Oil source
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">oil source</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)