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GEP-18 Compaction processes ? porosity, permeability and rock properties evolution in sedimentary basins - Tribute to Knut Bjørlykke
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Microcrystalline authigenic quartz in fine-grained mudstones of the northern north sea
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Brit Thyberg, University of Oslo (Norway)
Jens Jahren, University of Oslo (Norway)
Knut Bjørlykke, University of Oslo (Norway)
Turid Winje, University of Oslo (Norway)
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Quartz will for kinetic reasons, not precipitate in sediments before a temperature of about 60-80 °C is reached in compacting silisiclastic sediments. In fine grained silisiclastic sediments like mudstones quartz formation will activate the smectite-to-illite reaction where quartz is one of the reaction products. The precipitation of quartz contributes to volume loss and cementation of the mudstone and can be termed chemical compaction. In the Late Cretaceous fine-grained mudstones of well 33/5-2 in the Northern North Sea smectite is altered to mixed-layered smectite/illite, chlorite and illite at burial temperatures above about 60-80 °C. These mineral reactions have produced significant amounts of microcrystalline authigenic quartz within the fine-grained clay matrix. The finely dispersed micro-quartz is interpreted to be derived from local release of Si related to the progressive alteration of smectite-to-illite during burial. The positive identification of microcrystalline authigenic quartz has been obtained by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and mono-chromatic wavelength dispersive cathode luminescence (CL).
The microcrystalline authigenic quartz typically produces a characteristic broad luminescence peak between 580nm and 620nm during the monochromatic CL analysis. The microcrystalline authigenic quartz mainly occurs as sub-micron to 1-3 m spherical grains. Some euhedral microcrystalline quartz grains have also been identified. The microcrystalline authigenic quartz occurs both as single grains within the micro-pores of the fine-grained clay matrix and as networks of several authigenic quartz grains. The network quartz crystals appear to form bonds to other quartz authigenic micro-crystals or bridges to the clay matrix consisting mainly of illite-smectite and illite. Supporting this observation is the common presence of illite/smectite aggregates or pure authigenic illite and finely dispersed authigenic micro-quartz after disintegration of the mudstones in an ultrasonic bath. Short chains of micro-sized authigenic quartz are found as well supporting quartz and quartz-illite network formation. The presences of microcrystalline quartz cement within the fine-grained clay matrix probably contribute significantly to stiffening and strengthening these mudstones. This is also indicated by the well logs showing a clear separation between the mechanical part of the compaction development above 60-80 °C and the chemical compaction described herein below this temperature interval. Formation of larger more pervasive quartz networks in these mudstones could explain the shift in compaction related parameters like porosity and velocity.
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