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ABSTRACT: Defining dynamics of hydrocarbon zones in reservoir strata using petroleum inclusion distribution and geochemistry - Elucidating petroleum migration into the Gullfaks Sør field and the nearby Gullfaks Gamma structure, Norwegian continental shelf

Karlsen, Dag A.1, Kristian Backer-Owe1, and Reidar Helland2
(1) University of Oslo, N-0316 Oslo, Norway 
(2) Statoil, N-4035 Stavanger, Norway

The Gullfaks Sør and the Gullfaks Gamma structures are rotated clastic faultblocks with complex arrays of hydrocarbon and water zones in their compartmentalized stacked multi-layered reservoir configurations. Examination of core extracts, drill stem tests and fluorescent petroleum inclusions in authigenic quartz cements in 5 wells from the Gullfaks Sør and the Gullfaks Gamma structure has showed that these structures have experienced considerable movements in oil-water, gas-water and gas-oil contacts over time, revealing in this way the dynamic nature of intra-field petroleum migration. This is evident from the existence of oil type petroleum inclusions in the present water zones and gas zones, observations substantiated by the associated rich bitumen core extracts from these zones.

We illustrate how such geochemical data are used to construct a filling direction model for the Gullfaks Sør and the Gullfaks Gamma structures.

The gas chimney above Gullfaks Sør is most likely associated with episodic leakage from a partly eroded cap rock or reservoir strata, which in the near past were close to fracture pressure. We display chemical signatures from core extracts which we believe to illustrate fractionative gas stripping through the erosionally thinned cap rock, whilst for Gullfaks Gamma we propose a loss of the oil column due to displacement by gas and/or by remigration because of seal failure, as the reservoir today is close to fracture pressure.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia