The beach body of Lower Triassic Feixianguan Formation in northwestern Sichuan can be divided into platform marginal oolitic beach and intraplatform point beach. The water is relatively deeper at the forming of most of platformmarginal oolitic beach which is not transformed by the diagenesis of fresh water and mixed water and has poor reservoir properties. However, a part of oolitic beach formed under shallowwater environment, and it is transformed by the diagenesis of fresh water and mixed water and well dolomitized, which can form excellent reservoir. The intraplatform point beach experienced good dolomitization and has good reservoir properties, with little development and thin thickness. Beach facies is only the basic condition for reservoir development. The forming of effective reservoir in the beach body depends on the later diagenesis. The beach body is dolomitized when it exposes above the water, which is favorable for the later buried denudation and forming of good reservoir.