Geological description of flow units in channel sandstones in a fluvial reservoir analogue (Loranca Basin, Spain)

Lithofacies heterogeneity in channel sandstones in a labyrinth-type reservoir outcrop are expressed by the nested correlation structure of experimental permeability semivariograms. First-rate correlation lengths are related to average set size or the average thickness of laterally accreted beds, second-rate correlation lengths to the average spacing of cosets or beds, and third-rate correlation lengths to the average spacing of surfaces truncating individual point bars within a meander loop. Three flow units are defined. Flow unit I comprises ribbon sandstone bodies and is characterized by large trough cross-stratified sets. Second-order bounding surfaces are dominant. Permeability is interdependent between two second-order bounding surfaces. Flow unit II comprises the lower part of meander loop sandstone bodies without grain size differences visibly expressed at the lateral accretion surfaces. Lithofacies and permeability characteristics are similar to flow unit I although the average set size and mean permeability are low. The average volume of flow unit II is largest. Flow unit III comprises the heterolithic upper part of meander loop sandstone bodies built up by ripple lamination in laterally accreted beds with distinct grain size changes. The latter form pronounced bounding surfaces and, significantly, determine two-dimensional permeability interdependency. The mean permeability is three times smaller than that of flow unit I. The spatial distribution of the flow units is mainly determined by climatic fluctuations from relatively humid to relatively arid conditions.