抄録
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, 500m and at a depth interval between 3, 800 and 4, 500m. In addition, conversion of random (R=0) I/S into R=1 ordered I/S records at depths of between 3, 800 and 4, 000m, which is compatible with a Tmax value of 435°C by Rock-Eval pyrolysis. Present geothermal gradient from temperature data, however, indicates that the conversion is likely to occur at a depth of 3, 000m. This discrepancy may be explained by a reverse fault at a depth of 2, 470m which resulted in a deeper burial of sediments up to 1, 000m. Deeply buried materials such as I/S and organic matter have not yet attained an equilibrium condition that corresponds to thepresent geothermal gradient. As a result, the rapid increase in percentage of illite layer below 2, 500m may be a discontinuity caused by the reverse fault. Anotherrapid variation in layer proportion between 3, 800 and 4, 500m is attributable toneoformation 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 adecrease in silica. This is expressed by the following reaction: smectite+Al3++K+→illite+Si4+
In this reaction, potassium is derived from K-feldspar, which is present in significant amounts throughout the well.
