Comparison of clay and zeolite mineral occurrences in Neogene age sediments from several deep wells

Clay and zeolite mineral assemblages were determined for five deep wells in volcano-clastic sediments (Japan) and for one well in mudstones (California). The clay mineral suites in these wells showed a gradual change with depth of illite/smectite (I/S) composition, which increases in smectite content in the upper portion towards a fully expandable mineral (2-3-km depth) and then decreases in smectite content with depth (2-5 km). The temperature of transformation or recrystallization to a fully expandable smectite mineral is about 60 degrees -70 degrees C in non-zeolite bearing rocks and 70 degrees -90 degrees C in zeolite-bearing rocks, with no apparent dependence on time. Comparison is made between the I/S smectite content in the lower part of the wells (i.e., below the occurrence of the fully expandable mineral) and the zeolite mineral zone boundaries. The 60% smectite composition was found at 108 degrees -118 degrees C maximum burial temperatures. The zeolite II/III zone boundary, i.e., the onset of the analcime zone, occurs between 85 degrees and 95 degrees C and may be slightly time-related in the span of 1-15 Ma. The clay and zeolite minerals can be used as temperature indicators in the range of the Neogene age.