The natural gases collected during production testing from nine oil and gas fields in the Niigata basin were characterized using parameters such as C2+, C2/C3, iC4/nC4, iC5/nC5, N2 and CO2. The chemical compositions of gases are related to the temperatures of the reservoirs from which the gases are produced.
Chemical composition together with isotopic composition of methane suggests that the natural gases in the area are of thermogenic and mixed thermogenic/bacterial origins. The iC4/nC4 ratio is considered to reflect both the type of source rock and its degree of thermal maturation.
In the Hirakida gas field the gases from the Nishiyama and Shiiya Formations may have undergone bacterial alteration.
