Step 1: Acquisition and processing
Some mCSEM data acquired along line 5001, in the form of source gathers at 1Hz are shown. A significant response to the accumulation encountered at Wisting Central can be clearly seen in the CSEM data, particularly in the phase response (lower panel around 611km Easting). This is observed across a wide band of frequencies.
The CSEM data for six frequencies (0.2Hz, 0.8Hz, 1Hz, 1.4Hz, 2.2Hz, 2.6Hz) were inverted using an Occam approach (Constable et al., 1987; Key, 2016) to derive anisotropic resistivity models. The inversion was performed in stages. Firstly, an unconstrained inversion was run in order to examine the resistivity structure obtained in the absence of any a priori information. However unconstrained inversions in general have poor resolution. Resolution can be improved by including structural information from the seismic data. This also ensures consistency between seismic and CSEM derived results, which is important for subsequent integrated interpretation. In this way, vertical (upper panel) and horizontal (lower panel) resistivity were recovered. Good RMS residual were achieved insuring proving that such recovered resistivity model honored the data.
Step 2: Analysis
Vertical resistivity for the unconstrained and constrained inversions run are shown in the interval of interest: The top one corresponding to the unconstrained inversion, and the bottom image shows the results of the constrained inversion previously shown. Both models are equivalent mCSEM wise (same mCSEM data fit) as they have the same transverse resistance. The constrained results are preferred given the a priori information available. A qualitative interpretation of the CSEM inversion results supports the outcome of the Alternative, Central and Bjaaland wells. A prominent resistivity anomaly is recovered at Central, in which there was a significant oil discovery, which is in agreement with the high resistivity values measured at the reservoir location. On the other hand, the Realgrunnen structures penetrated at Alternative and Bjaaland, two dry wells, are related to low resistivity values that support the petrophysical outcome.
Such analysis of the CSEM data in isolation does not allow to go beyond the previous qualitative conclusion. Only a quantitative approach that integrates the resistivity measurements with the seismic analysis can lead to reservoir properties.
