Why Are CSEM and Borehole Derived Resistivities in Basin Sediments Often Incompatible?
Abstract
In a number of reported cases in offshore hydrocarbon provinces, electrical resistivities of sea floor sediments determined by Controlled Source Electro Magnetic (CSEM) surveys are found to be significantly greater than those measured by electrical well logging of boreholes at the same locations. In some instances the ratio between CSEM and bore hole log resistivities are as much as 5:1. A possible cause of this discrepancy is strong electrical anisotropy of the sea bed. Borehole logging techniques generally measure electrical resistivity using currents circulating in horizontal planes. CSEM surveys collected using 'end-on' source-receiver geometries, in contrast, are far more sensitive to the resistivity encountered by currents circulating in vertical planes. Grain alignment, aligned fractures and thin layering all can cause electrical resistivities to be different in the vertical compared to the horizontal direction. To investigate this we have examined electrical well log data and sediment samples from the North Sea. We have compared vertical (in-series) resistivities to horizontal (in-parallel) resistivities for a number of cases. Also, many reservoirs in the North Sea have been drilled using both vertical and deviated wells. By correlating the same sedimentary unit between different well logs from deviated and un-deviated bore holes we are able to compare resistivities measured at different inclination angles. Both sets of results are compared against laboratory measurements. Results for shale units indicate that electrical anisotropy can account for much of the discrepancy observed between CSEM and borehole resistivities. However sand layers do not show the same level of electrical anisotropy.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMGP41A0788S
- Keywords:
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- 0600 ELECTROMAGNETICS;
- 0639 Nonlinear electromagnetics;
- 0900 EXPLORATION GEOPHYSICS;
- 0925 Magnetic and electrical methods (5109);
- 3006 Marine electromagnetics