Abstract: Statistics of reserves in the main oil layers of PY4 Sag suggest that the fault-controlled reserve is nearly up to 80% of the total found until 2012. It means that the exploration of fault traps in the PY4 Sag is low in risk. In order to better study the control factors of a fault sealed reservoir of the PY4 Sag, we proposed in this paper a new concept of Composition of Shalestone Development, CSD in brief. According to the research of the CSD for the main oil layers of the PY4 Sag, it is found that when the content of shale stones thicker than 2 m is above 60%, it is excellently sealed; otherwise, when the content of shale stone thinner than 2 m is above 40%, the sealing is very poor. Based on our study, the CSD is the key factor controlling fault sealing in the PY4 Sag.