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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.

Mean sonic transit times from eight stratigraphic units in forty-four wells in western and northern offshore Taiwan were plotted against depth. Apparent erosion (displacement on the depth axis from the normal compaction) was determined for each unit in each well. Cretaceous and Eocene units show greater apparent erosion than Miocene and Pliocene units in the same well, which in turn show more erosion than Pleistocene units. Apparent erosions derived from two different units are plotted against each other for correlation analysis. Based on the analysis, the main periods of erosion were during the Oligocene and Plio-Pleistocene.The timing of erosion inferred from sonic interval transit is consistent with recognized unconformities, and its areal distribution is consistent with the observed structural evolution of the region. The maximum true erosion (corrected for post-erosion burial) at the Oligocene unconformity was 3500 m on the Penghu Platform. Approximately 2500 m of true erosion occurred on the Kuanin Ulift and the Pengchiahsu Platform. Due to moderate-to-minor erosion and a great amount of post-erosion burial, true magnitude of erosion is masked at the Miocene unconformity in most of the area.Erosion of up to 500 m on the eastern side of the Penghu Island at the Pleistocene unconformity defines the shape of the present-day Penghu Waterway. In the northern offshore area up to 1000 m of Plio/Pleistocene, true erosion occurred in the Taiwan-Sinzi folded zone where Miocene folds are observed. The magnitude of true erosion decreased west of the Taiwan-Sinzi folded zone to 500 m in the Pengchiahsu Basin. A maximum of 500 m of true erosion is also observed in the west offshore area along Hsing-Chu to Miaoli coast. ᭧

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