The Effect of Progressive Cementation on the Mechanical Properties of a Quartz Arenite

Diagenesis converts loose sediment into rock and encompasses many processes, including mechanical and chemical compaction, chemical alteration and dissolution of existing phases, and the precipitation of authigenic phases including cements. These physical and chemical processes alter the sediment at the grainscale and result in significant mechanical changes. In a system dominated by rigid grains, such as quartz sandstones, mechanical changes related to diagenesis result primarily from compaction, precipitation of cements, and dissolution. Here, we examine the effect of progressive cementation on the mechanical behavior of St. Peter Sandstone, a mature quartz arenite with variable amounts of quartz cement, using ultrasonic velocities and microstructural analysis to characterize and quantify the mechanical impact of cementation.

Samples analyzed for this study have a range of cement abundances, from < 2% to greater than 15% cement, but have similar intergranular volumes (original porosities). To quantify the geometric changes associated with progressive cementation, detailed back-scattered electron and CL images were collected. Image analysis indicates that the St. Peter exhibits typical quartz overgrowth geometries but has considerable variation in cement abundance. Quartz cement morphology changes with progressive cementation. Grains in low-cement (< 5%) samples exhibit thin, encrusting layers of quartz with isolated crystal facets and pore spaces are relatively equant and interconnected. In contrast, high-cement (> 10%) samples have an interconnected network of cement with well-defined facets, more crack-like pores, and diminished pore connectivity. Results from ultrasonic velocity measurements and preliminary finite element modeling suggest that both the geometric evolution of cement distribution and the amount of precipitated cement are important controls on the elastic behavior of quartz sandstones.