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Delphine Croizé, University of Oslo (Norway)
Knut Bjørlykke, University of Oslo (Norway)
François Renard, Université Joseph Fourrier (France)
Dag Kristian Dysthe, University of Oslo (Norway)
Jens Jahren, University of Oslo (Norway)
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Porosity loss in carbonate and processes controlling it through burial diagenesis are poorly constrained, despite several descriptive studies. Compaction is both mechanical as a function of effective stress and chemical involving dissolution and precipitation of minerals. Among the main processes of porosity loss during carbonate's burial, pressure solution creep is recognized as playing a major role. The resulting loss of intergranular volume due to dissolution and porosity occluding cement formation will destroy reservoir properties like permeability and porosity. Due to the stronger time dependency of chemical compaction in comparison to mechanical compaction, the later is easier to study in laboratory. However the relative fast kinetics of carbonate enables experimental work on carbonate chemical compaction. Two set of experiments were realized on carbonate chemical compaction, where the aim was to better constrain the respective role of dissolution at grain contacts, diffusion and precipitation during carbonate burial compaction.
In the first set of experiments, high-Mg carbonate sands, consisting of mostly skeletal mollusc fragments, were compacted in uniaxial compression tests at 50 degrees. Their deformation during creep was monitored for one month, at constant effective vertical stresses of 30 or 50 MPa. Comparison of creep deformations obtained with reactive and non-reactive fluids allows us to separate mechanical and chemical effects. Enhancing carbonate solubility by adding NH4Cl to the pore fluid slightly increases creep strain. The sensitivity of creep deformation to the chemistry of the fluid indicates that pressure solution is an active compaction process in these experiments. In addition to the pore fluid chemistry, the effects of vertical effective stress and grain size on the system were studied.
In the second set of experiments, calcite dissolution was measured as a function of the contact area development. Single contact experiments were realized on calcite monocrystals on a micro-scale, i.e., contacts of few thousands of square micrometers. The interface's roughness was characterized using white light interferometry microscopy. With these techniques indentation rates as slow as 1 nm per hour could be measured. These experiments allow a better quantification of volumetric strain induced by calcite dissolution at the grain to grain contact as a function of stress.
The combination of the two sets of experiments provides a better insight on dissolution, diffusion and precipitation processes during carbonate burial diagenesis. These experiments help to constrain more specific strain laws for modelling carbonate sediments compaction as a function of stress, temperature and fluid chemistry.
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