USE OF THE SMECTITE TO ILLITE CONVERSION REACTION MODEL - EFFECTS OF ORDER OF MAGNITUDE

A model for the evolution of smectite-illite mixed layered minerals du ring diagenesis has been previously developed as a function of tempera ture and time variables using data from seven deep wells. Efficient us e of this model assumes certain restrictions, the greatest being a str ict use of the same methods to determine mineral composition as those used in developing the model. In such a model, implicit in the nature of kinetics, there are certain limits to its use in determining geolog ical events (the goal oi use for any such model). Among these limits i s the fact that the upper portion oi a sedimentary section is not very sensitive to differences in thermal gradient while the deeper portion s (> 2 000 m) are much more sensitive to gradient. As the reaction is driven by absolute temperature, a high geothermal gradient gives simil ar temperatures to a low one in the first 1 000 m of burial whereas th e differences in absolute temperature are much more significant at gre ater depth. Early erosion events (those which affect sediments at low absolute temperature) will be significantly ''annealed'' from the memo ry of a clay mineral reaction series by further burial, whereas late e rosion episodes are well recorded in the deeper sediments where erosio n changes the absolute temperature oi a rock layer to a great extent. Hence, events which affect the temperature of a rock layer when it lie s at great depth are fully recorded in the clay mineral reaction serie s while those which change the temperature at shallow depth will be po orly recorded. Certain other precautions and limitations to the use of the model are indicated.