Crossword Genius

Petroleum source (5)

Ross

I believe the answer is:

shale

'petroleum source' is the definition.
(I've seen this in another clue)

This is all the clue.

(Other definitions for shale that I've seen before include "Oil-bearing rock" , "Easily-split sedimentary rock" , "Soft sedimentary rock" , "Fine pieces of stone on leash" , "Easily split layered rock" .)

I've seen this clue in the Newsday.
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