The complexity of shale formation interpretation requires an accurate evaluation of a detailed petrophysical model in association with the analysis of the geomechanical properties. Mineralogy plays an important role in controlling shale’s mechanical properties, among which one of the most problematic parameters to establish is the Biot’s coefficient. Although, this parameter is necessary to determine the magnitude of the effective stresses acting in the reservoir, it is not included in the standard protocols used in Poland. This paper presents a comprehensive petrophysical and geomechanical evaluation of the unconventional reservoirs of lower Paleozoic age formation: lower Silurian and Ordovician deposits located in the onshore part of the Baltic Basin (Poland). In this study, the Biot’s coefficient from well-log data was calculated. Initially, a calibrated rock-physics model was derived to provide a set of relationships between the elastic and petrophysical properties. Based on an accurate, calibrated petrophysical model, the effective bulk modulus along with the Biot’s coefficient and horizontal stresses were calculated. Ultimately, the tectonic regime was determined. Using full-waveform sonic data analysis, the horizontal anisotropy was estimated. The directions of maximum and minimum horizontal stress were established based on several X-tended Range Micro Imager images of breakout structures and drilling-induced fractures.