FROM SAND TO SANDSTONE: EXPERIMENTAL SIMULATION OF EARLY CALCITE CEMENTATION

Authigenic carbonate minerals are one of the most abundant cement phases in clastic reservoirs. Mineral types, abundance and distribution patterns of such cements have major impact on the petrophysical properties and ultimately the reservoir quality of sedimentary rocks. Despite the significantly improved knowledge about calcite reaction kinetics, the upscaling of experimental rates remains challenging and reservoir quality predictions are not yet well constrained in terms of carbonate cementation. Hence, systematic investigations on carbonate cement formation do not only improve the accurate prediction of reservoir quality for the geothermal and petroleum industry, but also advance the knowledge of carbonate interactions with CO₂ for carbon capture and storage.

The objective of this study was to analyse, to compare and to link the precipitation and distribution of calcite cement in synthetically produce sandstones on core-scale. Precipitation experiments simulating natural early calcite cementation processes were integrated with X-ray tomography to quantify the evolution of porosity and permeability, while high resolution 2D optical and scanning electron microscopic were used to track micron scale features. To simulate early-calcite cementation, loose sediment with various composition and grain size was perfused by a calcite supersaturated solution for about 30 days at 20°C. The experimental results show the precipitation of grain-rimming, pore-bridging and pore-filling granular calcite cement with crystal sizes of up to 100 µm. Despite a clear positive correlation between carbonate grains and the amount of calcite crystals, calcite cement does not preferentially nucleate on bioclast surfaces, irrespectively of the favourable mineralogy. Furthermore, grain size variations within the sediment have a significant influence on the precipitation pattern of calcite. Pore flow simulations based on the 3-dimensional pore networks allow us to characterise permeability evolution. The findings of this study provide new insights into the precipitation mechanisms of calcite cement in siliciclastic and mixed carbonate-siliciclastic sequences and contribute to our understanding of the impact of diagenetic CaCO₃ formation.