Crossword Genius

Petroleum and bitumen found near the outcrop (4,3)

Ross

I believe the answer is:

rock tar

I'm a little stuck... Click here to teach me more about this clue!

'petroleum' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'bitumen found near the outcrop' is the wordplay.
I cannot quite understand how this works, but
'bitumen' could be 'tar' (tar is a kind of bitumen) and 'tar' is found in the answer.
'outcrop' could be 'rock' (synonyms) and 'rock' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).

'and' acts as a link.

Can you help me to learn more?

I've seen this clue in The Irish Independent.
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