Permeability is one of the most critical parameters characterizing unconventional (shale or tight) gas and oil reservoirs for resource evaluation and exploitation. Permeability is also perhaps the most difficult parameter to be accurately characterized because it is not just a single number but a complicate property with attributes that depend on many factors (namely sampling or testing scale, pore shape and size and distribution, different transport mechanisms, different test fluids, pore pressure, effective stress, and even temperature). Laboratory permeability measurements are mainly conducted on cores or smaller samples. The size and scale of laboratory measurements are hence severely limited as compared to the meters or kilometers scale of exploration or producing fields. Even in the scale of centimeters or less, for the same core samples, laboratory measurements likely yield variable permeability spanning several orders of magnitudes, leading to seemingly useless laboratory permeability for field applications. In this study, using samples from the Montney Formation in the Western Canada Sedimentary Basin as an example, we first present contrasting laboratory permeability measurements with different methods or experimental conditions. Explanations to the seemingly contradictory permeability measurements are then provided in context of different transport modes in highly heterogeneous microporous unconventional reservoir rocks, highlighting that the contrast laboratory measurements are not useless but full of information for understanding the complex characteristics of microporous unconventional rocks. Appropriate experiments to determine the appropriate permeability and their application to field problems are also discussed, which is helpful for petroleum geologists and engineers to better understand the unique permeability system of unconventional reservoirs and hence to make optimal decisions for successful unconventional resource exploitation.

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