Cavalcade, an investor group based in Lubbock, Tex., proposed to buy Tesoro Petroleum for cash and securities that the group valued at between $20 and $25 a share, or $274.6 million to $343.3 million. Tesoro, based in San Antonio, is an oil and gas producer whose financial health has deteriorated recently because of the sharp decline in oil prices. The company also refines and markets petroleum products and provides oil field services. A spokesman for Tesoro said the firm had no immediate comment on the proposal. Cavalcade said it owns 3.8% of Tesoro's stock.