新潟地域における温泉水と付随ガスの地球化学

抄録

Analytical results of waters and natural gases from seven hot spring wells in Niigata Prefecture are presented. Methane is the dominant hydrocarbon in these spring gases and the gases in the Nishiyama Formation contain about 15% CO₂. The hydrocarbon gases in the Shiiya and Teradomari formations are of thermogenic origin and partly biodegraded. On the other hand, those in the Nishiyama Formation are of microbial origin or mixed origin of microbial and thermogenic similar to production gases from oil and gas fields around the wells. The spring waters are derived from the fossil seawater. Although those in the Nishiyama Formation are the same as formation waters from oil and gas fields in Niigata, those in the Shiiya and Teradomari formations are distinguished from the formation waters based on I^－/Cl^－ and Br^－/Cl^－ ratios. Lithium contents of the spring waters and the formation waters range from 0.1 to 12.3 mg/l and their Li^＋/Cl^－ ratios are less than 0.001. Temperatures calculated from the Mg-Li geothermometer are higher than water temperatures in spring waters, but those are similar to reservoir temperatures in the formation waters.