Modeling smectite to illite transformation and the effect on compaction and overpressure development. 9th Middle East Geosciences Conference, GEO 2010.

Abstract

Smectite illite (I/S) transformation is part of the lithification process of fine-grained sediments. We constructed and calibrated a coupled kinetic I/S transformation and mechanical compaction model in which the Arrhenius Equation describes the rate of transformation and I/S grains collapse. The model accounts for porosity reduction and overpressure development contributed by the I/S transformation. The overpressure contribution results from the transfer of effective stress born by the I/S grains to pore water due to the collapse of I/S grains. The model is controlled by the initial expandable fraction in I/S and the temperature/time history. All together 320 mudstone samples were analyzed by high-quality X-ray diffraction (XRD) analysis for their mineral contents and expandable content in the mixed-layer illite/smectite. Below 70°C, I/S transformation barely starts. Our dataset shows that expandable fraction in I/S for the samples with the temperature less than 70°C, that is the value at deposition, can vary from 40 to 100%, a reflection of different sources of I/S. The large variation of expandable fractions in I/S at initial deposition impose difficulties in modeling the I/S diagenesis. We chose a well with a thick homogeneous mudstone section in our model calibration to minimize the effect of the uncertainty of initial expandable fraction. The predicted results from the constructed model agree well with the measured data for the calibration well and also a blind test well. For our dataset, the large range of expandable fraction can be modeled using a different range of initial expandable fractions and reasonable temperature/time histories. Since the blind test well has almost the highest initial expandable in I/S and high temperature in the history, we used it to investigate the maximum effect of I/S transformation on porosity and overpressure based on the assumption of no dissipation of overpressure contributed by I/S diagenesis. The maximum porosity reduction is 0.02 and the overpressure only 4 MPa. In reality, the over-pressure contributed by I/S transformation will dissipate and the effect on overpressure will be much less. Our study concludes: (1) the initial expandable content in I/S can vary in very large extent, in the range of 40-100%; (2) our constructed model describes the I/S transformation and its contribution to compaction and overpressure satisfactorily well; and (3) the effect to porosity and overpressure is very limited.