本邦油•ガス田地帯における天然ガスの起源に関する地球化学的考察

抄録

Chemical and isotopic compositions were analyzed to characterize the genetic origin of natural gases from various occurrences (production gases, water-dissolved gases, coal gases, surface gas seeps) in Japanese oil and gas fields.
Bacterial gases and thermogenic gases are characterized in two diagrams using the relationships of hydrocarbon compositions and methane ¹³C, and of methane ¹³C and ethane ¹³C, respectively. A significant proportion of production gases assosiated with oils is mixtures of bacterial gases and thermogenic gases. These diagrams can also reveal the influence of secondary processes such as migration and oxidation of hydrocarbons. Surface gas seeps are usually affected by such secondary processes.
Origin and migration of gases in the northern part of Niigata oil and gas fields might be different from those in the southern part of the area. Production gases in the south (Nagaoka-Kasiwazaki area) consist of only thermogenic gases and have no relation to their reservoir depth or age. On the other hand, gases in the north (Niigata-Kitakanbara area) mainly consist of mixed gases; the younger reservoirs contain a large quantity of bacterial gases, while the older reservoirs mainly consist of thermogenic gases. This areal distribution might be controlled by the geological factors which are discussed in this paper.