Summary This paper presents a method where fluid flow units are used in reservoir description. We developed the proposed method using core and well-log data from the Endicott field on the North Slope of Alaska. Sedimentary intervals of the cored wells are divided into major zones on the basis of core description information. The major zones are further subdivided into subzones to allow less variation in geologic and petrophysical properties within each subzone and more variation between the subzones. On the basis of the transmissibility, storativity, and net- to- gross-thickness data, the subzones are classified into four distinct fluid flow units by use of the statistical method of cluster analysis. We use a regression relationship established between the core and well-log data in the cored wells to estimate the permeability of the uncored wells, allowing for extension of flow units to these wells. We present stratigraphic cross sections to illustrate areal variations of the petrophysical properties in the Endicott field. Introduction Reservoir description is an important step in reservoir evaluation. A thorough description of reservoir heterogeneity leads to an accurate design of a reservoir simulation model and is essential for effective reservoir management. Reservoir heterogeneity is a predominant factor that affects oil recovery, especially during EOR production. Prediction of reservoir production performance is very sensitive to the method used in the representation of the reservoir geology in the simulation model. It is important to pay considerable attention to both the accuracy of the reservoir description and its representation in the simulation model. The prediction of reservoir performance depends, to a great extent, on the technique used to subdivide the production interval into subunits and on the characterization of these units. These units constitute the base for reservoir description. Thus, an important step in reservoir description is the definition of the subunits within the sedimentary interval. Several methods have been proposed for subdividing a sedimentary interval for reservoir description purposes. Using permeability data from a sedimentary interval, Testerman proposed a statistical reservoir "zonation technique" to identify and to describe naturally occurring zones in a reservoir. First, the interval was divided into two zones and then divided into three zones. The subdivision into additional zones continued until the zones had minimum variation in permeability internally and maximum variation between zones. Cant proposed the "slice technique" in which a sedimentary interval is subdivided into arbitrary slices, either of constant thickness or of thickness proportional to the thickness of the entire interval. Earlier, Bishop had described the slices of an arbitrary thickness within a rock body as "operationally defined units." The limitation of this technique is that the arbitrary selection of slices may cut across facies and depositional units. Cant also presented the "sequence analysis" technique in which the distinctive log patterns, such as the fining- or coarsening-upward sequences commonly observed in gamma ray logs, are recognized and correlated from well to well over a wide area. However, the application of the sequence analysis method is limited because the sequences may cut across the lithologic boundaries.