Is this heavy, green, blown glass object an inkwell?

The oil lamps we're thinking of have a hole that is sized so that the wick is self-supporting, no extra hardware needed. I imagine there are some that have a small wick-holder, not now present in yours. On-line search for "blown glass oil lamp" gives results such as these:



And I guess to fill these you pull the wick out and use a small funnel.

 

This is what I was thinking with the hole. Mine would fit a thumb or more and would not grasp the wick by itself. The overlap on the top has me puzzled as well. Ty

 

Sure. I've seen ceramic or glass oil-lamps where the hole was larger, like yours, with a smaller piece, loose, to hold the wick; think something like a large bead, with the central hole a size to fit the wick firmly, and an outer diameter slightly larger than the hole. (See first example, in ceramic; second example shows a glass oil lamp with a fixture in the hole)
This arrangement lets one easily remove the bead and wick assembly for refilling, no funnel needed. You might be able to find a bead that would do this for your item, if desired.