Abstrack Sandstones deposited in tide-dominated estuarine and deltaic environments are characterized by a distinct range of heterolithic facies, which display a hierarchical distribution of mudstone layers and gross sandstone/ mudstone content. In the subsurface, these heterolithic facies are interbedded strata of sandstone and mudstone, ranging in thickness from millimeters to centimeters. They cannot be resolved by conventional well logs. Consequently, facies discrimination in these successions is often crude and does not allow genetically significant rock types to be distinguished. This severely limits the analyses of facies sequence necessary to deliver a high-resolution model of reservoir architecture and predictions of rock properties appropriate for reservoir modeling. In this study, heterolithic facies identified in cores of a tidal-sandstone reservoir (Lower Jurassic Cook-3 reservoir, Gullfaks field) have been discriminated on conventional logs by undertaking a high-resolution electrofacies analysis. Application of multivariate statistical techniques to the conventional logs enabled probabilistic discrimination of heterolithic facies in each well, to thicknesses of only 0.25 to 0.5 m. The Lower Cretaceous Barnes High Sandstone Member, exposed along the southwestern coast of the Isle of Wight (southern England), shows a similar scale and range of heterolithic facies and is an excellent outcrop analogue for this type of tidal-sandstone reservoir. Detailed facies mapping in this outcrop illustrates complex facies relationships similar to those inferred in the Cook- 3 reservoir, but in considerably greater detail. In general, the tidal-bar succession coarsens upward. Vertical and lateral interdigitation of heterolithic facies provide a model for prediction of fluid-displacement patterns in similar tidal-sandstone reservoirs.