Abstract

A major limiting factor in efforts to develop a predictive capability for the distribution of clay-coat-derived positive reservoir quality anomalies, in deeply-buried sandstones, has been the lack of a reliable and user-independent method to quantify the extent of clay-coat coverage. Clay minerals attached to grain surfaces as coats (rims) have been reported to inhibit quartz cementation during prolonged burial heating and so preserve reservoir quality deep in sedimentary basins. The completeness of clay-coat grain coverage is the principal factor that controls the effectiveness of quartz cement inhibition and the preservation of elevated primary porosity in deeply buried sandstones. Being able to quantify extent of clay-coat grain coverage is thus of paramount importance in facilitating predictive models for the distribution of clay-coat-derived enhanced reservoir quality.

This study presents one qualitative and two new quantitative methods that are capable of detailing: (i) the extent of the grain covered by attached clay material, and (ii) the volume of clay minerals attached to grain surfaces as clay coats. This study focused on the surface sediments in the Ravenglass Estuary, UK, and involved the use of a combination of scanning electron microscopy (SEM) and scanning electron microscope energy dispersive spectrometry (SEM-EDS) to characterize clay-coat coverage. Datasets produced in this study document the distribution of clay coats across the marginal marine system and allow the assessment and comparison of each technique.

The results reveal that existing qualitative classification schemes poorly resolve clay-coat variability in sand-dominated sediment typical of sand flats, tidal bars, and outer estuarine depositional environments. A key outcome is that current predictive models based on qualitative data sets for the distribution of clay coats in deeply buried sandstones potentially underestimate the distribution and grain coverage in such settings. However, the two new methods presented here, using SEM and SEM-EDS images, for the quantification of clay-coat grain coverage and the volume of grain-coating clay minerals, produce comparable quantitative spatial distribution trends with the volume (thickness) and completeness of grain coverage decreasing with distance towards the open ocean. The novel SEM and SEM-EDS clay-coat quantification techniques reported in this study are applicable to both modern and ancient sediments, and provide a method to construct a robust predictive capability for clay-coat-derived reservoir quality in ancient and deeply buried sandstones.