While estimating reserves for coal, oil and natural [conventional] gas is relatively routine and standarised, resource calculations for gas in coal is neither. Because gas in coal is held in such a fundamentally different way than in conventional gas reservoirs, these variables should be derived commensurate with coal-gas properties. Thus, a methodology has been developed to determine resource size and to assign uncertainty around those estimates.The underlying tenet for assessing a resource should be to understand the uncertainty around the key components of the estimation process. For coalbed methane those components are rock volume and gas content. Rock volume needs to take into account the area, coal bed thickness and density of the organic material, while gas volume needs to assess the adsorption properties of the coal and likely saturation levels. A refined assessment also considers the level (%) of gas types such as CH_4, CO_2, C_2H_6 etc. By defining the uncertainty of each of these parameter estimates (most simply in terms of worst, base and best case, or more completely as parameter distributions), and potentially any correlation between these parameters, it is possible to undertake probabilistic analyses to quantify the uncertainty of the gas resources. Acceptable limits of uncertainty will be a reflection of individual investor's appetite for risk. Defining uncertainty allows a fairly objective means of distilling geological data into levels of probability.
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