Anadarko Petroleum Corp. said Monday it is going back to the table to discuss a mega-merger with Occidental Petroleum Corp. (Oxy), even though it has a definitive agreement with Chevron Corp. that includes a $1 billion breakup fee.

“Anadarko is resuming its earlier negotiations with Occidental because Anadarko's board of directors, following consultation with its financial and legal advisers, has unanimously determined that the Occidental proposal could reasonably be expected to result in a ‘superior proposal’ as defined in the Chevron merger agreement,” management said.

A “superior proposal” as determined by the board triggers the start of a four-day period during which Chevron may respond to Oxy’s offer with an enhanced proposal.

“The Occidental proposal reflects significant improvement with respect to indicative value, terms and conditions, and closing certainty as compared to any previous proposal Occidental made to Anadarko.”

Chevron’s cash-and-stock agreement, estimated to be worth a total of around $50 billion, including debt, would trade $16.25/share and 0.3869/share. Oxy, whose management team has said it spent two years wooing Anadarko, made a counter offer last week estimated at around $57 billion, a 50-50 cash-and-stock transaction that would trade $38/share and 0.6094/share.

Anadarko board's determined it could resume negotiations with Oxy under the terms of the Chevron merger agreement.

“The Chevron merger agreement remains in effect and accordingly, the Anadarko board reaffirms its existing recommendation of the transaction with Chevron at this time,” the super independent noted. “There can be no assurance that negotiations with Occidental will result in a transaction that is superior to the pending transaction with Chevron.

“Further, the terms of any transaction with Occidental may vary from those reflected in the Occidental proposal.”

During a conference call Friday to discuss first quarter results, Chevron management indicated it was open to revamping the agreement.

Tudor, Pickering, Holt & Co. analysts on Monday said Chevron’s management team during the conference indicated it “feels good about their bid despite a lower price and believes it will create a superior, more financially flexible pro forma company versus its competitor.

“When asked about the potentially defensive nature of the deal, management also outlined their backlog of projects in the Chevron-only portfolio, with shale growth extending well beyond the five-year guidance,” analysts said.

“We continue to believe that even when looking through the lens of Chevron's conservative $60/bbl Brent deck, Anadarko is accretive enough to justify a higher bid, but that it is unnecessary to match Oxy's bid with several suitable substitutes available” in the exploration and production space.

Anadarko’s financial advisers are Evercore ISI and Goldman Sachs & Co. LLC, while its legal adviser is Wachtell, Lipton, Rosen & Katz.

Meanwhile, the Securities and Exchange Commission (SEC) on Monday said there was possible insider trading in Anadarko before it formally agreed to a takeover by Chevron. U.S. District Judge Gregory Woods of the Southern District of New York granted a request to freeze overseas accounts linked to purchases made between Feb. 1 and April 1.

Chevron privately proposed a merger with Anadarko on Feb. 8, according to the SEC. During the private negotiations until April 1, the SEC said about 1,650 call options on Anadarko stock were purchased in four transactions by undisclosed buyers, which could amount to about $2.5 million in fraudulent purchases.

A call option gives a buyer the right to purchase shares at a fixed price in the future, and the value of an option may increase when the stock price increases. On April 12, the day Chevron publicly announced its merger agreement, Anadarko shares increased by almost one-third.

“The timing, size, nature, and profitability of the defendants’ trades, as well as the lack of prior history of significant Anadarko options trading in the subject accounts, make the trades at issue highly suspicious,” the SEC stated.