片貝地域のグリーンタフ火山岩貯留層に認められる熱水変質と二次孔隙の性質

抄録

The Katakai gas field produces gas from the Green Tuff volcanic rock reservoirs deeper than 4000m. The “Green Tuff” volcanic rocks of Miocene age are submarine felsic volcanic units and have undergone an extensive hydrothermal alteration which has improved the pore properties of the volcanic rock reservoirs. The timing of hydrothermal alteration in the volcanic rock reservoirs is a key issue in the argument on the formation of effective reservoirs. Two scenarios can be considered: the formation of the reservoirs might have occurred after deep burial or soon after the submarine volcanism. Based on the results of isotopic, mineralogical and fluid inclusion studies, the latter scenario seems to be more rational, which can explain the hydrothermal alteration process.
The alteration zoning of clay minerals and carbonates revealed two upflow zones of hydrothermal fluids. Basaltic highs in the lower half of the Green Tuff unit correspond to the presumed upflow zones. The microthermometry data of fluid inclusions show the mixing of high temperature brines with moderate temperature waters of very low salinity. It is presumed that the latter waters are of catagenesis origin in contrast to the former of modified sea water origin. Catagenesis waters are likely to have been expelled from overlying mudstones, which is conformable with the occurrence of oil inclusions in quartz amygdales.
It is concluded that the hydrothermal activity followed the felsic submarine volcanism and heated overlying mudstones to commence to generate petroleum in a shorter period of time than the maturation of organic matter during burial.