Compaction, Pressure Solution, and Porosity Evolution of Sandstones: Insights from Experiments and Numerical Modeling
Wenwu He1, Andrew Hajash2, and David
Sparks2
1Research and Engineering, M-I SWACO, Houston, TX
2Department of Geology and Geophysics, Texas A&M University, College Station, TX
Despite many years of study, diagenesis of sandstone is still not well understood. Natural sandstones are produced by complex coupled chemical and physical processes operating over long times. Here we report on long-term (4-7 months) creep compaction experiments on quartz sand at diagenetic conditions. Experimental measurements combined with theoretical considerations were used to model the rate-controlling process of pressure solution and porosity evolution as unconsolidated quartz sand undergoes lithification to sandstone. Our experiments indicate that the presence of water increases strain rates by several orders of magnitude. The presence of water activates intergranular pressure solution and also accelerates microcracking. Creep compaction rates decay exponentially under all conditions investigated; however they decrease most rapidly under dry conditions, less rapidly under vapor-dominated conditions, and slowest for water-dominated conditions. Theoretical modeling indicates that intergranular pressure solution is initially rate-controlled by dissolution. As strain increases, diffusion becomes the rate-controlling process. The amount of strain required for this transition increases with decreasing grain size and increasing diffusivity. Both experimental investigations and theoretical modeling suggest that creep compaction rate is related to pore-fluid concentration. Undersaturated pore fluid in open systems accelerates the diffusion of silica from contacts and enhances intergranular pressure solution. Conversely, supersaturated pore fluid will slow diffusion and retard pressure solution. This may explain why pressure solution textures are better developed in sandstones with little clay, since pore-filling or grain-coating clays lead to higher pore-fluid concentrations.
AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas