Experimental and Natural Compaction of Carbonates - Quantification of Mechanical and Chemical Compaction
Carbonate sediments are more reactive than siliceous sediments at low temperatures. Thus compaction of carbonate even at shallow depth may be strongly influenced by chemistry. Two different sets of experiments were conducted to evaluate the effect of chemical and mechanical compaction respectively on porosity loss.
Firstly experiments were performed on 16 samples cored during ODP Leg 194 on the Miocene Marion Plateau, offshore NE Australia. They were uniaxially compacted up to 70 MPa. The samples are bioclastic carbonate platform facies that were cemented with low-Mg calcite and 5 samples were dolomitized before burial to present depths of 39-635 mbsf. 10 samples tested under dry conditions had up to 0.22 % strain at σ'1=50 MPa, whereas 6 samples tested saturated with brine, under drained conditions, had up to 0.33 % strain. The yield strength was reached in 5 of the plugs. The compressibility and elastic constants show overall a positive correlation with porosity.
Secondly experiments were performed on high-Mg carbonate sand, consisting mostly of skeletal fragments of molluscs. Samples were uniaxially compacted up to 32 MPa at 50°C. Creep deformation was monitored for one month, at a constant vertical stress of 30 MPa. Comparison of creep deformations obtained with reactive and non-reactive fluids allowed separating mechanical and chemical effects. Enhancing carbonate solubility by adding NH4Cl to the pore fluid slightly increased the creep strain. Only small creep deformation occurred in the dry samples while the yield stress for carbonate sand was not reached. The effects of vertical effective stress and grain size on the system were also studied. Microstructures and intragranular cracks were observed in scanning electron microscope.
Early cementation of the Marion Plateau carbonates produced mechanically stable and over-consolidated sediments. These carbonates have a very small mechanical compressibility up to stresses equivalent to 4-5 km burial depth. Therefore further porosity loss would be due to chemical processes. Uniaxial compression tests on uncemented bioclastic sand showed that the mechanical compressibility is similar to the one in siliceous sands. Grain crushing occurred during mechanical compaction producing nucleation sites for further crack growth at constant stress. Results of the creep tests suggest that porosity loss in these creep experiments occurred by a combined effect of pressure solution and subcritical crack growth.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009