U.S. oil and gas major Chevron Corp has put all of its Myanmar gas block stakes up for sale, which at a combined likely valuation of $1.3 billion, would be the biggest deal involving Myanmar assets to date, financial sources familiar with the matter said. The sale, part of Chevron's efforts to preserve cash and retreat from non-core assets in the wake of sliding oil prices, is seen as setting the tone for future deals in a country that is opening up for business after historic elections last year.