Experimental approaches to the formation of early-diagenetic grain coats on quartz surfaces

Abstract

Grain coats can inhibit quartz cementation, thus preserving porosity in deeply buried sandstones. We focus here on the early-diagenetic formation of grain coats, such as illite coats, through adsorption of mineral colloids under electrostatically unfavourable conditions. Our experiments utilising quartz surfaces having variations in surface roughness indicate that submicron-sized surface structures enhance the efficiency of colloidal retention and thus the initial formation of grain coats. Negatively-charged latex spheres (d = 0.5 mu m, 1 mu m, 2 mu m) served as a simple analogue for natural colloidal particles. The range in surface roughness of our quartz collector surfaces is large (RMS roughness Rq from 100 nm to 10 mu m), making them appropriate analogues of natural quartz grains in sandy sediments. Our results show no direct correlation between colloidal adsorption and absolute surface roughness. Instead, the retention of colloids on rough quartz surfaces is governed by small variations in roughness. These variations reflect the distribution of submicron-sized surface features. Furthermore, results of long-term adsorption experiments reveal the lateral growth of grain coats on surfaces, following linear deposition kinetics. These results permit quantitative prediction of the size and distribution of grain coats in sandstones, as well as an evaluation of their potential impact on inhibition of quartz cementation. These variations thus offer a possible means of understanding variation in sandstone reservoir properties.