From the French carte ('card') + Greek -logy ('word', now meaning study).
Cartology is often found as a typo of cartography, the study of maps. But that's not right.
Cartology is sometimes used to mean the making (and, I assume, the study) of carts. (The Greek word for cart is zeno, but apparently zenology is the study of Jupiter). But even that's not quite right.
More properly, cartology is a sort of geological map making, a method of identifying and matching up coal seams. It's a little harder than it sounds; a geologic map needs to be in 3D. Seams meander, and it's very hard to tell if one seem is connected to another. Cartologists look at surrounding rock strata, embedded fossils, and chemical composition to try to match seams.
The more common phrase for cartology is 'coal-seam correlation.'