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The Role of Organic Acids in Feldspar and Quartz Dissolution at 100 degree C/347 Bars: An Experimental Study

FRANKLIN, STAN P., ANDREW HAJASH, JR., and THOMAS T. TIEH, Texas A&M Unversity, College Station, TX

Water-soluble organic anions are capable of dissolving feldspar and quartz under diagenetic conditions and maintaining high levels of Al and Si in solution. It has been shown that abundant organic acids are released prior to and possibly during hydrocarbon generation. The combination of dissolution and complexing capacity suggests that organic acids may be of significant importance in mineral dissolution and secondary porosity development in sandstones in the diagenetic environment.

Dissolution of albite and quartz was studied using flow-through hydrothermal vessels at 100 degrees C and 347 bars. These systems allow realistic simulation of subsurface flow rates and low water/rock ratios. Buffered 5000 ppm acetate/500 ppm oxalate solutions (pH = 4.7) were passed through sand packs of 250-500 um Amelia albite [Ab(97-99)] and Brazilian hydrothermal quartz. The solid starting material was cleaned ultrasonically to remove any adhering ultrafine particles. Flow rate was varied from 10, 5, 2.5, and 0.5 m/yr over a period of seven months. Pore-fluid chemistry of the reacted solutions was analyzed for Na, Al, and Si using atomic absorption spectrophotometry. Organic species were analyzed using ion chromatography.

Steady-state concentrations of SiO(2) and Al average 237 ppm and 50 ppm, respectively. These values are significantly higher than predicted by the solubilities of common aluminosilicate clay minerals in water. This suggests that the amount of Al held in solution by complexing is very high, significantly reducing the amount of Al available for clay precipitation. Following rapid initial dissolution, the system appears to have reached equilibrium as indicated by the lack of variation of Al and SiO(2) concentrations with flow rate. Al and SiO(2) concentrations remained high at a flow rate of 0.5 m/y over a period of 130 days. Molar ratios of Al/Si and Al/Na exceed stoichiometric values throughout the experiment.

Optical and electron microscope analyses of the solid run products show fairly extensive dissolution features which closely follow cleavage planes. Numerous pits were observed which probably reflect enlargement of fluid inclusions. Virtually no authigenic minerals were observed. The lack of authigenic aluminosilicates indicates the efficiency of the organics in removing and transporting Al and Si. Future work with aliphatic and aromatic organics, as well as inorganic acids, will allow comparison of the role of various aqueous species in mineral dissolution and secondary porosity development.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)