Shales account for 75% of the formations drilled worldwide, while drilling troubles associated with shale represents 90% of wellbore stability problems reported. In this paper, a traditional wellbore stability study has been implemented at an onshore gas field in a tectonically active region. Despite following the program however, the drilling experienced significant challenges, particularly in the shales where the sonic logs have shown high velocity and therefore high rock strength. To investigate the root cause, several mechanisms for the wellbore instabilities have been investigated, such as overpressure, weak bedding plane, chemical activity, insufficient mud weight, and fractured shales. The cavings collected and preserved from the wells, as big as several inches in size, have undergone XRD, SEM, thin-section, and strength tests. While most mechanisms have been ruled out, focus has turned to the fractured shales. With the evidence from the lab and collaborated with drilling experience in the field, it has found the troublesome shale formation has been extensively fractured during past and present tectonic movements. The presence of the fissure network becomes so significant that it dominates rock mechanical behavior and wellbore stability. More interestingly, it seems the sonic logs have overlooked the existence of fissures and still shown the response from a typical strong rock. Based on those findings, modified drilling strategies have been suggested and implemented in the region. The field case presents a new challenge to the log-based wellbore stability interpretation routinely implemented in current drilling society. Being open-minded, closely monitoring and testing cavings, and updating the model with drilling experience may be as important as the model development itself.