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PETR�OLEO PETROLEUM 20 PETROLEUM INDUSTRY AND TRADE 12 HISTORIA 5 ECONOM�IA 4 HISTORY 4 PETROLEUM ENGINEERING 4 PETROLEUM LAW AND LEGISLATION 4 ASPECTOS ECON�OMICOS 3 CHEMISTRY 3 ENERGY POLICY 3 INDUSTRIA Y COMERCIO 3 LAW AND LEGISLATION 3 LEGISLACI�ON 3 POL�ITICA ENERG�ETICA 3 QU�IMICA 3 REFINING 3 BUSINESS AND ECONOMICS 2 CHINA 2 ECONOMICS 2 ENVIRONMENTAL ASPECTS 2 GENERAL 2 HYDROCARBONS 2 PETROLEUM RESERVES 2 POL�ITICA 2 PRODUCCI�ON PETROL�IFERA 2 PROSPECCI�ON 2 PROSPECTING 2 TECHNOLOGY 2 TECNOLOG�IA 2