ABSTRACT: Petroleum Migration, Accumulation, and Biological Degradation in the Tampen Spur Area, Norwegian North Sea

I. Horstad, S. R. Larter, N. Mills

The Tampen Spur area in the northern part of the Norwegian North Sea has been intensively studied during the last decade, with many wells having been drilled within quadrants 33 and 34. Despite the large amount of data from the area, no satisfactory model has been proposed to explain the petroleum migration and reservoir filling processes for the oil accumulations in the area, including Gullfaks, Snorre, and Statfjord.

Based on detailed petroleum column geochemistry analysis (GC, GC/MS, GC-Isotope-MS) of more than 70 drill-stem test oil samples,

screening analysis (TLC-FID latroscan) of several hundred reservoir core extracts, and the use of standard reservoir PVT data, we have come up with a more realistic migration/accumulation model for the Tampen Spur region.

The petroleum accumulated in the area ranges from light oils and gas condensate fields to biodegraded and undersaturated medium-gravity oil fields. The recognition of the migration and accumulation patterns in the area is complicated by the fact that all of the oils are derived from the same source rock layer. However, careful investigation, using statistical testing of the biomarker distributions, has enabled discrimination of the reservoired petroleums into several populations, related to generation and filling of structures from different source rock basins. Tentative gradients in the biomarker maturities across one of the fields are believed to reflect filling directions. Later, in-reservoir biodegradation selectively removed n-alkanes in the petroleum within some of the structure during partial exposure of the area in the Cretaceous-Tertiary. Pervasive gradients in the extent of biological degradation show the scars of the direction of water flow in the structures during the period of in-reservoir degradation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990