estimating reserves

I have a question regarding the estimation of reserves. Before I got involved in the practical aspects of O and G business I always assumed that the size of an oil or gas field was done by measuring the thickness of the gas column in a well and then estimating the lateral extent using multiple wells in conjunction with structural analysis. However, obviously judging from the posts on this forum, people are expecting some estimate of reserves based on 1 well. Can someone that is scientifically inclined please explain how you do this? I suspect it must be done in a similar way to a transient hydrological drawdown test where you match up Theis curves etc. However, in such a case you need some pressure drop during the test...but could you really expect a transient pressure drop from a 10 Tcf field over the course of a test. I am very confused here. Would love to be enlightened on this. It's very interesting...