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Modeling Elastic Properties Changes and Overpressure Generation Due to Smectite to Illite Transformation in the Thunder Horse Mini-Basin, Mississippi Canyon, Gulf of Mexico


We integrate in this study basin modeling with rock physics to quantify the changes in elastic properties due to smectite to illite transformation, as well as the implications for predicting the generated overpressure. We used a dataset from the Thunder Horse mini-basin in Mississippi Canyon (Gulf of Mexico), which includes both Thunder Horse and Thunder Horse North Fields. The dataset consists of 3D seismic data and velocity cubes for depth conversions, well-log data and core XRD data from two wells. We built a 2D basin model that transects two wells in both fields to simulate the smectite to illite transformation. After calibrating the model to temperature data from the two wells, the simulation results of the transformation match the XRD data of clay samples in these two wells. The simulations suggest a shallower depth of the completion of transformation in Thunder Horse (i.e. 24960 ft) compared to that in Thunder Horse North (i.e. 25376 ft) due to the total coverage of Thunder Horse North by a shallow salt body that rapidly absorbs heat from the sediments below it. Combining the simulation results with the well-log data and pore pressure measurements in the two wells, we observed changes in the relationships between P and S waves velocities (Vp and Vs), porosity, density and effective stress. The zone of the completion of transformation in each well shows a change of the Vp and density relationship marked by a decrease of velocity and a slight increase in density. When we modeled the relationships between seismic velocities and porosity, the same zone showed steepening of velocity-porosity trends. Compaction trends of Vp and porosity with effective stress show a shift toward a more compacted rock after the transformation as higher Vp and lower porosity become achievable at a lower effective stress. These results suggest the need to modify compaction curves when predicting pore pressure from seismic velocities at the transformation zone. Detection of the transformation zone can come from basin modeling results. Another potential detection method based on the findings of this study is the change in amplitude variations with offset (AVO) due to changes in elastic properties driven by the transformation.