|
Methods |
1 School of Earth Sciences, The University of Birmingham Birmingham, B15 2TT, UK
2 Hamilton Brothers Oil and Gas Ltd, Devonshire House, Mayfair Place, London WIX 6AQ, UK
3 Departamento de Estratigrafia, Facultad cc. Geologicas, Universidad Complutense, 28040 Madrid, Spain
A detailed palaeomagnetic study has been made of the Skagerrak Formation (Triassic) in two wells from the Crawford Field in the northern North Sea. In 9/28a-9 (unorientated) a well-defined magnetostratigraphy is present comprising up to four reversed and three normal zones. There are secondary magnetizations (Tertiary 8 Recent in age) particularly in areas which are oil stained. In 9/28a-13 (orientated), three normal and four reverse magnetozones can be isolated. Despite the existence of orientation data it is more difficult to establish a reliable Triassic magnetostratigraphy. This is partly due to the fact that the sandier sequence in this location is impregnated with oil and associated diagenetic effects have resulted in pervasive magnetic overprints which are difficult to separate from the original Triassic magnetization. The effects of oil emplacement included widespread dissolution and the precipitation of a suite of secondary iron minerals including pyrite and possible pyrrhotite or magnetite. These later phases carry a secondary component of magnetization which overprints the original Triassic characteristic remanence. Directions of secondary magnetization are consistent with oil emplacement in Tertiary times.
In attempting to correlate the two wells a number of factors have to be taken into account, in particular variations in sedimentation rate and the lack of precision in the published Triassic polarity time scale. The results of this study strongly suggest that the cored sequences in the two wells are not time equivalent. However, the sequence in 9/28a-9 compares most closely with the Upper Triassic polarity time scale.
