Ian,
I would suggest the first thing you do is take it easy, take a breath, take your time.
Now, go back and look carefully at the questions you've been asked so far. Perhaps list them out and then provide simple answers in your reply/post. Just keep in mind, you are not under any pressure at the moment and there is no need to rush. Do not let anyone make you feel like you have to act immediately.
It seems that you have known about this land for a long time because you say you signed a gas lease many years ago. Actually, I am not sure how you executed a lease as a teenager, but that is another story.
So you have a lease, so you must have some mineral rights and per your posts possibly 160 acres. Yet the County Clerk cannot find your deed. Can they find your gas/oil lease? It should be on record as well (be sure you are talking to the County Recorder). We need to understand how you acquired the acreage, but it would appear an inheritance that you received as a teen.
It also appears you are under lease to the same gas and oil company you leased with as a teen. Is there a well on your property? are you part of a drill unit? Perhaps neither of these and that is why they are sending you documents about drilling and spacing - maybe they are trying to put your land in a drilling unit. Are the documents coming from the company that you are currently leased with?
The more I type, the more of a mess this is. It is very hard to communicate in this fashion since what you have provided is not clear.
But it seems as though you inherited land when you were a kid and you leased that land. Perhaps there has been no drilling activity and now they want to drill so they are sending you information about that. Others may be getting wind of this activity (drilling) and hope to buy your land and mineral rights so they could get the royalties if a well was successful. But I am just rambling and speculating. So I am going to stop for now.... you'll have to provide more clear background about your situation.
Carefully look at questions that have been raised by Charles, MJ and myself and type a response listing our questions and your answer. That might be the best starting point.
But take your time, do not sign, do not sell, do not do anything until you CLEARLY understand what you are getting into. You say you want to sell the land, but is that because you'd get 160 x $1,200 = $192,000 - that isn't anything compared to walking away from a well with future royalties.
A lot of people here would be happy to help you. Good luck.
Wilson