ALBERT

Description: 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. 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.

Description: Vertical and lateral changes in physical properties in Cenozoic mudstones from the northern North Sea Basin reflect differences in the primary mineralogical composition and burial history, which provides information about sedimentary facies and provenance. Integration of well-log data with mineralogical information shows the effect of varying clay mineralogy on compaction curves in mudstones. The main controlling factor for the compaction of Eocene to early Miocene mudstones within the North Sea is the smectite content, which is derived from volcanic sources located northwest of the North Sea. Mudstones with high smectite content are characterized by low P-wave velocities and bulk densities compared to mudstones with other clay mineral assemblages at the same burial depths. Smectitic clays are important during mechanical compaction because they are less compressible than other types of clay minerals. A comparison between well-log data and experimental work also shows that smectite may be a controlling factor for overpressure generation in the smectite-rich Eocene and Oligocene sediments. At greater burial depths and temperatures (〉70–80°C), the dissolution of smectite and precipitation of illite and quartz significantly increases velocities and densities. Miocene and younger mudstones from the northern North Sea have generally low smectite contents and as a result have higher velocities and densities than Eocene and Oligocene mudstones. Lateral differences in the compaction trends between the north and south for these sediments also exist, which may be related to two different source areas in the Pliocene. The log-derived petrophysical data from the northern North Sea Basin show that mudstone lithologies have very different compaction trends depending on the primary composition. Simplified compaction curves may therefore affect the outcomes from basin modeling. The amplitude-versus-offset response of hydrocarbon sands and the seismic signature on seismic sections are also dependent on the petrophysical properties of mudstones and will vary depending on the mineralogical composition. Øyvind Marcussen received his M.S. degree in 2003 from the University of Oslo. He is presently a Ph.D. student in petroleum geology at the same university. Brit I. Thyberg received her M.S. degree in geology in 1993 from the University of Oslo. She is presently a researcher in petroleum geology at the University of Oslo. Christer Peltonen received his B.S. in geology from California Lutheran University (1996) and his M.S. and Ph.D. in petroleum geology from the University of Oslo, Norway (2007). He is currently working as a development geologist for Venoco Inc. located in Santa Barbara County, California. Jens Jahren received his M.S. degree (1988) and his Ph.D. (1991) from the University of Oslo. He has been an associate professor first in mineralogy and petrology (from 1994) and then in petroleum geology (from 2003) at the same university. His research focuses on mechanical and chemical compaction processes in sediments. Knut Bjørlykke is a professor at the Department of Geosciences, University of Oslo. He has worked in the field of sedimentology and clastic diagenesis. In recent years, he has led a research group on sediment compaction and rock physics at the University of Oslo. Jan Inge Faleide is a professor at the Department of Geosciences, University of Oslo. He has been project leader and principal investigator for several interdisciplinary and international research projects focusing on the formation and evolution of sedimentary basins and continental margins. Most of his studies have been located offshore Norway and conducted in close collaboration with the petroleum industry.

Description: Fault systems in extensional basins commonly display geometries that vary with depth, reflecting depth- and lithology-dependent mechanical strength. Using an experimental approach, we investigate this relationship by deploying physical analog models with stratified sequences consisting of brittle–ductile (sand–silicone polymer) sequences subject to single and polyphase deformation. The experiments were used as analogs for a sandstone sequence interlayered by beds of evaporates or overpressured or unconsolidated mudstone in nature (the latter being representative of decollement horizons).Experiments (series 1 [S1]) using homogeneous and stratified quartz and feldspar sand produced asymmetric, composite single grabens with diverse fault frequencies and fault styles for the graben margin faults.For the mechanically stratified experiments with one decollement level (series 2), contrasting graben configurations were produced, in that the lowermost sequence was characterized by graben geometries of similar type to that of the S1 experiments, whereas the sequence above the decollement was characterized by large fault blocks, delineated by steepened or oversteepened faults.The experiments with two decollements (series 3) were displayed similarly but included graben geometries that widened upward, with each level being characterized by independent fault systems.The results can be used to explain strata-bound fault patterns and depth-dependent extension as seen in several places along the Norwegian continental margin and elsewhere.