Magnetic characterisation of the Norwegian continental shelf
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The purpose of the present compilation of the magnetic characteristics of the Norwegian continental shelf is to provide background data for developing a system to reduce the well-bore positional uncertainty in offshore areas. IKU Petroleum Research wants to consider the potential influence from magnetic rocks adjacent to offshore exploration and production wells. An aeromagnetic map at the scale of 1:3 million, depth to magnetic basement maps and compila- tion of available petrophysical data are presented. The magnetic anomaly pattern on the Norwegian continental shelf is interpreted to reflect three different types of magnetic sources; a) Magnetic basement continuing from mainland Norway below the continental shelf b) Tertiary and Permian volcanic rocks c) Subcropping and buried sedimentary units of late Palaeozoic, Mesozoic, Tertiary or Quaternary age causing low amplitude, high frequency anomalies. The latter is only seen in modern, high-sensitivity data. The depth to magnetic basement estimates indicate a general depth of 7-10 km under the basins of the Norwegian continental shelf. The magnetisation of the basement decreases to zero at a depth of approximately 20 km where the rocks reach the Curie temperature for magnetite (580oC) We conclude from available petro- physical data that the volcanic rocks in the sedimentary basins are the most important contributor to inaccuracy in the positioning of offshore wells. The magnetic basement is in most cases several kilometres deeper than the reser- voirs of interest to the petroleum industry and their influence will there- fore be relatively small. The magnetisation of the sediments within the basins are one to two orders of magnitude lower than the magnetisation of basement rocks and volcanics.