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 coarser-grained 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 detrital-clay-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.