BOND, KATIE J.¹, ANDREW C. APLIN¹, STEVE R. LARTER¹, and ROLANDO di PRIMIO^2
1Fossil Fuels and Environmental Geochemistry Postgraduate Institute, University of Newcastle upon Tyne, UK
²Saga Petroleum, Oslo, Norway

Abstract: Vertical Migration through the Snorre Mudstone Caprock

Although generally a barrier to petroleum movement, a caprock, if breached, can become an important pathway for the vertical migration of petroleum, from a reservoir into the overlying horizon.

This process can occur in one of two ways; firstly, through the capillary network of the caprock if the hydrocarbon buoyancy exceeds the capillary resistance of the caprock. Alternatively, rock fracture initiates caprock failure when the forces of buoyancy exceed the tensile strength of the rock before the capillary resistance of the caprock can be breached. Migration through a fracture network results from this type of caprock failure.

The Snorre Field, Norwegian North Sea is capped by more than 600 m of mudstones. There is a seismically defined gas chimney above Snorre and geochemical analyses show that liquid petroleum is present in the caprock.

Physical and chemical analyses are used (1) to define the anatomy of the Snorre caprock and (2) to determine the distribution and composifion of the petroleum in the caprock, in order to assess the mechanism by which petroleum has moved into the caprock.

The extracted organic matter content and geochemical analyses indicate that a significant volume of petroleum is present throughout the caprock and is likely to have originated from the Snorre reservoir. Pore size distributions yield capillary entry pressure values, which suggest the caprock is currently behaving as an efficient seal.

It is, therefore, proposed that the Snorre caprock has leaked through a capillary network.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas