Isopach

http://pubs.usgs.gov/of/2003/ofr-03-037/htmltext/introduc.htm

This isopach map shows the thickness variation of a the rocks that contain shale across Illinois, Indiana.  The different colors indicate the type of stage the shale maturity is currently in so they know where to drill to extract it.  The red region is the area is very dense and concentrated and is currently ready for extraction for oil production.