Yesterday I posted a directed graph of US oil company mergers and spin-offs. The edges run left to right, showing the transition from the historical little players like Skelly and DX (recalling from my youth) assimilated into the giants of today. I started with a list of about 2500 transitions and way over to the right side of the graph, you can see the top dogs emerging: ExxonMobil (snippet at right), ChevronTexaco, RoyalDutchShell, and BP-Amoco. Although not yet a funnel in shape, if we get a few more mergers, the list of 2500 will transition to one (note to self: I forgot to merge Sohio with BP). Monopolistic practices work that way. Quite telling that the original Standard Oil breakup in the early 1900's barely shows up in the net. 
As this exercise served as more of a discovery process than anything else, I did notice an odd straggler in the midst of the network. You will typically find during the process of merging, that enough company tentacles (spin-offs, subsidiaries, etc.) reach out to make it virtually impossible for a large subnet to break out of the pack. Yet that did happen at least once in the layout process. And in retrospect, I find it quite understandable that the largest of the splinter networks, known as Enron Oil&Gas, did not get assimilated.
Why does this make sense? Well, just read Greg Palast or some other Enron watcher, and you find that Enron built itself off of many sham organizations, existing in name only, and then collapsed into nothingness after its descent into bankruptcy.
Take your pick: Corruption or Monopoly. In corporatist America, we don't have a heck of a lot of choices as we reach peak oil. On the backside, if we have more than one big oil company per super-power, I'll merge into a monkey's uncle.