How Important Are Organic Acids in Generating Secondary Porosity in the Subsurface?

Abstract

The hypothesis that organic acids are responsible for the creation of significant volumes of secondary porosity and the enhancement of aluminium mobility in the subsurface is reviewed against data from reservoir intervals, laboratory experiments and mass-balance considerations. The hypothesis appears to fail on all counts, notably: Observational evidence does not support a sudden increase in abundance of secondary porosity over the temperature range at which carboxylic acids are most abundant. Experimental data on the dissolution of feldspars in the presence of organic acids show little evidence to support either enhanced dissolution kinetics or increased A1 mobility unless the pH of the experiment was unrealistically low for natural pore fluids. Increased A1 mobility due to complexing by organic acids is unlikely in natural pore waters due to the competing effects of other ions for the organic acid ligands. Simple mass-balance considerations show that source rocks would have to be unreasonably abundant to generate enough carboxylic acids to account even for a few percent secondary porosity.