That will depend on the soil and be a question for a local engineer to determine who was been on site. This is not internetable.

I wouldn't place a footing directly over it. There are very old houses with dug wells in their basement. It could make for a great sump!

old houses with dug wells in their basement

A renovator friend was having his basement floor lowered in his home just east of downtown Toronto. Suddenly, the jackhammer fell from his grip into a covered-over well under the floor. (Fortunately, it wasn't The Cabin in the Woods.)

Yes, done all the time. Cut the well casing to the desired depth, insert mesh to a desired level and pour concrete right over the top of that to seal.

I almost confused B T (B space T) as BT.

B space T, please elaborate what you mean by dug well. Is it a well people dug by hand large enough for a human to go down and has a bucket you use to get water? (Or make a wish throwing a coin down.) Or is it a PIPE (well casing) which was installed from drilling a well?

More details are needed from you explaining exactly what you have and even some pictures to get any reasonable answer from anyone here, otherwise all we can do is guess what you have. If your Dug well is deep and big enough to fit a body down it's very old. Old wells like this are full of bodies and treasures as seen on TV and the movies, I'm just guessing but you should check.

If it's one of those 6" dia. pipe wells, no problem. Seal it up and build.

Thanks all.

@kevin9408, in Ontario (not like a horror movie type of well) …. A dug well is defined as an excavated well to a depth of usually less than 50’. Then It’s encased at the top with a well cap, and a pvc or steel hose protruding that goes to the bottom.
Well diameter is usually around 2-3’

Ontario, oh. I guess I'll tip toe out of this discussion because I have no knowledge of Canadian rules and regulations. But good news! You can do it yourself if you like according to Ontario's water supply wells, requirements and best practices. I read the requirements and doesn't seem very complicated, but if you hire someone make sure they're licensed and retain the information. Try and locate a well tag if it has one. Have fun!