June 9 (Reuters) - Exxon Mobil Corp said on Wednesday it has made a new discovery at Longtail-3 in the Stabroek Block offshore Guyana, as the U.S. oil major continues to develop one of the world's most important new oil and gas blocks in the last decade. (Reporting by Arunima Kumar in Bengaluru; Editing by Shailesh Kuber)