Posted 12/09/2009 11:18 a.m. EST A big oil discovery could lift the largest holding in iShares MSCI Brazil ( EWZ). Petrobras ( PBR) has completed testing on the Lara oil field, and findings show that the massive field off Rio de Janeiro has the capability to produce between 3 billion and 4 billion barrels of oil. This discovery further strengthens the nation's chances of becoming a major oil exporter. The Lara oil field is just one of the offshore reserves the Brazilian firm has been focusing on. In 2007, Lara's sister field Tupi was found to hold between 5 billion and 8 billion barrels of oil. Petrobras is EWZ's top holding, making up more than 22% of the fund's total portfolio. Besides Petrobras, EWZ's performance is heavily reliant on the growth of its energy and materials industries, which together account for nearly 55% of the fund. Other top holdings include Vale ( VALE) and Gerdau ( GGB). While the outlook appears bright, I advise investors looking to include the EWZ in their portfolio to follow my lead and keep exposure small. Year to date for the period ended Dec. 8, EWZ has gained more than 117%. The fund's standout performance and strong momentum led me to include it in my ETF Action portfolio, and I remain bullish on the instrument heading into the end of the year, but with nearly a quarter of the fund dedicated to a single firm (along with other industry concentration), this fund will be susceptible to energy- and commodity-driven swings. >>See our new stock quote page.