PALEOMAGNETIC CHARACTERISTICS OF DRILL CORES FROM MIDDLE JURASSIC DELTA PLAIN SEDIMENTS, THE BRENT GROUP, NORTH-SEA

Low-level palaeomagnetic signals in hydrocarbon-producing delta plain reservoir sediments from the Ness Formation of the Middle Jurassic Brent Group, west of the Gullfaks Sor field, are the result of low concentrations of magnetic constituents. Analysis of magnetic extracts suggests the presence of single to pseudo-single domain magnetite in addition to pyrrhotite. Scanning electron microscopy confirms the presence of detritus-derived magnetite, minor quantities of haematite/goethite and spherical magnetite possibly of diagenetic origin. Acid dissolution experiments suggest that reactive iron is responsible for 90% of the magnetic susceptibility. The stable remanent magnetization in a 75 m core section is carried by detrital magnetite, which is probably of post-depositional origin. The presumably diagenetic pyrrhotite carries a predominant chemical remanent magnetization which it is suggested was acquired more or less contemporaneously with the detrital magnetization.