UPDATE 1-Venezuela sets minimum $500 mln premium for Carabobo

(Adds tax details)

CARACAS, July 2 (Reuters) - Venezuela will demand that oil companies pay an high premium of between $500 million and $1 billion to participate in the development of the Carabobo oil block in the OPEC nation's giant Orinoco heavy crude belt.

The premium is to be paid by consortiums that win a tender round, the results of which are expected to be announced on August 14, and worsens already tough conditions for companies hoping to build upgraders to extract the region's tar-like oil.

According to a revised document on the Oil Ministry's website, the price will depend on the location of the blocks and how difficult it is to develop them.

The last time the Oil Ministry tendered property was in 2005, when it sold off several natural gas blocks in the Venezuelan Gulf. On that occasion, the highest premium was $40 million paid by Russia's Gazprom.

The heavy premium requirement adds to financing problems that many of the companies interested in Carabobo are facing against a backdrop of relatively low oil prices and Venezuela's tough tax regime for oil companies, which includes a windfall tax when prices top a monthly $70 per barrel average.

Venezuela charges a 30 percent royalty on oil projects, although in the Orinoco belt that is sometimes dropped to 20 percent.

Some of the companies participating in the Carabobo round say they are still hopeful the ministry will soften some of its tax requirements to guarantee the profitability of the projects.

Among the firms expected to participate in the tender are Chevron (CVX.N), BP (BP.L), Statoil (STL.OL), Total (TOTF.PA), CNPC, Repsol (REP.MC) and Petrobras (PETR4.SA)

Other interested companies include Italy's Eni (ENI.MI), Mitsubushi (8058.T), Galp Energia (GALP.LS), Shell (RDSa.L) and Gazprom and Colombia's Ecopetrol ECO.CN. (Reporting by Marianna Parraga; Writing by Frank Jack Daniel; Editing by Christian Wiessner)