THE SMECTITE-TO-ILLITE TRANSITION IN THE KOYOSHIGAWAOKI WELL IN THE AKITA SEDIMENTARY BASIN, NORTHEAST JAPAN

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概要

• 論文の詳細を見る

• Mineralogical and chemical examinations on interstratified illite/smectite (I/S) from the Koyoshigawaoki well in the Akita basin provide an excellent example to document the diagenetic change of smectite to illite. X-ray diffraction analysis shows a stepwise increase in percentage of illite layer with depth: two abrupt increases in illite layers are recognized at depths just below 2,500 m and at a depth interval between 3,800 and 4,500 m. In addition, conversion of random (R=0) I/S into R=1 ordered I/S records at depths of between 3,800 and 4,000 m, which is compatible with a Tmax value of 435℃ by Rock-Eval pyrolysis. Present geothermal gradient from temperature data, however, indicates that the conversion is likely to occur at a depth of 3,000 m. This discrepancy may be explained by a reverse fault at a depth of 2,470 m which resulted in a deeper burial of sediments up to 1,000 m. Deeply buried materials such as I/S and organic matter have not yet attained an equilibrium condition that corresponds to the present geothermal gradient. As a result, the rapid increase in percentage of illite layer below 2,500 m may be a discontinuity caused by the reverse fault. Another rapid variation in layer proportion between 3,800 and 4,500 m is attributable to neoformation of R=1 ordered I/S. Chemical analysis also illustrates a remarkable compositional variation in I/S with burial depth, demonstrating an increase in potassium and aluminum and a decrease in silica. This is expressed by the following reaction: smectite+Al^<3+>+K^+→illite+Si^<4+>. In this reaction, potassium is derived from K-feldspar, which is present in significant amounts throughout the well.

• 日本粘土学会の論文

著者

• YOSHIMURA Takahisa

  Department of Material Science, Faculty of Science, Kochi University

• Son B‐k

  Korea Inst. Geosci. And Mineral Resources (kigam) Kor

• Son Byeong-kook

  Korea Institute Of Geology Mining And Materials

• Yoshimura T

  Department Of Environmental Sciences Faculty Of Sciences Niigata University

• Yoshimura Takahisa

  Department Of Environmental Sciences Niigata University

• Yoshimura Takahisa

  Department Of Environmental Science Faculty Of Science Niigata University

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