Updated from 11:59 a.m. EDT

Unocal ( UCL) offered about $400 million in stock to buy the remaining shares of oil and natural gas producer Pure Resources ( PRS) from its stockholders. Unocal already owns 65% of the company's shares.

Unocal will offer stock worth about $22.25 share, or about 12 million new shares, to Pure Resources' holders, to complete the transaction.

Pure Resources' management and board will meet this week to consider the offer, according to a press release, and make a recommendation to shareholders by Sept. 19.

Pure Resources' second-quarter earnings, announced this month, were down significantly from last year due to weaker oil prices. The company posted $8.7 million or 17 cents a share, compared with $25.6 million, or 50 cents a share, a year ago.

But its shares soared on word of the buyout offer and closed up $6, or 34.25%, to $23.52 on the New York Stock Exchange. Unocal closed down 24 cents, or 0.70%, to $33.85.