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The AAPG/Datapages Combined Publications Database
AAPG Special Volumes
Abstract
Chapter from:
(Publication Subject: 
Rock
Hydrocarbons,
Snorre Field, Norwegian North Sea
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
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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