Subsurface Microbiofacies of the Kazusa Group in the Western Part of Chiba Gas Field, Japan

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Microbiofacies of the Kazusa group in the western part of the Chiba gas field are extremely different from those of the eastern part. The Umegase and the underlying formations indicate mostly stable bathyal environment in the east side area of Chiba city. But in the west side area of Chiba city, the fossil foraminiferal faunas are of a shallow water habitat. Concerning these problems, the writers examined many samples collected from the gas wells. From the foraminiferal sequence of an exploratoly well P-1 and the biofacies maps (FIG. 2-FIG. 7) of the area, they have come to the following conclusion.The sedimentary environment in the Kiwada stage was bathyal one in the both side areas, except in Chiba district, where the f oraminiferal fauna shows a relatively shallow and marginal facies. It may be considered that some barrier was present in this district in the stages of lower Kazusa formations. The sedimentation behaviors in the west side area of this barrier were fairly different from those in the east side area, and these different characters have caused the different mode of natural gas accumulation. And also the presence of some relief of basement rocks in the district is more possible than it was imagined before.
石油技術協会の論文