Studies on Clay Minerals in the Oil-bearing Neogene Tertiary Sedimentary Rocks of Northwest Honshû, Japan, with a Proposal of Criteria for Analysis of Clay Minerals in Sedimentary Rocks

Abstract

The author has been studying on clay minerals in the oil-hearing Neogene Tertiary sedimentary rocks of Northwest Honshû, Japan. In this paper he describes his provisional criteria for analysis of clay minerals in sedimentary rocks, with his basic considerations and laboratory experiments, which he conducted. He adds supplementary notes on the qualitative analysis to his preceding paper20). He also summarizes some noteworthy sedimentological relations, which have been considered by him to exist in sedimentary sequences of many geological basins. Contents of this paper are as follows: 1) Progress of basic experiments for the establishment of the criteria and some result of qualitative analysis are described. a) X-ray diffraction analysis applied to centrifugal fractions of mudstones due to random powder method. b) X-ray diffraction analysis combined with various reagent-treatments and heat-treatment, applied to centrifugal fractions of mudstones due to powder orientation method. 2) He determined clay mineral suit of montmorillonite-illite-chlorite (± minor amount of montmorillonite-illite mixed layer), for the oil-bearing Neogene Tertiary sedimentary rocks in Northwest Honshû, Japan. 3) He proposed a classification of mudstones due to relative amount of clay minerals and quartz. 4) He found the striking negative correlation between the montmorillonite log (or clay mineral log) and the log of “Cl'-concentration of interstitial water” in mudstones, prepared for the exploration well Mitsuke SK. 1 (or Tamugiyama SK. 1). Based on consideration of it, he interpreted that a part of the so-called interstitial water, is composed of that delivered from interlayer water of montmorillonite crystals, through the pretreatment or heating up to 110°C of core samples. This interpretation is supported by the result of another experiment of his own, in which the oscillation method of X-ray diffraction analysis during heating up to 900°C, is applied to a centrifugal fraction of a mudstone cored from the depth of 1098m of the well Tamugiyama SK. 1. 5) The author determined clay-mineralogical divisions on the montmorillonite log, and he found that these divisions coincide with fossil foraminiferal subzonules. 6) On the Q-M (quartz-montmorillonite) diagrams characteristic distribution curve can be drawn for each fossil foraminiferal biotope unit in the Mitsuke and Tamugiyama sections. Existence of a kind of sedimentological relation is suggested by the comparison of relative positions of these characteristic distribution curves on Q-M diagrams with their corresponding foraminiferal faunal assemblages. 7) The author describes the possibility to determine characteristic distribution curves on the M-I-Chl (montmorillonite-illite-chlorite) triangular diagrams for geological formations or members. 8) The criteria for analysis of clay minerals in sedimentary rocks, which he proposed, are described. 9) In conclusion, present status and future problems of the sedimentary clay mineralogy, especially that concerning petroleum exploration and exploitation, are summarized.