The natural gas of accumulations exploited in the Niigata field and in the Mobara and smaller fields of the southern Kanto gas region is not associated in any way with crude oil, but consists of CH 4 (generally over 90 percent) and CO 2 (generally up to 5 percent) produced by bacterial decomposition of organic material in sediments, together with some N 2 of atmospheric origin, and occurs in solution in connate brines in poorly consolidated late Tertiary or Pleistocene sediments in the deeper parts of basinlike structures where percolating meteoric waters have not gained access. Until recent years, much of Japan's gas production came from deposits of this type. At present, oil-field gas is the major source of production. Limited amounts are also produced from coal-bearing deposits.

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