A part from the natural gas of the oil fields, there are methane rich natural gas in the Pleistocene and Pliocene marine formations in the Niigata and Chiba districts, practically, being used for gas industry.
The gas occurrs associating with the formation water in the reservoirs, and is called the suiyosei gas, which might be originated in the formations soon after their deposition.
On the other hands, however there are also the methane rich natural gas pools of the Miocene marine formations, the Shiiya and Teradomari, non-associated with oil in the Niigata district, which also being used for gas industry.
Most of the methane gas of these gas pools non-associated with oil, might, hypothetically, be originated in the Miocene formations, soon after their deposition, to form the suiyosei gas pools, in the early geological time of deposition of the formations as methane rich gas pools shown in the younger formations as those of the Pleistocene and Pliocene, and during geologic historic time, which might be transformed to form the gas pools non associated with oil at present.
