I'd stick with the FTSE 250 plays rather than the high risk/reward tiddlers, since the downside from exploration failure is cushioned by production, and these companies are useful takeover targets for 'big oil'. Hence Tullow (TLW), SOCO (SIA), Premier (PMO) and Dana (DNX).
Back on the potential ten bagger theme I am of course keeping an eye on Global (GBP) which is currently drilling offshore Kenya. I am already exposed via DNX (30% interest in the well) but the gearing is in Global which would rocket on any good news. It's high risk, however, and both the GBP and DNX directors were recently exercising sizeable options and selling shares ahead of the drilling which did not inspire me. They've hedged their own bets (or more accurately, taken profits with the help of genuine investors and then diluting them).