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Models of Faults and Fluid Flow |
Department of Geosciences University of Oslo, Box 1047, 0316 Oslo, Norway knut.bjorlykke{at}geo.uio.no
Modelling of sediment compaction requires that the rate limiting processes are understood. The compaction of uncemented sediments at relatively shallow burial depths should be modelled as a function of effective stress following soil mechanical principles and using experimental compaction data for calibration. In siliceous rocks chemical compaction is dominant at depths greater than 2–3 km (80–100°C). Chemical compaction should be modelled as a function of the temperature history and the mineralogical and textural composition of the sediments. The rate of chemical compaction for siliceous sediments is to a large extent a function of the quartz cementation, which is an exponential function of temperature, while the effective stress plays a minor role. In the case of carbonate sediments the kinetics of precipitation of cement is much faster and the effective stress is more important than temperature.
The magnitude and distribution of effective situ stresses is a complex function of external tectonic stresses, gravitational forces and fluid pressures. Sediments undergo mechanical compaction when subjected to high effective stress and are much more compressible than basement rocks. Chemical compaction also results in a reduction in rock volume and this has a strong feedback on the in situ stresses. If the horizontal stress is greater than the vertical stress, both mechanical compaction and chemical compaction will also occur in the horizontal direction, thus relaxing in situ stresses unless there is significant basin shortening. Calculations show that relatively large in situ stress anomalies (10 MPa) may be relaxed in 5–10 ka by chemical compaction during basin subsidence. Chemical compaction may also continue during uplift; it is fundamentally different from mechanical compaction and must be modelled separately.
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