Four people familiar with the matter have told Reuters that an arbitration panel that will decide a case by Exxon Mobil (NYSE:CVX) seeking to block a merger between Chevron Corp. (NYSE:CVX) and Hess Corp. (NYSE:HES) will try to determine the real value of Hess' Guyana stake. In May, Hess shareholders approved its proposed $53B merger with Chevron despite Exxon's attempts to stop the deal. Exxon claims that Hess should have first given it the opportunity to purchase its stake in the prized Guyana asset and that Chevron structured the…