Step 1: Methodology used to estimate water saturation from Seismic and CSEM
In order to allow direct comparison between seismic and CSEM results, the next step is the estimation of resistivity models from seismically derived properties. With this goal in mind, we used the seismically-derived litho-fluid facies, clay content and total porosity sections with a calibrated rock physics model to transform these petrophysical properties into the electrical domain. The calibrated rock physics model used was the Simandoux equation. However, the seismically and mCSEM derived resistivity models are inherently at different resolution and consequently cannot be directly compared. To overcome such vertical resolution problem, the transverse resistance seismically and CSEM derived is compared. This is on the basis that 2-different resistivity models with the same Transverse Resistance can be considered as equivalent.
In this way the mCSEM derived Transverse Resistance could be inverted for water saturation Sw using the Transverse resistance seismically derived as forward modelling. This method seeks the value of Sw that provides the minimum misfit between seismically and CSEM derived transverse resistance, using a grid search algorithm. It is important to mention that only potential reservoir rocks as indicated by the seismic litho-fluid facies, were considered to have variable Sw during the inversion process. In this way the quantitative seismic interpretation result not only provides information about the clay content and total porosity of the rocks, necessary for the Simandoux equation, but also about the location and thickness of the potential pay sand, thus maintaining seismic resolution in the final result. However, note that since only Sw varies during the inversion, it is implicitly assumed that the porosity and Vclay as defined by the seismic data are correct.
Step 2: Conclusions
Finally, the resulting Sw profile was mapped back in its correct position using the seismically-derived litho-fluid facies volume to generate a hydrocarbon saturation section along the line. Excellent correlation with known well results was achieved. The integration of seismic, CSEM, and well data predicts very high hydrocarbon saturations at Wisting Central, consistent with the findings of the well. The slightly lower saturation at Hanssen predicts the well outcome correctly. There is no significant saturation at Wisting Alternative, again consistent with the findings of the well. At Bjaaland, although the seismic indicate the presence of hydrocarbon bearing sands, the integrated interpretation result again predicts correctly that this well was unsuccessful.
