Grease thoroughly from a petroleum site (3,4)

I believe the answer is:

oil well

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'a petroleum site' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'grease thoroughly' is the wordplay.
'grease' becomes 'oil' (I've seen this before).
'thoroughly' becomes 'well' (I've seen this in another clue).
'oil'+'well'='OIL-WELL'

'from' acts as a link.

Can you help me to learn more?

(Other definitions for oil well that I've seen before include "What extracts riches" , "How to minimise friction" , "supporter of black economy?" , "Bore for energy source" , "not a source of 8 [RENEWABLE ENERGY]" .)

I've seen this clue in the Sydney Morning Herald.

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