The smectite-to-illite transition in the Koyoshigawaoki well in the Akita sedimentary basin, northeast Japan

Abstract

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°C by Rock-Eval pyrolysis. Present geothermal gradient from temperature data, however, indicates that the conversion is likely to occur at a depth of 3,000 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 = 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.