Mechanisms of compaction of quartz sand at diagenetic conditions

The relative contribution of cracking, grain rearrangement, and pressure solution during experimental compaction of quartz sand at diagenetic conditions was determined through electron and optical microscopy and image analysis. Aggregates of St. Peter sand (255±60 μm diameter grain size and porosity of approximately 34%) were subjected to creep compaction at effective pressures of 15, 34.5, 70, and 105 MPa, temperatures of 22 and 150°C, nominally dry or water-saturated (pore fluid pressure of 12.5 MPa) conditions, and for times up to one year. All aggregates displayed transient, decelerating creep, and volume strain rates as low as 2×10⁻¹⁰ s⁻¹ were achieved. The intensity of fracturing and degree of fragmentation increase with volume strain and have the same dependence on volume strain at all conditions tested, indicating that impingement fracturing and grain rearrangement were the main mechanisms of compaction throughout the creep phase. The increase in fracture density and decrease in acoustic emission rate at long times under wet conditions reflect an increase in the contribution of subcritical cracking. No quantitative evidence of significant pressure solution was found, even for long-term creep at 150°C and water-saturated conditions. Comparison of our findings to previous work suggests that pressure solution could become significant at temperatures or times somewhat greater than investigated here.