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Øyvind Marcussen, University of Oslo (Norway)
Brit I. Thyberg, University of Oslo (Norway)
Jens Jahren, University of Oslo (Norway)
Knut Bjørlykke, University of Oslo (Norway)
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Clays and mudstones are the most common lithology found in sedimentary basin, and may act both as a source and seal for hydrocarbons. In addition high pore pressures in mudstones may cause drilling problems. Little is known about how different mudstones change their physical properties during burial. Based on well log data from 42 wells in the northern North Sea basin it is evident that velocities and densities vary greatly with depth. Difference in velocities may be up to 500-700 m/s and densities may vary as much as 0.4-0.6 g/cm3 at burial depths less than 2 km within the North Sea. Very low velocities and densities are observed in mudstones of Paleocene to Oligocene age and these sediments have high smectite contents. Mudstones of Miocene to recent age have much lower smectite content and velocities and densities are higher than the underlying Paleocene-Oligocene mudstones. Variations in velocities and densities can be related to the primary mineralogical composition and different burial histories. The smectite content is one of the main controlling factors for mudstone compaction due to its low compressibility compared with other clay minerals. High porosity and low density compared with experimental compaction curves may indicate intervals with overpressure. Mechanical compaction of smectite is limited even at 50 MPa vertical effective stress and the observed increase in velocities at burial depths greater than 2 km (> ∼70°C) must be due to chemical compaction. Simple exponential or linear compaction curves are often used to represent shale and mudstones in basin modeling, but they are very far from those observed from well data in the northern North Sea basin. Compaction curves closer to reality for basin modeling may be obtained by trying to predict the distribution of the primary facies, mineralogy, textural relationships and the burial histories. This may provide a better basis for more accurate input to basin modeling techniques. The different properties of the two mudstone sequences from the North Sea are also apparent in seismic sections. Variations in acoustic impedances appear as different seismic signatures, and the integration of well log and mineralogical data may be used to predict the distribution of different mudstone lithologies. In addition, varying mudstone composition also affects the AVO response of hydrocarbon sands. This important to keep in mind when doing AVO analysis in areas with limited information about mudstone compositions. The present study documents that mudstones have a wide ranges of compaction trends, and that well log and mineralogical data may be used to better predict how fine-grained sediments change their properties during burial.
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