The role of carboxylic acids in albite and quartz dissolution: An experimental study under diagenetic conditions

Abstract

Simple water soluble organic acids may promote secondary porosity development in sandstones during diagenesis by increasing feldspar solubility and dissolution rate. To test this hypothesis, Amelia albite and Brazilian hydrothermal quartz were reacted with 0.07 m acetate and 0.07 m acetate-0.005 m oxalate solutions at selected pH values, and distilled water. Pore fluid chemistry was monitored through time at various flow rates to obtain both solubility and dissolution rate data. The experiments were conducted in large volume, semi-static, flow-through systems at 100°C and 347 bars. These systems simulate subsurface flow rates, low mass water/rock, and high surface area/fluid mass. Acetate and acetate + oxalate solutions significantly increase albite solubility at temperatures, pressures, and pH values typical of diagenetic environments. Albite solubilities increased in acetate and acetate + oxalate solutions by factors of 2 and 3.4, respectively, compared to distilled water. In these same solutions, Al concentrations were ≈ 140 and ≈480 times higher than that calculated for kaolinite solubility at the same conditions without organic species. These enhanced solubilities occur at pH values (4.6–4.8) that may overlap with formation waters. In contrast to albite, quartz solubility was essentially identical in all solutions investigated. Dissolution rates in the acid region decreased with increasing pH in the acetate and acetate + oxalate solutions. Slopes of log rate vs. pH curves were ≈0.6 for acetate and ≈0.3 for acetate + oxalate. Although the effects of acetate on the dissolution rate are small, the effects of oxalate are significant. A rate law valid for albite dissolution at 100°C, oxalate concentrations to 0.01 m, and pH values ranging from 3.4 to 5.5 is given below (assuming activity coefficients = 1 and acetate rate ≈ the proton-promoted rate): R total = 5.88 × 10 −11+ 5.01 × 10 −8m 0.56 H+ + 6.7 ×10 −102.3 × 10 −4m O x /(1.0+2.3 × 10 −4 m O x ), where m O x and m H+ are oxalate and H + molal concentrations, respectively. Reacted albite grains examined by SEM show extensive dissolution concentrated along cleavage planes and structural imperfections such as twin boundaries and fluid inclusions, consistent with surface-controlled reaction kinetics. No authigenic aluminosilicate minerals were observed. The lack of authigenic clays indicates the efficiency of oxalate and acetate in mobilizing and transporting Al. The combination of enhanced solubility and increased dissolution rates indicates that carboxylic acids may play major roles in feldspar dissolution and secondary-porosity development during diagenesis of feldspathic sandstones.