A Venoco Inc. shareholder has sued the Denver oil and gas company to block Timothy Marquez’s bid to buy up outstanding shares and take the company private. Marquez, Venoco’s chairman and CEO, already owns just over half of its outstanding shares. On Friday he submitted an offer to the company of $12.50 a share for the remaining stock. Venoco (NYSE: VQ) disclosed the offer Monday and said that a special board committee is evaluating the bid.