Look I’ve read the comments and I’m a master mason with over 30 years, almost 40, of experience. First the picture isn’t good enough to make any suggestions with a clear conscience.Without a full picture I wouldn’t give you any advice except to go get the original blueprints at the abatement company in your county or town that will show the construction of it. If it’s not original then you should get a professional chimney service to look at it. The cost is minor considering what damage could be done by having no information. I’m sure you can DIY this even if it’s gas. Most of these are just facades with a galvanized double tube that was already removed when the roof was done. There are just to many variables for me to give you advice. I read how you should do several different things all of which could cause serious damage and injury. I can give you a list of my certifications that show how experienced I am. There’s nothing that’s worse than to have a collapse because you took out a structural member. I hope this helps you and at least gives you some direction. Chimney companies will charge for their time but they will also give you a lot of information and of course having the prints are great to see exactly what your home footprint is and if you’re going to need permits. That can get expensive if you change something on your home and the city hits you with a fine and you have to use a licensed contractor. In other words get information from the sources available not here. Good luck and god bless.