The occurrence of gas confined in shales allows us to consider it as a component of the host rock. During drilling wells, the gas is released into the drilling fluid from finely ground gas-bearing rock particles. The amount of gas released can be determined on the basis of mud-gas logging; in addition, it is possible to determine the gas-content in shales expressed by the volume of gas released per mass unit of rock [m3/ton]. The gas content in the Ordovician and Silurian shales (Sasin formation and Jantar member respectively) in two selected wells in northern Poland was determined using this method. It has been found that clearly distinguishable, highly gas-bearing sections, which are separated by very poorly gas-bearing ones, can be determined in the well log. The increased gas content in shales can be observed in zones generally enriched in TOC. No direct correlation between TOC and gas-bearing capacity was found however, but the structure of TOC variability and the gas-bearing capacity described using variograms is identical. Correlations of the distinguished gas-bearing layers in the wells under consideration suggest a multi-lens or multi-layered reservoir model. The lack of natural boundaries in the shale gas reservoir means that they must be determined arbitrarily based on the assumed marginal gas-bearing capacity. In the case of several gas-bearing zones, numerous variants of interpretation are possible. In any case the low, best and high estimated resources may be evaluated, assigned to each borehole in the area with radii equal to the range of variogram of gas content in horizontal part of the well.