Particle alignment during sedimentation – a 4D approach

The microstructural evolution of clay-rich sedimentary rocks starts with the settlement and alignment of particles. With the accordance between shape preferred orientation and crystallographic preferred orientation (CPO) in the case of disk-like clay particles, the parallel alignment can be quantified measuring the clay CPO.

In order to quantify the influence of the sedimentation conditions on the CPO of primary layering we performed sedimentation experiments combined with in-situ synchrotron diffraction measurements. The experimental procedure involved a sediment suspension drip inserted at the top of a 30 cm water column at regular intervals. The fluid in the column was either deionized water or seawater, and the sediment suspension contained either kaolinite with of disk-like particle shapes or a mixture of kaolinite and polycrystalline illite, the latter with more compact particle shapes. Time-resolved CPO development in the experiments was measured at ESRF, beamline ID22.

The formation of a CPO is readily observed at the water-sediment interface with an initially higher CPO strength in deionized water experiments than in seawater. The resulting sediment shows a pronounced layering in both fluids when using pure kaolinite. In the layered sediments in deionized water the CPO strength varies strongly between different layers. At some measurement positions, a high initial CPO strength drops fast in the first 150 minutes, interpreted to result from dewatering-related reorganisation of the microstructure. A stable CPO strength can be observed after ~200 min. In the seawater experiments the CPO strength does not vary in different layers and increases slowly but constantly with time and overburden indicating a successive rotation of particles. CPO in kaolinite experiments is higher than in kaolinite + illite experiments as compact particles locally inhibit the alignment.

The evolution of clay particle orientation and therefore microstructure can be quantified in space and time. It is suggested that the initial microstructure is crucial for the progressive development of the rock during diagenesis and hence e.g. resulting physical properties.