千葉県茂原ガス田の地下地質と水溶性天然ガス・ヨウ素の生産性について

抄録

Several natural gas and iodine companies have developed the Mobara gas field for 70 years. In this field, the natural gas dissolved and the iodine are contained in the formation brine. More than 1000 wells were recorded by electrical logging that consists of sponteneous potential and resistivity data when they were drilled. In this study, subsurface geology of the Mobara gas field is integrated with stratigraphy of the Kazusa Group by correlating well logs. Productivity of natural gas and iodine is indicated on a contour map, then the relationship between geology and productivity is evaluated. Main conclusions are as follows: (1) Distribution of turbidite sandstones indicates that the area is covered with the spread of a submarine fan. In the Umegase and the Otadai Formations, many turbidite successions are deposited in the submarine fans. The central parts of the submarine fans are located at the southwestern part of the Mobara area. (2) In the central parts of the submarine fans of the Umegase and the Otadai Formations, both the gas water ratio and the iodine chlorine concentration ratio are relatively high. It means that productivity of natural gas and iodine corresponds with the area with the thickest part of the turbidite sandstones. Consequently, the central parts of the submarine fans are highly productive. (3) The Shibahara area, which is the most productive area for natural gas, is located on the west side of the normal faults. The depths of the production zone are relatively shallow (between 200m and 600m below sea level). It may serve as evidence that the faults and the depths of the gas reservoirs influence natural gas productivity.