It is generally known that shale layers are the source rock layers of hydrocarbons. The shale gas is a kind of methane gas which is adsorbed on the surface of shale particles. The weight-volumetric method is used to estimate the geological resources of shale gas. Due to the extremely low permeability of the shale, it is represented by the nano Darcy levels （1 nD=10^-9 D=10^-6 mD）, and the adsorbed gas is in a saturated adsorption state. Therefore, the development of shale adsorbed gas requires horizontal well drilling, multi-stage fracturing, depressurization and desorption before putting into production. Since the horizontal well is an independent closed production unit for the shale gas fields, it is necessary to estimate the well-controlled recoverable reserves firstly, and then to estimate the recoverable reserves of the shale gas field. Based on the normalized production decline curve shifting method proposed by Baihly, the well-controlled recoverable reserves and the reserves of the estimation areas from the three large shale gas fields （Barnett, Woodfort, and Haynesville） in the United States have been effectively predicted using the generic exponential production decline（GEPD） model proposed by Chen Yuanqian in recent years. Furthermore, the production decline rate of shale gas wells is theoretically analyzed, and the results completely accord with the actual production decline characteristics of shale gas wells. It shows that the production of a gas well decreases rapidly in the initial stage, and gradually slows down after another 20 months of production. At the same time, five kinds of decline models are studied and estimated in this paper.