The lower Miocene strata of the foothills and western coastal areas of northwestern Taiwan consist, in ascending order, of the Wuchihshan, the Mushan, and the Taliao Formations; the Kungkuan Tuff is a lateral equivalent of the lower part of the Taliao. The middle Miocene series is divided into two units, the Shihti Formation below and the Tiehchenshan Formation above. The latter includes the Peiliao Member, the Talu Shale Member, and the Kuanyinshan Sandstone Member. The upper Miocene also is divided into two units, the Nanchuang Formation below and the Kueichulin Formation above. The former includes the Tungkeng Member and the Shangfuchi Sandstone Member, and the latter consists of the Kuantaoshan Sandstone Member, the Shihliufen Shale Member, and the Yutengping Sandston Member.

The thicknesses of the type Wuchihshan, Mushan, and Taliao Formations are 1,188.5 m, 595.5 m, and 382 m, respectively. The Shihti Formation has no complete exposure section at the type locality. The thicknesses of the three type members of the Tiehchenshan Formation are 474 m, 332 m, and 156 m, respectively. The thicknesses of five type members of the Nanchuang and the Kueichulin Formations are 586 m, 102 m, 176 m, 38 m, and 271 m respectively.

The isopach map of the lower Miocene Taliao Formation and Mushan Formation shows the total thickness of 1,150 m in the northeast foothills region to decrease westward to 900 m near the Hsinchu coast. The isopach map of the middle Miocene shows the thickest part to be 1,500 m in the foothills region 20 km southeast of Hsinchu; the unit thins northward to 700 m on the Kuanyin coast. The isopach map of the upper Miocene shows the thickest part to be more than 2,400 m in the foothills region 20 km southeast of Hsinchu; the upper Miocene thins northward and southwestward at the coast to about 700 m.

As a whole, the sediments of the Miocene formations in northwestern Taiwan were deposited in mixed shelf and geosynclinal environments. Structures suitable for drilling should be present in the foothills region, under the coastal area, and beneath the sea bottom offshore. Wells to 3,500 to 5,500 m should test the Miocene oil and gas objective sandstones.