He bought all of the domestic competitors. Which is why they broke up standard oil into different oil companies. Then they all floated on the stock exchange and made him monstrously richer. Or at least he tried to. The idea was once you discover a lot of oil, its harder to control.

but then what happened was international oil companies came around. Venezuela was very rich until they ruined it. RDS was immortalized by Tony Curtis in Some Like It Hot. https://www.youtube.com/watch?v=udZgIsIKU30