(FORTUNE Magazine) – FOR SALE signs are going up all over the oil patch in what could become the biggest sell-off of oil and gas reserves in U.S. history. The sellers are major companies, such as Amoco and Chevron, which are spending heavily to explore abroad and financing that push by disposing of domestic assets that are too depleted or far-flung to manage profitably. According to some estimates, properties worth $10 billion are on the block. Lining up to buy are a lot of the small and medium-size independents bent on getting bigger, and wealthier. Investors who latch on to these up-and-comers early stand to make a bundle. Small, smart companies can make money on old properties if they focus on a niche, such as a geographical region or a unique technology. Among the buyers that look strong in the early rounds, according to Brian Lidsky of John S. Herold, a consulting firm, is Apache Corp., which doubled its reserves with a $545 million purchase from Amoco last year and is readying to buy more. He also likes Gerrity Oil & Gas, which specializes in Colorado fields and is one of the lowest-cost operators in the business. Paul Liebman, a principal at the Denver brokerage Petrie Parkman & Co., thinks that Noble Affiliates is poised to make a major acquisition. The property sales have been slow to develop because depressed gas prices earlier this year made buyers wary. But with gas prices rebounding, sales should soon pick up.