The origin of clay-coated sand grains and sediment heterogeneity in tidal flats

Abstract

The presence and distribution of clay minerals attached to grain surfaces as coats (also known as rims) are of great interest because they affect petroleum reservoir quality via the inhibition of the porosity-occluding quartz cement during prolonged burial and heating. Being able to predict the distribution of clay-coated sand grains in petroleum reservoirs is thus important to help find and exploit anomalously high porosity sandstones deep in sedimentary basins. The few studies focused on the distribution of clay coats in marginal marine sediments derive from surface sediment-based data sets, with limited emphasis placed on the preservation of the surface trends in sediment undergoing eodiagenesis in the near-surface environments. The post-depositional processes of bioturbation (sediment homogenization) and infiltration of clay into sand-grade sediment have been widely invoked as potential mechanisms that produce clay coats in modern sands and ancient sandstones. However, the potential for such processes to alter surface trends and govern clay-coat distribution in the subsurface remains unconstrained. In this study, we report the development of a novel, quantitative model of clay-coat coverage in order to identify the controlling mechanisms that govern clay-coat distributions. This study has focused on surface and near-surface sediments in the Saltcoats tidal flat deposits of the Ravenglass Estuary, UK. This unique bio-sedimentary study involved geomorphic mapping, core logging, a range of scanning electron microscopy techniques, and quantification of grain-size, clay fraction content, biofilm abundance (total carbohydrate and biomarker analysis), clay-coat coverage, and clay-coat mineralogy. In this study, against established convention, we demonstrate that infiltration and bioturbation do not significantly affect clay-coat grain coverage in near-surface sediments. Instead, we show that the extent, distribution, and mineralogy of clay coats in near-surface sediment are governed by the surface-based hydrological segregation of the clay mineral assemblage and biological clay-coat formation.