Application of magnetostratigraphy to Brent Group reservoir zonation in the Visund Field

Abstract

Magnetostratigraphy has been applied to an increasing number of oil fields in the North Sea region with promising results for detailed chronostratigraphic reservoir correlation. However, the magnetization processes in hydrocarbon reservoir successions, and the potential use of magnetostratigraphy for reservoir zonation purposes, are not very well known. Therefore, aspects of these subjects are discussed on the basis of a magnetostratigraphic study of three different wells covering the Middle Jurassic Brent Group at the Visund Field, northern North Sea. Results from scanning electron microscopy and remanent coercivity analysis suggest that the primary magnetization defining the magnetozones in the Brent Group is a combination of detrital remanent magnetization and a chemical remanent magnetization, probably acquired contemporaneously shortly after deposition. Detailed thermal demagnetization has uncovered an important secondary magnetization component in most of the core samples. This magnetization is probably due to cooling of the cores from a reservoir temperature of about ll0~ to surface temperature, and it causes the main disturbances in the palaeomagnetic results. Based on analyses of individual core samples, characteristic remanent magnetization components have been defined. The magnetopolarity stratigraphy for the individual wells is based on these components and the results are comparable from well to well. The well correlation covers a rock sequence with a thickness of about 200m, and both magnetostratigraphic and biostratigraphic results have been used in combination with the lithostratigraphy. The acme of Corollina represents the most important palyno-event, and the top and bottom of this palynozone is used as the main tie-level for two of the wells. Sixteen additional correlation lines have been derived from the magnetostratigraphic results, six of these considered as tentative. The correlation reveals that the magnetostratigraphic results are facies independent and some of the lithostratigraphic formation lines appear to be diachronous within this deltaic mega-sequence. Although the magnetostratigraphy only provides a relative time framework for the Brent Group, the results are encouraging and suggest the possibility of achieving high resolution chronostratigraphic well correlations for Middle Jurassic reservoir sequences in the northern North Sea area.The application of magnetostratigraphy in the oil industry is rapidly increasing and the method has been used in several oil fields world-wide (e.g. Rey et al. 1993; Hauger et al. 1994; Belkaaloul et al. this vol.; Bryant et al. this vol.).The potential uses of magnetostratigraphy are many and in hydrocarbon reservoir research may reflect: (1) on a regional scale, magnetopolarity stratigraphy can be used for dating and duration assessments of sequences and individual formations (Haq et al. 1988;Harland et al.