Due to its large thickness, homogeneity, and lack of a major tectonic deformation, the lower Paleozoic shale sequence of the Baltic Basin in northern Poland provides an excellent laboratory for the investigation of natural hydraulic fracturing associated with hydrocarbon (HC) expulsion from source rock to its overburden. The study integrates structural and geomechanical analysis and HC generation modeling to determine fractures and HC volumes, as well as pressure evolution during natural hydraulic fracturing. Structural interpretation of borehole scanner images allowed the determination of the maximum vertical extent of expulsion-related natural fractures, which varied from 60 m (196.9 ft) to more than 800 m (2625 ft), following trends in the thermal maturity of the source rock. The geomechanical model allowed the determination of the paleo-fracture gradient, which indicates a significant decrease in paleo-pressure in the shale source rock, as the vertical extent of hydraulic fractures increases. These results were incorporated into a model that compares the theoretical capacity of natural hydraulic fractures and the volumetric effects of decompression during HC expulsion. We estimated the effects of the HCs’ volume expansion, pore space reduction, and the magnitude of pore overpressure that provide the best fit between fracture capacity and the volume of gas expelled into the fractures. We demonstrate that the volumetric effects of decompression can play a significant or even dominant role in HC expulsion, when the hydraulic fracture zone is several hundred meters high.