10 wells studied. Several transformation stages: smectite disappears at 149 to 167 degrees F, whereas transformations in the mixed-layer illite-smectite clay minerals are observed between 113 and about 212 degrees F. At temperatures above 113 to 149 degrees more than 30% illitic layers are found in the mixed-layered clay minerals, whereas temperatures from 140 to 167 degrees F indicate the transition zone toward more than 50% dehydrated lattice positions. Temperatures above 176 to 212 degrees F mark the lower limit for mixed-layer clay minerals containing more than 70% illitic layers. The major part of the transformations takes place in the mesodiagenetic zone, in which most of the hydrocarbons are also formed--Modified journal abstract.

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