Abstract
Mineralogical and chemical examinations were made on interstratified illite/smectite in the mudstones taken from three drill holes penetrating the Tertiary Yeonil Group to study its reaction during diagenesis. The Yeonil mudstones are dominated by large amounts of the randomly (R=0) interstratified illite/smectite with a maximum illite layer of up to 38%. Chemically the interstratified I/S was formed by input of K and Al ions originated from the destruction of detrital illite and kaolinite in compensate for the release of water and exchangeable cations from interlayers during burial diagenesis. This can be represented simply by the following reaction.
4.5K+8Al3++smectite → illite+3Si4++H2O+Na, Ca
However, the illitization terminated at the randomly interstratified I/S with illite layers of 38% because the Yeonil Group has shallow burial depth. On the other hand, R=1 ordered I/S appears in only one sample from the deepest depth of the F hole with different occurrence and organic content from the mudrocks including the randomly interstratified I/S. Judging from the high potassium content and the deeper part of the hole, the illitization may be accelerated further. The Yeonil Group shows a very low degree of the diagenesis, probably result from the shallow burial depth. The maximum temperature of the Yeonil Group is estimated at 50-70°C based on the illite/smectite diagenesis and organic maturation
