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When do faults in sedimentary basins leak? Stress and deformation in sedimentary basins; examples from the North Sea and Haltenbanken, offshore Norway

¹ Department of Geosciences, Box 1047 University of Oslo, 0316 Oslo, Norway; knut.bjorlykke{at}geologi.uio.no
² Department of Geosciences, Box 1047 University of Oslo, 0316 Oslo, Norway
³ Department of Geosciences, Box 1047 University of Oslo, 0316 Oslo, Norway
⁴ Department of Geosciences, Box 1047 University of Oslo, 0316 Oslo, Norway

Knut Bjørlykke received his Ph.D. from the University of Oslo, became a professor of petroleum geology at the University of Bergen (1976–1984), and is presently a professor at the University of Oslo. He has worked on different aspects of sedimentology, sedimentary geochemistry, and petroleum geology. Recently, he has attempted to apply the principles of diagenesis to rock mechanics.Kaare Høeg received his Doctor of Science from the Massachusetts Institute of Technology (United States) in 1965 and was professor of civil engineering at Stanford University (1968–1974). He was managing director of the Norwegian Geotechnical Institute (1974–1991) and is presently a professor at the University of Oslo. He is an elected member of the Norwegian Academy of Science and Letters and the U.S. National Academy of Engineering.

Jan Inge Faleide is a professor at the Department of Geosciences, University of Oslo, where he is the leader of the Passive Margin Research Group and coordinator of the new M.Sc. study in petroleum geology and geophysics. He is mainly involved in research projects covering the Norwegian continental shelf and margin and adjacent areas in close collaboration with international partners from academia and industry.

Jens Jahren received his M.Sc. degree (1988) and his Ph.D. (1991) from the University of Oslo. He has been an associate professor in geology in the same university since 1994. His research focuses on rock properties with applications to petroleum reservoirs.

Faults may be barriers or conduits for fluid flow in sedimentary basins. The properties of faults, however, depend on stress conditions and rock properties at the time of deformation and subsequent diagenesis of the fault zone. Several recent publications have suggested that petroleum reservoirs in the North Sea and at Haltenbanken, offshore mid-Norway, have experienced leakage along faults caused by imposed stresses, related to glacial loading during the Quaternary. The Jurassic reservoirs in these areas are, however, bounded by faults produced during the Upper Jurassic rifting, when the sediments were still soft and, for the most part, uncemented. These faults do not represent zones of weakness. Because of strain hardening and later diagenesis in sandstones and cementation in mudstones, the fault zones are commonly stronger than the adjacent rocks. They are therefore not likely to be reactivated tectonically. Furthermore, there is little evidence of glacial deformation in the Quaternary sediments overlying these oil fields. It has been proposed that very large horizontal stresses, inferred to be related to periods of glacial loading, caused shear failure at pore pressures below fracture pressure and subsequent leakage along these shear zones. We argue that this is not a likely mechanism during progressive burial in sedimentary basins. Very high horizontal effective stresses, up to 60 MPa, at about 3 km (1.8 mi) depth, at Haltenbanken would have caused more mechanical compaction and grain crushing than that observed in situ. External stress, i.e., plate-tectonic stress from spreading ridges (ridge push), will be transmitted primarily through the basement and not through the much more compressible overlying sedimentary rocks.

During progressive basin subsidence, chemical compaction, i.e., caused by quartz cementation, causes rock shrinkage, which will relax differential stresses. This makes brittle deformation (shear failure), resulting in open fractures less likely to occur at stresses below the fracture pressure. In subsiding sedimentary basins with progressive compaction, horizontal stress will normally not exceed the vertical stress except when there is significant shortening of the underlying basement.

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