Earlier today, Marathon Oil Corp.  (MRO) provided an interim update of its first-quarter 2009 results, with contribution from better than expected oil and natural gas production offset by lower realized commodity prices. We continue to like Marathon for its revitalized upstream business, top-tier Midwest-centered refining business, and very cheap valuation.As we mentioned in our comments on Chevron Corp's  (CVX) preannouncement yesterday, look for negative revisions to earnings estimates for the entire group in the coming days. Marathon is scheduled to report first-quarter results on April 30th.Marathon performed very well in its upstream business, with oil and natural gas production above its own guidance range. The company reported production (available for sale) of 429,000 oil-equivalent barrels per day (BOE/d), significantly above its guidance range of 400,000 to 415,000 BOE/d.This is a 7% sequential production growth, reflecting strong operating performance from the company's ...