Oil group Rockhopper Exploration has been awarded a 40% stake in Block 9 offshore Croatia, joining operator Eni in an area with proven hydrocarbons.After announcing last month that it had applied for acreage in licencing rounds in Croatia, the company will now take part in the first exploration phase of Block 9, expected to take three years.The first phase consists of seismic acquisition, processing and re-processing. The second phase, if Rockhopper elects to proceed, will consist of drilling of a well.Rockhopper's chief executive Sam Moody said the deal represents an "outstanding low cost opportunity" for the company."We are delighted to be partnering with the leading international producer in Croatia as we build our position in our second core area of the greater Mediterranean region following the acquisition of Mediterranean Oil and Gas [last year]," Moody said.Rockhopper is also currently awaiting the outcome of its application for acreage in the first offshore licensing round in Montenegro.The stock was up 0.8% at 65p in early deals on Monday.