Clay coated grains can inhibit ubiquitous, porosity-occluding quartz cement in deeply buried sandstones and thus lead to anomalously high porosity. A moderate amount of clay that is distributed in sandstones as grain coats is good for reservoir quality in deeply buried sandstones. Being able to predict the distribution of clay coated sand grains within petroleum reservoirs is thus important to help find and exploit such anomalously good reservoir quality. Here we have adopted a high resolution, analogue approach, using the Ravenglass Estuary marginal-shallow marine system, in NW England, UK. Extensive geomorphic mapping, grain size analysis and bioturbation intensity counts were linked to a range of scanning electron microscopy techniques to characterise the distribution and origin of clay-coated sand grains within surface sediment. Our work shows that grain coats are common within this marginal-shallow marine system but they are heterogeneously distributed as a function of grain size, clay fraction and depositional facies. The distribution and characteristics of detrital-clay coated grains can be predicted with knowledge of specific depositional environment, clay fraction percentage and grain size. The most extensive detrital-clay coated grains are found within sediment composed of fine-grained sand containing 3.5 to 13.0 % clay fraction, associated with inner estuary tidal flat facies. Thus, against common convention, the work presented here suggests that, in deeply buried prospects, the best porosity may be found in fine-grained, clay-bearing inner tidal flat facies sands and not in coarse, clean channel fill and bar facies.

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The presence of clay-sized particles and clay minerals in modern sands and ancient sandstones has long presented an interesting problem, because primary depositional processes tend to lead to physical separation of fine-and coarse-grained materials. Numerous processes have been invoked to explain the common presence of clay minerals in sandstones, including infiltration, the codeposition of flocculated muds, and bioturbation-induced sediment mixing. How and why clay minerals form as grain coats at the site of deposition remains uncertain, despite clay-coated sand grains being of paramount importance for subsequent diagenetic sandstone properties. We have identified a new biofilm mechanism that explains clay material attachment to sand grain surfaces that leads to the production of detrital clay coats. This study focuses on a modern estuary using a combination of field work, scanning electron microscopy, petrography, biomarker analysis, and Raman spectroscopy to provide evidence of the pivotal role that biofilms play in the formation of clay-coated sand grains. This study shows that within modern marginal marine systems, clay coats primarily result from adhesive biofilms. This bio-mineral interaction potentially revolutionizes the understanding of clay-coated sand grains and offers a first step to enhanced reservoir quality prediction in ancient and deeply buried sandstones.

The spatial distribution of clay minerals and authigenic-clay-coated sand grains in ancient and deeply buried petroleum reservoirs, which can enhance or degrade reservoir quality, is poorly understood. Authigenic clay coats are reported to originate from the thermally driven recrystallization of detrital clay coats or through in situ growth from the authigenic alteration of precursor and early-diagenetic minerals during burial diagenesis. To help predict the spatial distribution of authigenic clay coats and clay minerals in estuarine sandstones, this study provides the first modern-analogue study, using the Ravenglass Estuary, UK, which integrates the distribution patterns of lithofacies, Fe-sulfide, and precursor detrital-clay-coats and clay-minerals. X-ray-diffraction-determined mineralogy and the extent of detrital clay-coat coverage of sediment in twenty-three one-meter cores was established, at an unprecedented high resolution. The output from this study shows that detrital clay mineral distribution patterns are controlled principally by the physical sorting of clay minerals by grain size. Chlorite is most abundant in coarsergrained sediment (e.g., low-amplitude dunes), whereas illite is most abundant in finer-grained sub-environments (e.g., mud flats). Kaolinite abundance is relatively homogeneous, whereas smectite abundance is negligible in the Ravenglass Estuary. This study has shown that distribution patterns of detrital-clay-coats and clay-minerals are controlled by processes active during deposition and bio-sediment interaction in the top few millimeters in the primary deposition environment. In the Ravenglass Estuary, distribution patterns of detrital-clay-coats and clay-minerals have not been overprinted by the postdepositional processes of sediment bioturbation or mechanical infiltration. Optimum detritalclay-coat coverage and clay mineralogy, which might serve as a precursor to porosity-preserving authigenic clay coats in deeply buried sandstone reservoirs, is likely to occur in low-amplitude dunes in the inner estuary and central basin. Furthermore, bioturbation in low-amplitude dunes has reduced Fe-sulfide growth due to oxidization, meaning that iron remains available for the formation of authigenic Fe-bearing clay minerals, such as chlorite, that can lead to enhanced reservoir quality in deeply buried sandstones.