For distributed relational data sources in oilfield domain, we propose a semantic integration method of building virtual knowledge base using ontology mapping technology. In this method, two kinds of mappings are created and two ontologies designed specially to record these mappings, by which semantic query can be rewritten into SQL statements to be executed on data sources, and results will be returned as knowledge instances. Moreover, a knowledge exchange center is provided to cache some results to improve query efficiency. Since query results directly come from data sources rather than by instances transformation, this query-driven integration system can provide a unified, virtual knowledge base which can be served to achieve decision support for oilfield production.