--> Abstract: Organic Geochemistry of Cap-Rock Hydrocarbons, Snorre Field, Norwegian North Sea, by T. L. Leith; #91012 (1992).

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ABSTRACT: Organic Geochemistry of Cap-Rock Hydrocarbons, Snorre Field, Norwegian North Sea

LEITH, T. L., IKU (Continental Shelf and Petroleum Research Institute), Trondheim, Norway

An understanding of the relationship between hydrocarbon-bearing reservoirs and their cap rocks is of fundamental importance to both prospect generation and reservoir studies. This study investigates the organic geochemistry of the Tertiary-Cretaceous cap-rock succession that overlies the Triassic-Early Jurassic sandstone reservoir of the Snorre field in the Norwegian sector of the North Sea.

The results obtained confirm that vertical leakage of oil from the Snorre field into the overlying claystones and marls of the Late Cretaceous Shetland Group has occurred. These rocks were found to contain relatively high concentrations of residual oil that was compositionally related to the Snorre oil but was typically enriched in asphaltenes and polar compounds. The concentration of residual oil gradually decreased toward the top of the cap-rock unit with increasing distance from the reservoir horizon. Molecular biomarker data allowed monitoring of mixing between leaked residual oil and traces of immature indigenous bitumen in the generally organic-lean cap-rock succession.

The distribution of residual oil in the cap-rock succession immediately overlying the Snorre reservoir has more similarity to a very poor reservoir than to a "seal." Residual oil found in the cap rocks must have been emplaced directly from the reservoir by bulk-flow processes, rather than by diffusive processes. Although the occurrence of a fracture zone in the cap rocks may lead to locally high residual oil concentrations, there is no evidence that major fracture systems are responsible for emplacing the oil found in the cap rocks. It is suggested that the sealing capacity of the cap rocks is related to a combination of lithological variation in the cap rocks, microfracturing, and hydraulic equilibrium with the reservoir.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)