Abstract: The rate of penetration (ROP) of some oilfields in the western South China Sea is very slow in recent years, which restricts their exploration and development process. In order to increase the ROP and reduce the drilling costs, the mechanical properties of rock and the rock breaking mechanism should be known precisely. We identify the lithological characters of the complex formations (mostly pebble and interbed formations) by mineral composition identification device (X-ray diffractometer). Laboratory experiments of core uniaxial compressive strength (UCS), hardness, plasticity coefficient, drillability and abrasive property were done; and the prediction models of rock mechanical parameters were established. We plotted the continuous sections of rock mechanical parameters of 30 wells and three dimensional drillability planes using the log data to reveal the complex formations distribution and their rock mechanical properties. According to the complex formations breaking mechanism of the PDC bit, we studied the influence law of the tooth shape, weight on bit (WOB) and revolutions per minute (RPM) of PDC bit on penetrating efficiency of pebble and interbed formations. The results show that for the Northern Gulf Basin oilfields, the bit aggressivity should be improved and the downhole drilling motor with high rotary speed should be used; for the Pearl River Mouth Basin oilfields, the more wearlessness of the PDC bit under higher bit weight and lower rotary speed is suggested, which can reduce the drilling cost and improve the drilling efficiency.

Key words: western South China Sea, complex formation, lithology, rock mechanical properties, breaking mechanism, drilling parameter