Mechanisms of Compaction of Quartz Sand at Diagenetic Conditions

Judith S. Chester¹, Steven C. Lenz², Frederick M. Chester¹, Andreas K. Kronenberg¹, and Richard A. Lang³. (1) Center for Tectonophysics, Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115, phone: 979 845-1380, fax: 979 845-6162, chesterj@geo.tamu.edu, (2) Unocal Corporation, 14141 Southwest Freeway, Sugarland, TX 77478, (3) Geoscience Data Management, 10003 Woodloch Forest Drive, The Woodlands, TX 77380

The relative contribution of critical cracking, subcritical cracking, and intergranular 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 mm diameter grains, 34% porosity) were subjected to creep compaction at effective pressures of 15, 34.5, 70, and 105 MPa, temperatures of 25 and 150 °C, nominally dry or water-saturated (12.5 MPa pore fluid pressure) conditions, and for times up to one year. All aggregates displayed transient, decelerating creep, and volume strain rates as low as 2 x 10^-10 s^-1 were achieved. Intensity of fracturing and degree of fragmentation increase monotonically with volume strain in a consistent fashion at all conditions tested. Impingement fracturing and grain rearrangement were the main mechanisms of compaction throughout the creep phase. Critical cracking dominated during the short-term wet tests, and during the short- and long-term nominally dry tests. Subcritical cracking dominated during long-term creep under water-saturated conditions. These interpretations are consistent with the formation of circular, 10-20 mm diameter ring cracks, formation of linear grooves, and tensile crack openings, oriented in the direction of inferred slip, noted on prism faces of a single crystal of quartz that was embedded in one long-term experiment. No quantitative evidence of significant intergranular pressure solution was found, even for long-term creep at 150 °C and water-saturated conditions. Comparison of our findings to other work suggests intergranular pressure solution should become significant at temperatures or times somewhat greater than investigated here.