Abstract Chemically enhanced oil recovery depends on the phase and interfacial properties of the crude phase and interfacial properties of the crude Oil-brine-surfactant-rock system of each reservoir. Various chemical and physical properties of crude oil components can determine the performance characteristics of crude oil-surfactant mixtures. Using a convenient computer processing technique to compare component analyses, thirty-five crude oils are separated into smaller groups with similar chemical properties. These clusters of crude oils characterize properties. These clusters of crude oils characterize different prospective targets for chemically enhanced recovery processes. The procedures used to cluster the crude oil properties are outlined, and implications for appropriate surfactant formulation design are discussed. Introduction The performance of a chemically enhanced oil recovery (EOR) process, especially a micellar or microemulsion-type surfactant flood, depends on the phase and interfacial properties of the crude oil phase and interfacial properties of the crude oil brine-surfactant rock system of each reservoir, The complexity of these interactions has forced many practical simplifications in recovery research. For practical simplifications in recovery research. For example, with surfactant-synthetic brine-hydrocarbon mixtures, recent progress has revealed several fundamental relationships. Very low interfacial tension systems have been described for refined, essentially nonpolar hydrocarbon mixtures in terms of alkane scaling rules or optimal brine salinity. Real crude oils, however, present entirely new problems. Crude oil components not found in refined problems. Crude oil components not found in refined hydrocarbon systems can determine the chemical and physical properties most important to chemically physical properties most important to chemically enhanced recovery processes. A new classification scheme for crudes, which is based on a closer examination of the types and distribution of certain crude oil components, can help account for the special differences observed in crude oil-surfactant systems. This new classification for crude oils can be used to advantage in design of appropriate chemical formulations for EOR. A considerable history of crude oil analysis already exists for hundreds of oils sampled by the API. In addition, modern refinery engineering practice commonly requires a battery of physical practice commonly requires a battery of physical tests for each crude stock run. Essentially all of these analyses are directed toward physical characterization of the crude or product mix. Distillation profiles and contaminant concentrations (e.g., sulfur, profiles and contaminant concentrations (e.g., sulfur, nitrogen and metals content) are typical data collected. Few crudes are analyzed on a fractional component level, especially with chemically enhanced recovery in mind. Although total paraffins and aromatics contents describe the whole crude, the molecular distributions of the paraffins and aromatics can influence the surfactant-stabilized emulsions formed during a surfactant flood. Both whole crude (physical) and component (chemical) analyses are needed to sufficiently describe a crude oil for chemical EOR. A convenient computer program has been developed which compares crude oils using chemical and physical analytical data. The program searches for the "nearest neighbor pair" of crudes with similar properties, and averages the analytical data for the properties, and averages the analytical data for the pair. New neighboring pairs are joined at larger pair. New neighboring pairs are joined at larger distances, and component analyses are added together, until all the crudes are connected in a loose network. Natural concentrations or clusters of similar crudes are observed within this network, which provides a new basis for classification. This technique can be evaluated experimentally by testing surfactant formulations on members of a cluster containing a crude type already known to respond to that formulation. Other clusters can be treated with modifications of existing formulations, or entirely new surfactant systems. Thus, the differences between clusters of crude oils can be used to advantage to help formulate appropriate surfactant blends for chemical EOR. EXPERIMENTAL PROCEDURE Materials Thirty-five crude oil samples were obtained from major producing regions throughout the U.S. Crude samples 2, 20, 21, 22, 26, and 31 are foreign crudes with unusual physical properties.