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 as grain coats is good for reservoir quality in deeply buried sandstones. Being able to predict the distribution of clay-coated sand grains in petroleum reservoirs is thus important to help find and exploit anomalously good reservoir quality. Here we have adopted a high-resolution, analogue approach, using the Ravenglass Estuary marginal-shallow marine system, in NW England, U.K. Extensive geomorphic mapping, grain-size analysis, and bioturbation-intensity counts were linked to a range of scanning electron microscopy techniques to characterize the distribution and origin of clay-coated sand grains in surface sediment. Our work shows that grain coats are common in 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 in 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 might be found in fine-grained, clay-bearing inner-tidal-flat-facies sands and not in coarse, clean channel-fill and bar facies.