Complex sentence: Following the opening of the Panama Canal, Royal Dutch Shell built what was the world's largest petroleum refinery in the port of Willemstad due to its naturally deep harbor, its stable government, and its relative proximity to Venezuelan oil reserves.
Split into simpler sentences: Following the opening of the Panama Canal, Royal Dutch Shell built what was at the time the world's largest petroleum refinery in the port of Willemstad. The city was selected due to its naturally deep harbor, its stable government, and its relative proximity to Venezuelan oil reserves.