I'm making the following assumptions:
Briscoe's mission has been to move the project forward to drilling and development (drilling confirms hypothesis)
Briscoe does not want to / can't effectively be THE GUY beyond the discovery / proof phase.
The other players (BOD) have put together a plan / deal that Briscoe sees as acheiving the objective sufficiently well to allow him to give over to the people who will take it forward into the next phase(s).
More info by year end.