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Mechanical Compaction of Sand and Clay: Constraints From Experimental Compaction, Chemical Reactions and Fluid Flow During Burial – An Overview


Mechanical compaction of sediments can be performed in the laboratory and the results agree well with data from natural compaction in sedimentary basins. The porosity loss as a function of burial indicated by well logs corresponds well with experimental compaction of sediments with similar mineralogical and textural composition. The porosity loss due to mechanical compaction is controlled by the distribution of the overburden stress on grain contacts. Experimental compaction of sand show that well sorted coarse sand is much more compressible than fine grained sand. Experimental compaction of clays with different mineralogical and textural composition also shows this pattern. Coarse clay minerals like kaolinite compact much more than fine grained clays like smectite and illite. At the same stress (20MPa) the porosity found for kaolinite samples may have been reduced to 20-25 % while smectitic clay may still have porosities around 40- 45%. Mechanical compaction controls the porosity at relatively shallow depth (<1.5-2.0 km) before the chemical compaction sets in during further burial. Chemical compaction involving dissolution and mass transport of minerals in solution during burial has been a subject of much debate. However, at temperatures above 70-80 °C, the pore water is beginning to approach equilibrium with the most reactive mineral phases and mineral reactions are driven by thermodynamics and kinetics. Some carbonate is present in most marine sediments buffering the pore water also with respect to acid solutions. The upwards component of fluid flow is limited by the rate of compaction in the underlying sedimentary sequence and may also be constrained by the salinity distribution in sedimentary basins. At shallow depth sediments may be flushed by meteoric water which may dissolve feldspar and mica and this is controlled by climate, head of ground water table and the connectedness of permeable sandstone layers. During deeper burial quartz cementation in sandstones is a function of temperature and time, but also of textural factors and clay coatings. Also in clays quartz cementation play an important role precipitating layers of quartz cement thus changing clay and mudstones to shales with a preferred mineral orientation and pronounced schistocity.