Using the Illite/Smectite Paleo-Thermo-Chronometer

D. R. Pevear and P. J. Vrolijk

In the last decade extent of smectite-to-illite reaction in shales, % illite (%I) in mixed-layer illite/smectite (I/S), has achieved status as a paleothermometer approaching vitrinite reflectance (%Ro). Scores of examples show downhole increase in %I that parallels %Ro. As with %Ro, published kinetic models predict %I as a function of time (t) and temperature (T). Comparison of predicted and measured %Ro and %I is used to calibrate burial/thermal history. Unfortunately, there is no chronology in %I or %Ro that is independent of an assumed burial/thermal history. Long t at low T can give the same values as short t at higher T. Obtaining K-Ar dates on newly formed illite, with the ability to model the date and %I using burial history and kinetics, provides a true paleotherm chronometer for constraining thermal history. Even if illite formed over a time interval, giving a mean age, the model can also calculate a mean age. The difficulty with shale illite K-Ar ages is that the diagenetic component is mixed with detrital illite (mica) which is older than depositional age. The mixture age is meaningless. Illite Age Analysis (IAA), Exxon's patent (# 5,288,695) for obtaining end-member ages of the mixture, provides a measured date for illite which can be compared, like %I and %Ro, with that predicted by the model. IAA uses linear extrapolation to get end- member ages from dated size fractions whose illite components (I/S + I, or polytypes) are quantified by XRD or electron diffraction. Integrating %Ro, %I, IAA and Apatite Fission Track Analysis measurements with heir respective kinetic models provides powerful constraint on thermal history, especially in areas with complex depositional, erosional and structural history.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California