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Abstract

Chapter from:
AAPG Memoir 67: Seals, Traps, and the Petroleum System, Edited by R. C. Surdam
(Publication Subject: Oil Methodology, Concepts)
AAPG Memoir 67: Seals, Traps, and the Petroleum System. Chapter 8: Organic Geochemistry of Cap-Rock Hydrocarbons, Snorre Field, Norwegian North Sea, by T.L. Leith and A.E. Fallick, Pages 115-134

Copyright © 1997 by The American Association of Petroleum Geologists. All rights reserved.

Chapter 8

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

T.L. Leith

IKU Petroleum Research Trondheim, Norway

A.E. Fallick

Scottish Universities Research and Reactor Centre East Kilbride, Glasgow, Scotland

ABSTRACT

This study investigates the organic geochemistry of the cap-rock succession overlying the Triassic-early Jurassic sandstone reservoir of the Snorre Field in the Norwegian sector of the North Sea. Understanding the relationships between hydrocarbon-bearing reservoirs and their cap rocks is of fundamental importance to prospect generation and reservoir studies.

The results confirm that vertical leakage of oil from the reservoir of the Snorre Field into overlying claystones and marls of the Late Cretaceous Shetland Group has occurred. The relatively high concentrations of residual oil present in the cap rock are compositionally related to the reservoired Snorre oil, but are enriched in asphaltenes and polar compounds. The concentration of the residual oil gradually decreases toward the top of the cap-rock unit. Molecular biomarker data allowed monitoring of mixing between leaked residual oil and traces of immature indigenous bitumen in the generally organic-lean cap rock.

Residual oil in the cap rocks overlying the Snorre Field must have been emplaced directly from the reservoir by bulk-flow processes. Although the occurrence of a fracture zone in the cap rock 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 rock. The sealing capacity of the cap rock is therefore suggested as being related to a combination of lithological variation in the cap rock, microfracturing, and hydraulic equilibrium with the reservoir. The occurrence of calcareous cements with a partially organic carbon isotope signature suggests that the transit of oil through a cap rock succession may enhance the sealing capacity of that cap rock under certain circumstances.

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