津川―会津区および片貝ガス田における中新世珪長質水中溶岩の規模・形態・分布特性

抄録

Information on the size, morphology and distribution of individual volcanic rocks is important in volcanic reservoir evaluation. This paper statistically analyzes the characteristics of Miocene felsic volcanic rocks in the Tsugawa - Aizu Province (TAP), and a comparison is made with Miocene felsic volcanic rocks of the reservoirs of the Katakai gas field (KGF) in Niigata, Japan.

TAP is a structural province of NW-SE trending graben that consists of numerous depressions (basins) divided by uplift zones, and buried by large amounts of volcanic rocks that resulted from early to middle Miocene submarine felsic volcanism. The felsic lava is comprised of massive and autobrecciated lava facies, and accompanied by pyroclastic rocks. The average thicknesses of lavas range from 10 to 1000 m, and the estimated volume of a single felsic lava is 1.3×10^-4−1.9×10²km³.

The Miocene felsic volcanic rocks in KGF form structural rise trending NNE-SSW; however, in the southern portion of the rise the extensional axis of the rise shifts to the southwest. Within this portion of the rise, the north-northeast is dominated by massive (coherent) lava with hyaloclastite, whereas the south-southwest gradually shifts to volcanoclastic rocks. According to well data, each lava body ranges from several to 200 m for the thicknesses, and from 1 to several km for the lengths, respectively.

The entire scale of the felsic volcanic rocks comprising the reservoirs of KGF is smaller than that of TAP, and the individual lava bodies are comparable to the small to medium size lava bodies found in TAP. It is suggested that the volcanism in KGF occurred in a similar submarine environment between depression and uplift in TAP.