Uncertainties in upfront predictions of hydraulic fracturing and gas production of underexplored shale gas targets are important as often large potential resources are deduced based on limited available data. In this paper, uncertainties are quantified by using normal distributions of different input parameters in wellbased models for hydraulic fracturing and gas production. Series of simulations are performed to determine variations in properties of hydraulic fractures and gas production due to uncertainties in geological input parameters (e.g., rock mechanical properties and reservoir permeability) and due to variation in treatment schedules (e.g., injection volumes and fluid type). Relations between stimulated reservoir volume, cumulative gas production, spacing of wells, fracturing stages along horizontal well sections, injection volumes and rock properties are derived from the simulations. The Posidonia Shale Formation in the South of the Netherlands (West Netherlands basin) is used as a demonstration field case to illustrate the approach. By addressing uncertainties in input parameters, more realistic assessment of cumulative gas production can be made. Relations between key parameters such as number of frac stages and cumulative gas production can be used to derive the optimum design of frac jobs as well as optimum placement of frac stages.