Hydraulic fracturing is critical to economic production of shale oil and gas from unconventional reservoirs. Success of completion is closely related to gas production. The brittleness index is widely used as a measurement of ability of hydraulic fracturing. However, the index may not be a unique and explicit indicator for estimating "fracability". In the Marcellus Shale of our study area, there is discrepancy between brittleness index calculated from mineralogy and that from elastic moduli. In addition, the brittleness index does not explain why the large majority of microseismic events were generated well above the Marcellus Shale. Two other factors appear to control fracture stimulation. One is the presence of pre-existing natural fractures. Natural fractures, measured and interpreted from the FMI log, show that fractures developed in similar patterns with high dip angle in the Marcellus and overlying Mahantango shale units, which are favorable for vertical fracture propagation. The other factor is geomechanical characterization. The Marcellus Shale has obvious presence of overpressure. According to the study on stress state using a Mohr's circle, overpressure in the Marcellus Shale increases the possibility of frictional sliding of pre-existing fractures especially at lower value of least principal stress, and will keep some fractures at certain dip angles open, although some of them are in non-active state. Unconfined compressive strength (UCS) is used to estimate rock strength in overpressure strata. The result calculated from empirical equations shows that the Marcellus Shale has significantly lower rock strength, which continues up into the Mahantango Formation, and explain vertical hydraulic fracture growth.