HOUSTON — Occidental Petroleum unveiled an audacious $38 billion takeover bid for Anadarko Petroleum on Wednesday, challenging Chevron — a company four times its size — in a battle for dominance of the biggest and most productive oil field in the United States.

This is the Houston-based company’s fourth bid for Anadarko in two years, but the first since Anadarko and Chevron entered a deal this month. This newest offer could set off a bidding war for Anadarko’s oil and gas assets that range from West Texas’ giant Permian Basin to deepwater oil wells in the Gulf of Mexico to a large natural-gas export project in Mozambique.

The offer attracted immediate skepticism from Wall Street analysts as Occidental’s shares declined. But Vicki Hollub, Occidental’s chief executive, said in an interview that she was committed to the battle and urged Anadarko shareholders to back the new bid.

“I believe the Anadarko shareholders will love this offer and give feedback” to the company’s board, Ms. Hollub said. “We believe that will make a difference.”