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Effects of Porosity Models on Thermal History Estimated by Geohistory Models


Paleotemperature modeled by geohistory simulation is controlled by ancient surface temperature, heatflow (HF), and thermal conductivity (TC). The problems associated with estimating grain TC are well recognized by the literature, but the influence of porosity uncertainty on TC and paleotemperature is not so well recognized. In commercial geohistory software, porosity is calculated as an exponential function of maximum burial depth or effective stress, but porosity loss is actually a function of effective stress and time (by cementation and mineralogical conversion). Porosity models calibrated to current porosity may underestimate ancient porosity. Lower porosity causes higher TC, so paleo temperatures estimated from a modeled HF will be underestimated. This effect can be substantial for mudrocks. One solution is to modify the porosity models to better match observed physical processes. Transient sandstone and limestone porosity loss can be modeled using published algorithms. For mudrocks, the most robust approach is to divide porosity into free water and bound water. Model the free water loss using the standard compaction equilibrium approach. Model the transient bound-water loss by clay reactions. Bound water content is related to the cation exchange capacity (CEC) of the rock. The CEC and bound water will decrease significantly as smectite converts to illite. As CEC decreases, total porosity decreases. Smectite-illite conversion kinetics are available in the literature. Unfortunately, transient porosity models cannot be implemented into the current commercial geohistory programs, but they can be incorporated into research software. Transient models reduce porosity with time, so models calibrated to modern porosity correctly estimate current temperatures and modern HF but underestimate paleotemperatures where HF is constant. If modern porosity is estimated from literature mechanical compaction models and strata are old, modeled modern TC and modern HF may be underestimated. As an empirical short-term solution for commercial software users, porosity models can be calibrated to modern porosity-depth, and the effects of lower paleo TC can be compensated by increasing paleo HF and surface temperature. We do this anyway when HF history is modified to match thermal indicators. Thus, geodynamic interpretations of higher paleo HF derived by fitting HF history to thermal indicators using current porosity-depth models may be invalid.