The Nagaoka Plain is located in the central part of Niigata Prefecture, Japan. It is gently folded area and is bounded by intensely folded hilly areas. Many oil and gas fields have been discovered and developed in these areas. Some of them are characterized by the abundance of pyroclastic reservoir rocks.
The writer studied geological development of these areas using some restored stratigraphic sections and isopach maps, and he obtained the following conclusions. (1) Gently folded anticlines in the eastern half of the plain area may have been originated from the synsedimentary upheavals (synchronous highs) which consisted of basement complex and "Green Tuff" (pyroclastic rocks of early Miocene). (2) Foldings of surrounding hilly areas may have been farmed since Nishiyama age early Pliocene). This tectonic movement was possibly caused by block movement of the basement. Thrust faults have extended along the boundary between the folding area and the gently folded area with down thrown anticlines under them. Down thrown anticlines have pyroclastic rocks in their cores and some gas reservoirs have been found in them. (3) There are two types of petroleum deposits of Green Tuff. In the first case, oil and gas migrated at the early stage into the Green Tuff which was making synchronous high by itself (Mitsuke Oil Field type). On the other hand, in the case of Yoshii Gas Field, gas migrated into the Green Tuff at the Teradomari or Shiiya age (late Miocene) when the Green Tuff got to reveal itself as the structural high (Yoshii Gas Field type).
