Borehole stability is the basis of realizing safe hydration exploitation. However, marlstonestone reservoir is prone to borehole instability to challenge drilling operations. Taking the marlstonestone reservoir of X well in southern Sichuan as the example, this paper analyzed the rocks with poroperm characteristics, acoustic characteristics, mineral composition, mechanical characteristics, thermal dispersion, thermal expansion, and the sensitivity. Through the characteristics of poroperm and acoustic, it was clear that the target layer rock was in low permeability and with developed cracks. All the rocks were mainly composed of calcite, a small amount of rock contained a lot of talcum and sepiolite, and the clay content was very low overall. The linear expansion and rolling recovery at high temperature indicated that the rock had a certain but little water absorption, and the oil-based drilling fluid had a lower effect than that of polysulfonate drilling fluid. The change of crack width under different confining pressures indicated that the rock was brittle, while the rock cracked and disconnected when the confining pressure was higher than 20 MPa. Uniaxial and triaxial compressive tests further showed that the rock was brittle and with developed cracks, high confining pressure enhanced the rock plasticity and reduced the brittleness. However, the brittleness of the rock was decreased after the soaked with polysulfonate drilling fluid, the compressive ability was greatly reduced and micro-cracks appeared, while oil-based drilling fluid had little effect on rock. The sensitivity analysis confirmed that containing large amount talcum made the rock susceptible to the water, resulting in the stress sensitivity, speed sensitivity and water sensitivity appeared in different degrees. The collapse pressure was predicted after testing the ground stress, and the borehole instability was easy to occur along the direction of the minimum horizontal stress. Combined with rock characteristics and talcum, it was clear that water had a strong effect on the talcum, leading the rock stability reduction and wellbore instability. The study of reservoir rock composition and the effect of drilling fluid on rock can guided the realization of safe drilling effect. For example, reducing the use of water-based drilling fluid and formation action time can effectively reduce the occurrence of wellbore instability in reservoirs with high talcum content.