The MITI exploratory test well “Tomikura” was drilled 6, 000m in depth by Japan National Oil Corporation. The geology of this well is mainly composed of the middle Miocene sedimentary rocks. These rocks are very hard, and have undergone high grade diagenesis. Illite/smectite interstratified minerals (I/S) are well known indicators of diagenetic and/or burial metamorphic grade. Increase of illite layers depends on burial depth.
The experimental results show that the illite layers in I/S increase with increasing burial depth. The changes of I/S are divided into five steps as follows:
I) slow conversion to illite layer with 20-30% I,
II) rapid conversion from 20-30% I to 70% I,
III) slow conversion with 70% I,
IV) rapid conversion from 70% I to 80% I,
V) slow conversion with 80% I.
The rapid conversion zones (II and IV) are important in the structural changes of I/S interstratification. In “Transition zone 1” (1, 700-3, 000m in depth), randomly interstratified structure (R0) has converted to the regular structure with R1 by transition, containing 30-70% I. Conversion of R1 to R3 interstratification occured in “Transition zone 2” (3, 400-4, 000m in depth), containing 70-80% I.
As a result of conversion of smectite to illite, the released water from the interlayers has had an influence on the interparticle condition. Log interpretations suggest that the water dehydrated from the interlayer of smectite has had an effect on the interparticle pore. Especially, dehydrated water in “Transition zone 1” is associated with the formation of the abnormal pressure zone.
