Whenever you say "the [noun]" in English, you can ask "Which [noun]?" in turn. That means, objects with a definite article in front of it have to be one discrete object or one discrete portion (in case of mass nouns). But there isn't anything discrete about this sentence. It's just any arbitrary undefined amount of petrol.
But "which petrol?" is a valid question in English. Eg "which petrol should I put in the car? Unleaded or 4-star?".
Obviously that implies "which KIND of petrol?" but that usage would be perfectly natural.
Likewise, I think "the petrol in Denmark" is perfectly natural because it implies a comparison with the petrol elsewhere. Eg "we were in Germany and Denmark; the petrol in Denmark was very expensive".