Low resistivity pay is common in China and presents a difficult interpretation challenge. One possible cause is sand-shale laminations where the presence of conductive layers dominates the resistivity measured by conventional resistivity tools which mainly sense the horizontal resistivity component. Those low resistivity layers may consist of shale or fine grain sands with high bound water. They create resistivity anisotropy and if not detected, it may result in missing or underestimating the pay.

In order to characterize laminated formations, the vertical resistivity needs to be measured and this can be achieved using a 3D induction tool. This tool uses triaxial transmitters and receivers at various spacing to measure a tensor which is inverted to extract horizontal and vertical resistivity.

Those resistivities are then used in a model based interpretation to compute the resistivity of the sand lamination and thus the saturation. Being a model based interpretation, the model needs to be validated. Some effects can be accounted for such as intrinsic shale anisotropy or dispersed clay in the sand layer. Other effects such as highly resistive cemented layer may not be compensated for and need to be identified. This paper describes some of the effects through log examples from offshore China.

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