Compaction and porosity reduction in carbonates: A review of observations, theory, and experiments

International audienceCarbonates are major sedimentary materials found in many upper layers of the Earth's crust. Understanding their compaction behaviour is important for porosity prediction in sedimentary basins and to improve the knowledge about sealing of active faults at shallow depths, where the faults cross-cut limestone formations. In carbonates, as opposed to siliciclastic sediments, diagenesis starts at shallow depths (<1 km) and can contribute to the formation of a mechanically stable solid framework. Vertical stress, grain size and clay content are the main parameters influencing mechanical compaction. Mechanical compaction of unconsolidated carbonate sands in laboratory occurs mostly at low stress and is mainly controlled by mineralogy and initial packing of grains. It can explain porosity reduction down to about 30%. Conversely, very little porosity loss (< 1%) is obtained by mechanical compaction of cemented rocks under laboratory conditions. In sedimentary basins, however, much lower porosity values are usually encountered, down to zero. Given that mechanical compaction does not explain satisfactorily porosity-depth trends observed in sedimentary basins, the effect of chemical compaction on porosity must be considered. Among chemical mechanisms, pressure solution creep involves local mass transfer by dissolution, diffusion and précipitation processes at the grain scale. Subcritical crack growth is also a fluid assisted process contributing to grain fragmentation and compaction by rearrangement of particles. Pressure solution creep strain rate depends on grain size, porosity, applied stress, fluid chemistry, and temperature. Chemical compaction by pressure solution creep becomes an effective process of porosity reduction from less than one kilometer of depth, as soon as fluids are present. The main parameters controlling porosity loss then become vertical stress, temperature, diffusive flow and pore fluid chemistry. Both mechanical and chemical compaction can lead to either pervasive compaction or localized deformation. The effect of the different parameters cannot easily be differentiated in observations of natural samples, as various deformation processes occur and interact simultaneously. However, control parameters may be separated in specially designed theoretical studies and laboratory experiments. So far, few experimental studies have been performed on pressure solution creep and subcritical crack growth in carbonates. Creep experiments on calcite powder and indenter experiments have shown that time-dependent compaction requires the presence of water. Even though the different controlling parameters were tested, no clear consensus exists on the rate limiting step of deformation and, consequently, on the creep law. Individual processes leading to porosity loss in carbonates are rather well identified. However, their respective importance during burial is still debated. Even at shallow burial (<1 km) chemical compaction is needed to explain the gap between porosity loss obtained during experimental mechanical compaction and porosity-depth curves from sedimentary basins. The present study reviews various processes at work during carbonate compaction and synthesizes the current understanding on the respective importance of thermodynamical and petrophysical parameters at different stages of carbonate compaction