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Abstract

AAPG Bulletin, V. 93, No. 1 (January 2009), P. 127-150.

Copyright copy2009. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/08220808044

Physical properties of Cenozoic mudstones from the northern North Sea: Impact of clay mineralogy on compaction trends

Oyvind Marcussen,1 Brit I. Thyberg,2 Christer Peltonen,3 Jens Jahren,4 Knut Bjorlykke,5 Jan Inge Faleide6

1Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway; [email protected]
2Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
3Venoco Inc., 6267 Carpinteria Ave., Ste 100, Carpinteria, California
4Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
5Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway
6Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway

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 (gt70–80degC), 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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