With the exception of places like Costco and large truck stops, the vast overwhelming majority of gas stations are franchises where the owner is just some local guy selling under the Chevron or Shell or whatever name. This has some promotional value, of course, but the owner sets the prices just like any other station operator. Ultimately, they all buy fuel from the same local refineries or wholesalers, regardless of the name on the fuel truck that delivers it.
Standard Oil encountered some legal issues back in 1911 by using its size and vertical integration to undercut competitors at the retail level. As a result the petroleum production industry is now intentionally very much NOT involved in retail fuel sales.