. There are many submarine channels in southwestern Taiwan, so similar stratigraphic hydrocarbon trap conditions may occur elsewhere in southwestern Taiwan.