The concept of diagnosis used in this paper has a similar meaning as it used in well test analysis, and essentially refers to the identification, and its sequence as the exploitation time increases, of the production mechanisms of a gas reservoir. This paper discusses an approach for the diagnosis of the gas reservoir production mechanism, derived from a general gas material balance equation (GGMBE), that considers the possibility that the reservoir could be naturally fractured: z/p vs (z/p)Gp. This diagnosis of the production mechanisms can be especially very useful for conditions where the reservoir characterization is limited. Once this diagnosis process has been accomplished, the original gas in place (OGIP) can be more accurately estimated through the present methodology, especially when combined with the available techniques (specific graphs, such as p/z vs Gp, Havlena and Odeh, etc.). In addition, this method allows the estimation of the cumulative effective compressibility, ce(p), of the associated water volume ratio, M, and of the water influx, We(t). The new method is illustrated through its application to the most discussed gas field behavior examples published in the literature, among them the McEwen's water influx case, the volumetric Begg's case and the overpressured Duggan's Anderson L case. Introduction During recent years, it seems that it is no longer fashionable to apply the material balance equation (MBE) to oil and gas fields, the belief being that it has now been superseded by the application of the more modern technique of numerical modeling1. Acceptance of this idea has prevented engineers of using their most powerful tool for studying reservoirs and understanding their performance rather than imposing their wills upon them, as is often the case when applying numerical simulation directly in history matching. One of the best discussions on the subject of this paper, the MBE, is that presented by Dake1. This basic model includes no geometrical considerations (geological models), hence it can be used to calculate the hydrocarbons in place and define the drive mechanism. The diagnosis of the production mechanisms or drive mechanisms in a gas reservoir sometimes may not be an easy task2–5, especially when the characterization of the system (reservoir, aquifer and associated shale [non pay] volume 6,7) is limited. The diagnosis of the production mechanisms of a reservoir has a similar meaning as used in well test analysis; essentially being for the latter the identification of the various flow regimes present during a particular well test, which is ussually accomplished through log-log graphs of the pressure response and of the pressure derivative versus elapsed time8,9. A review of the literature indicates that the diagnosis of the production mechanisms of gas reservoirs has been sarcely discussed10,11. The importance of a proper diagnosis can be made clear based on the previously referenced papers2–5, regarding gas reservoirs where the production mechanism was water influx or abnormal pressure. Again following the analogy with the well testing theory, where the next stept after the diagnosis is the application of specific graphs of analysis, once the production mechanism(s) of a gas reservoir has (have) been determined, the proper MBE can be used to estimate the important parameters of the system, like the original gas in place, G, water influx, etc. The purpose of this paper is to present based on a general gas material balance equation (GGMBE), a simple method for the diagnosis of the production mechanism(s) and material balance analysis: z/p vs (z/p)Gp. This new theory is illustrated through its application to some of the most discussed gas field behavior examples published in the literature, among them the volumetric Begg's case, the McEwen's water influx case, and the overpressure Anderson L Duggan's gas condensate reservoirs.