In this gas region, Tertiary sedimentary rocks are covered by Quaternary volcanic ash. Natural gas of the region can be classified into the following three groups.
I. CO₂-(N₂) group II. CH₄-(N₂) group III. CH₄-CO₂-(N₂) group
CO₂-(N₂) group occures in the western district, where the thickness of volcanic ash is large.
And the writers think that the gas of this group originated probably from volcanic activity.
On the contrary, CH₄-(N₂) group is recognized in the district, where Tertiary sedimentary rocks or the old lake deposits are exposed.
Except the Alluvial gas, the vertical destirbution of the natural gas quality is as follows.
Shallower part........................CO2 rich gas (I group) ................................................................ Deeper part.......................CH4 rich gas (III group)