SYSTEMATIC DIAGENETIC CHANGES IN GRAIN-SCALE MORPHOLOGY IN A QUARTZ-CEMENTED QUARTZ ARENITE AND THE RESULTING EFFECTS ON PERMEABILITY, MECHANICAL PROPERTIES, AND ULTRASONIC VELOCITY

Sandstones are important aquifers and hydrocarbon reservoirs, and have been identified as potential CO₂ sequestration sites. Modeling the hydromechanical response of sandstones to pumping and injection stresses is needed to effectively manage and maintain the integrity of these resources. Accurate modeling of these systems, however, requires a thorough understanding of controls on their hydrologic and mechanical properties. In sandstones, such properties are determined by a range of variables, including grain size, sorting, and modification of the original sediment through diagenesis. Of these parameters, the effects of diagenesis are arguably least well understood, yet are the most likely to be modified as large volumes of fluids, particularly reactive fluids, are moved through a given sandstone. To isolate the effects of diagenesis, and explore how they affect permeability and both elastic and inelastic mechanical properties, we quantified the changes in grain and pore morphology accompanying progressive diagenesis of a simple system: a well sorted, variably cemented quartz arenite that experienced a diagenetic history dominated by physical compaction and quartz cementation. This work demonstrates that these processes modify the grain framework in consistent, predictable ways. With progressive diagenesis, both the number and length of grain contacts increase as the number of pores increases and the number of large, well-connected pores decreases. These changes systematically alter pore shape and reduce pore size variability and bulk permeability. At the same time, they regularly increase elastic moduli, confined compressive strength, and ultrasonic velocity. The consistent, progressive nature of these changes allows us to calibrate the proportionality constant of the Kozeny-Carman relationship, improving our ability to predict permeability in quartz-cemented quartz arenites. This calibration spans a range in porosity and shear modulus that previous work has shown to correlate with variations in fault structure and permeability in sandstone, suggesting fault character can be viewed in this broader context. The strong correlation between the measured physical properties and ultrasonic velocity offers an avenue to extend observations to the subsurface.