This paper deals with the Neogene formations distributed in the foothills region and under the coastal plain of the Chiayi-Hsinying area of west-central Taiwan, and is one part of the regional stratigraphic study of the late Cenozoic sedimentary basin of western Taiwan.

The type section of the lower Miocene Peikang Formation is selected in the Peikang PK-3 stratigraphic test well in the coastal plain, and the thickness of the type formation is 444 m. The Chutouchi Formation, Maupu Shale, Ailiaochiao Formation, and Yenshuikeng Shale of the upper Miocene series in the foothills region have been found only in the CL-1 wildcat on the Chunlun structure. The type locality of the lower Pliocene sequence, which includes the Chunlun, the Niaotsui, and the Yunshuichi Formation, is along the Yunshui-chi.(FOOTNOTE 3) The thickness of these formations at the type locality are 485, 650, and 450 m, respectively. The total thickness of the upper Pliocene Liuchungchi Formation in its type locality at the Liuchung-chi is 760 m; of the upper Pliocene Kanhsialiao Format on at the Kueichung-chi is 540 m; of the Plio-Pleistocene Erhchungchi Formation (also at the Kueichung-chi) is 440 m; and of the Pleistocene Liushuang Formation at the Liushuang-keng (FOOTNOTE 4) is 1,040 m.

Only minor variation in lithofacies is observed in the lower Miocene Peikang Formation in the different wells in the coastal plain. From the isopachous pattern and the ecologic aspect of the foraminiferal assemblages, it is clear that the Peikang Formation was deposited on the buried shelf and basin slope in a moderately deep-water (neritic) environment. The several formations from the upper Miocene to the Pleistocene gradually thin in the whole area from the foothills region westward across the coastal plain to the coast, and their lateral lithofacies variations reveal that, in general, the sand content progressively decreases and the mud content gradually increases from northeast to southwest. The upper Miocene sequence was deposited in a nearshore, shallow-marine environment with r stricted access to oceanic influence, whereas the depositional environment of the lower Pliocene sequence was moderately shallow-marine open-sea conditions. The ecologic aspects of the foraminiferal assemblages from the upper Pliocene to the Pleistocene suggest that deposition was in a shallow-marine environment with limited access to oceanic influence. Growth faulting appears to have been important during deposition of the lower Pliocene and upper Miocene formations.

Under the coastal plain, hydrocarbon accumulations may be found in structural traps or possible stratigraphic traps developed toward the Peikang buried shelf as a result of overlapping of the lower Miocene, the upper Miocene, and the lower Pliocene series on the basin slope. In the foothills region, the potential oil and gas reservoirs in the upper Miocene series in the Chunlun structure are considered to be the most promising.

The 300-m and 700-m gas zones of the Niushan gas field are sandstone members in the upper Pliocene Liuchungchi Formation. Sandstone members equivalent to them are the drilling objectives in all the other structures of this area. Natural gas is found in the water wells on the coastal plain, and potential shallow reservoirs of dissolved gas are judged to be present within the Erhchungchi and the Liushuang Formations in the shallow subsurface structures.