ILLITE SMECTITE DIAGENESIS AND ITS VARIABLE CORRELATION WITH VITRINITE REFLECTANCE IN THE PANNONIAN BASIN/

The correlation between illite/smectite (I/S) diagenesis and mean vitr inite reflectance (R(o)) data is examined in mudrocks from a hydrocarb on exploration well (geothermal gradient 35 degrees C km(-1)) from the Great Hungarian Plain of the Pannonian Basin System. The expandabilit y of I/S decreases with depth and there is a change from random to ord ered mixed-layering at about 2500 m depth. At this depth R(o) is about 0.6%. Comparison of the correlation of expandability and R(o) from th is study to published data for the Vienna Basin and the Transcarpathia n Basin, sub-basins of the Pannonian Basin System, shows that the corr elation is systematically different for each sub-basin, according to t heir thermal histories. In the Vienna Basin (geothermal gradient 25 de grees C km(-1)), for any given value of R(o), the expandability of I/S is less than in the Transcarpathian Basin (geothermal gradient 55 deg rees C km(-1)) and the sediments are older and more deeply buried. Dat a from the present study are intermediate. This variation is believed to be due to the effect of time on the smectite-to-illite reaction. Re sults of an optimization procedure to calculate the kinetics of the sm ectite-to-illite reaction, using as input the expandability depth prof iles, and thermal histories constrained by comparison of observed and calculated R(o) data, showed that I/S diagenesis in the Pannonian Basi n System can be modelled by a single first order rate equation: -dS/dt = e(log(A)-E/RT). S where S = fraction of smectite layers in I/S, t = time (Ma), e = exponential function, log(A) = frequency factor = 7.5 (Ma(-1)), E = activation energy = 31.0 kJ mol(-1), R = universal gas c onstant, and T is temperature in Kelvin. This result also suggests an important role for time. By combining the kinetics of the smectite-to- illite reaction with a kinetic model of vitrinite maturation it is pos sible to define a domain within which all 'normal' (burial diagenesis) correlations between R, and I/S diagenesis should lie. Such diagrams can be used to identify different thermal histories related to differe nt geotectonic settings and 'anomalous' data such as that affected by igneous intrusions.