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Before seismic and CSEM data can be integrated, they must be transformed to a consistent scale, and converted into a common domain. Here we convert the seismically derived results to the electrical domain by applying Archie’s law to the seismically derived porosity. The resistivity at other saturations can be calculated by fluid substituting in areas which are identified as potentially hydrocarbon bearing in the seismic facies classification.
Comparing the CSEM derived transverse resistance with the water wet seismically derived case, it can be observed that many of the resistivity variations can be explained by variations in porosity
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Step 1: Integrated analysis. The figure shows:
- Left: Transverse resistance within the top Realgrunnen-Fruholmen interval, derived from 3D anisotropic inversion of the CSEM data.
- Middle: Seismically derived transverse resistance at wet conditions.
- Right: Seismically derived transverse resistance at 60% hydrocarbon saturation.
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Step 2: Litho fluid facies analysis. The figure shows the results of a litho-fluid facies classification incorporating both seismic and CSEM results.
- Left: Cross plotting the probability of hydrocarbon charge from seismic against transverse resistance from CSEM gives a domain in which commercial and noncommercial saturations can be distinguished from potential seismic and CSEM pitfall cases.
- Right: Litho-fluid classification from seismic and CSEM data, suggesting only residual hydrocarbon saturation in the area.
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Combining seismic and CSEM results can resolve the ambiguities that are present when only a single data type is considered. In this example, although seismic data identified potentially hydrocarbon bearing sands, the saturation could not be determined. Adding the CSEM data resolved this ambiguity to an extent: it is clear that saturations similar to those observed at Wisting (>90%) are not present in this area. However, because of limitations on the sensitivity/recoverability of the CSEM data in this high resistivity environment, it is less clear whether the saturation is very low (<20%) or higher (potentially up to 40-50%). This remains an uncertainty in the analysis.
