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1 Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway; oyvima{at}geo.uio.no
2 Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
3 Venoco Inc., 6267 Carpinteria Ave., Ste 100, Carpinteria, California
4 Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
5 Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
6 Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
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.
ABSTRACT
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.
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