Oil & Gas - Refining & Marketing (R&M) companies refine petroleum products, market oil and gas products, and/ or operate gas stations and convenience stores, all of which comprise the downstream operations of the oil and gas value chain. The types of refinery products and crude oil inputs influence the complexity of the refining process Thus lower prices are actually positive for oil marketing companies. OMC revenues will improve if oil price continues to fall as this results in (i) reduction in their purchase price (of crude oil) (ii) improves refining margins i.e. difference between the price of what refineries produce and the price of crude oil; (iii) improves marketing Diamond Shamrock Corp. or Diamond Shamrock Refining and Marketing was an oil refinery and gas station company in the United States, headquartered in San Antonio, Texas. Since 2001, it operates as a brand of Valero and has withdrawn the brand from a majority of Texas gas stations and had converted most of the stores to Valero or sold them off to Texaco or other companies.