Oil source (4)
'oil source' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for shale that I've seen before include "Fine pieces of stone on leash" , "Layered sedimentary rock" , "Clayey rock" , "Stone that splits easily into fine pieces" , "Soft rock; heals (anag.)" .)
(Other definitions for pore that I've seen before include "Small hole" , "Where sweat appears" , "Minute passage in skin" , "Closely study" , "Minute opening in the skin" .)
(Other definitions for well that I've seen before include "Lift-shaft" , "Bore; extremely" , "Bore; very much" , "The - at the World's End (William Morris)" , "Very much; healthy" .)