Private property sitting on valuable resources hard to find elsewhere. This plus economic games resulted in Standard Oil with it's owner one of the richest and most powerful men in history.
> Although Standard had 90 percent of American refining capacity in 1880, by 1911 that had shrunk to between 60 and 65 percent, due to the expansion in capacity by competitors [1]
Standard Oil did become hugely rich and powerful. It did reach massive marketshare as a result of cutting prices, but I wouldn't call it a natural monopoly. I see a monopoly as having no competitors.
[1](https://en.wikipedia.org/wiki/Standard_Oil#cite_note-47)