The terminology used in classifying petroleum substances and the various categories of reserves has been the subject of much study and discussion for many years. The need for an increased degree of standardization has been long recognized but significant difficulty has been encountered in achieving this objective. This is not surprising given the global nature of the petroleum industry and the many languages in which the industry operates and reports petroleum statistics.

A Study Group, comprised of representatives of five member countries of the World Petroleum Congress (Canada, The Netherlands, the United Kingdom, the United States and Venezuela), has reviewed the classification and nomenclature systems for oil and gas and for reserves as used by various countries and has recommended systems for universal adoption. The recommended systems are scientifically sound, and yet at the same time, practical and simple enough to be readily understood even by those not involved on a daily basis in technical aspects of the industry. The recommended terminology is as close to current common usage as possible in order to minimize the changes necessary to bring about wide acceptance.

Due to the many forms of occurrence of petroleum, the wide range of its characteristics, and the lack of accurate knowledge respecting the principles of its origin, migration and accumulation, a precise system of classifications and reserves nomenclature which will satisfy petroleum scientists is not practical. Furthermore, the establishment of a precise system would detract from its understandability by the average layman who is becoming increasingly interested and concerned with petroleum matters. The recommended systems thus stress known facts and the classification of petroleum substances is based primarily on the state of occurrence and the density and viscosity of the substances. For reserves, the recommended system is not a major change from systems which are well established in many countries but it is hoped that the recommendations will help in achieving better consistency in the reserves data presented by different authors and organizations.

The recommended systems are broad in nature and petroleum scientists will likely further sub-divide the defined categories in order to facilitate detailed use. In summary, the Study Group defines petroleum as a naturally occurring mixture of predominantly hydrocarbons in the gaseous (natural gas), liquid (crude oil), or solid (natural tar) phases. The characterization of petroleum as a crude oil or natural tar is recommended to be made on the basis of a viscosity of 10 O00 mPa s (centipo

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