Experimental and natural compaction of clays and shales: Consequences for stress and a fluid flow

Knut Bjørlykke, Nazmul Mondol, Chris Peltonen, Øyvind Marcussen, and Jens Jahren
Department of Geosciences, University of Oslo, Norway

Mudstones are very heterogeneous with respect to grain size, mineralogical composition and their physical and chemical properties vary within wide ranges. Mudstones may contain significant amount of silt and sand sized clastic grains in addition to variable carbonate cement. The clay minerals range from relatively coarse grained clastic mica and kaolinite to very fine grained illite and smectite. Experimental compaction tests show that the compressibility of clays is inversely related to grain size and that the composition of the fluid also plays an important role. This can be explained by surface area and double layer theory where compressibility is sensitive to pore-water salinity. Compaction trends in sedimentary basins, as indicated by well log data, show that density and velocity vary greatly at shallow to moderate depths (<2 km), depending on the primary clay mineral composition and the content of carbonate cement. Low densities and velocities may be both a function of the mineralogical composition (i.e. high smectite content) and low effective stress due to overpressure. This is clearly illustrated by log data from wells in the northern North Sea and Norwegian Sea. At greater depth (>2.5-3 km) the densities and the velocities are markedly higher and less variable. This is due to chemical compaction involving some clay mineral diagenesis (smectite to illite) and quartz cementation. Low velocity shales observed at these depths may be attributed to source rocks. The compaction driven fluid flux in sedimentary basins is a function of the rate of compaction, which may be calculated based on sedimentation rates and porosity/depth curves. Mechanical compaction requires increasing effective stress which is reduced by overpressure, thus usually preventing overpressures from reaching fracture pressure. Chemical compaction in siliceous sediments is however a function of temperature and quartz cementation is continuous at temperatures above 70-80 °C regardless of effective stress. The process of chemical compaction will also relax differential stresses.
Permeability may vary greatly in mudstones and shales, both perpendicular and parallel to bedding. Development of overpressure is mainly a function of permeability, which is almost impossible to determine accurately enough to predict high overpressures. Modeling of overpressure requires very detailed information about distribution of permeability which can not be obtained. Calculations show that permeability below 0.1 and 0.01 nD are required to reach fracture pressure at about 3 km depth assuming vertical compaction driven flow. The fact that overpressure may be preserved for a long time in exhumed basins, which are not undergoing compaction, indicate that shales may be almost impermeable (<0.001nD).
Prediction of shale properties as a function of temperature must be based on mineral composition and grain size, which can be linked to provenance and facies following the principles established.

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands