UPDATE 1-Anadarko to offer 30 mln shrs, stock drops

* Shares down nearly 5 pct post-market

* Offering 30 mln shares

HOUSTON, May 11 (Reuters) - Anadarko Petroleum Corp (APC.N) on Monday said it plans to offer 30 million of its common shares to pay for future capital expenses, an announcement that pushed its shares down almost 5 percent in post-market trading.

Anadarko, based in Houston, said it will grant the underwriters a 30-day option to purchase up to 4.5 million additional shares of its common stock.

UBS Investment Bank and Barclays Capital are acting as joint book-running managers for the offering, the oil and gas company said.

Shares of Anadarko fell to $46.50 in extended trading, down from a New York Stock Exchange close of $48.84. (Reporting by Anna Driver in Houston; Editing Bernard Orr)