Dissolution and compaction of albite sand in distilled water and pH-buffered carboxylic acid solutions: experiments at 100 degrees and 160 degrees C

Abstract

Experimental studies were conducted using albite sand (250- 500 mm) with distilled water, 0.07m acetate, and 0.07m acetate+0.005m citrate solutions in a hydrothermal flow-through system at conditions that simulate diagenesis. Pore-fluid chemistry and compaction were monitored to quantify the effects of organic acids on time-dependent compaction rates of albite. The effects of stress and fluid chemistry on the dissolution kinetics were also examined. Compared to distilled water, Si-based dissolution rates are 2 times faster in 0.07m acetate and 5 times faster in 0.07m acetate+0.005m citrate. In addition, the dissolution rate increases (=10%) when an effective pressure is applied, probably due to increases in total surface area caused by grain breakage at grain contacts. Albite dissolution appears to be controlled by surface complexation reactions at Al sites. The dissolution rate is strongly affected by the ability of specific organic acid ligands to form both surface complexes and aqueous complexes with Al. Time-dependent compaction was observed at 100 and 160'C at an effective pressure of 34.5 MPa. Strain rates increase with temperature and decrease with increasing strain in all pore fluids, especially at 100C. Strain rates (corrected for strain differences) were similar in distilled water and the acetate buffer; however, even small amounts of citrate species appear to enhance compaction compared to the other fluids Pore-fluids during time-dependent compaction studies have compositions that are near saturation with respect to albite. The fluids may be slightly supersaturated, but silicon concentrations show no unequivocal evidence for supersaturation, primarily because of the uncertainty in albite solubility. Consequently, any chemically-based interpretations of deformation mechanisms are severely limited. Textural data and the lack of supersaturated fluids indicate that time-dependent compaction occurs primarily by brittle mechanisms (fracture), at least at 100C. However, the time-dependent deformation is clearly thermally activated and may be chemically assisted by the aqueous pore fluid.