A simplified dissolution-precipitation model of the smectite to illite transformation and its application

By combining the compaction effect E-C from a dissolution-precipitation model (DPM) and the thermal effect E-T from a kinetic model (KM), a simplified DPM is presented. A pair of apparent activation energy E and frequency factor A to be used is proposed in the model, which is chosen to cause the predicted smectite to fit X-ray diffraction (XRD) data. The proposed model is then applied to the illite-smectite interstratifications of mudstone in well Tazhong4 of the Tarim Basin in western China. A KM is also applied for the sake of comparison. This case study shows that the most sensitive parameter governing smectite to illite (S-I) transformation is the pair of E and A. To calculate an accurate S-I transformation history, it is vital, therefore, to calibrate the E and A by using XRD data as a constraint. In this specific case study, the calibrated E ranges between 5 and 10.28 kcal mol(-1) while A is between 9.0623 x 10(-11) and 3 x 10(-7) s(-1) L mol(-1) for the simplified DPM, and E ranges between 3 and 5 while A is between 1.3888 x 10(-11) and 3.5052 x 10(-9) for the KM. The relation of E versus ln(A) appears nonlinear. By using these E and A, the present-day values of the calculated smectite history agree well with the XRD-determined data, with mean absolute relative residuals of 4.45% for the simplified DPM and 5.67% for the KM. Moreover, the case study indicates that E-C and E-T have the same importance for S-I transformation as contributions of E-C of 43.5% and of E-T of 56.5%.