The coal in Taiwan is mainly of Tertiary age and all the commercial coal deposits are in Miocene beds. There are 28 Miocene coalfields in Taiwan, three in central Taiwan and the rest in northern Taiwan. Nearly all the producing coalfields are concentrated in northern Taiwan at present. Three Miocene coal-bearing formations are recognized, represented by littoral sediments probably deposited in a tidal-flat, lagoonal to deltaic environment. These shelf-type sediments are in rhythmic alternation with basin-type marine sediments in the Tertiary geosyncline of western Taiwan. These coal-bearing formations gradually are replaced by, and grade into, marine beds as they are traced southward so that no workable coal deposits are known in southern Taiwan. The Miocene coal beds wer formed largely in paralic coal basins. Most of them vary greatly in thickness and lateral extent. They are markedly lenticular and often quite limited in areal distribution. Local pinching and swelling of the coal beds are common. The Upper Coal Measures have a maximum of seven workable coal beds; the Middle Coal Measures, a maximum of five workable coal beds; and the Lower Coal Measures, three workable coal beds. Each individual coal bed ranges in thickness from one meter or more to several millimeters with an average thickness of 30-40 cm. In some leading coalfields of northern Taiwan, only the main coal bed in the Middle Coal Measures attains a persistent thickness of one meter. The structure of the Miocene coalfields is complicated by abundant asymmetric folds and thrust faults of v ried magnitude. Steeply dipping coal beds are rather common. The Taiwan coals generally fall into two rank categories: low-rank bituminous and subbituminous. Semianthracitic coals are known only in small limited areas where andesitic intrusions are present. The rank of coal increases slightly with its geologic age. The original coal reserves of Taiwan total 659 million metric tons. The remaining coal reserves as of the end of 1973 total 465 million metric tons of which the estimated recoverable reserves may reach 220 million metric tons.

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