Puguang gas field and other major discoveries in northeastern Sichuan Basin obtained in recent years prove the correctness of the strategy searching for large gas pools in reservoirs of reef and oolitic shoal facies according to the exploration idea of structural-lithologic combination gas pools and lithologic gas pools occurring in deep marine sequences.Theories and methodologies of sedimentology

sequence stratigraphy and seismic sedimentology are applied to systematically study the building-up process of carbonate platforms and the formation and distribution of super-thick complexes of reef and oolitic shoal facies in sequences from the Upper Permian Changxing Formation to the Lower Triassic Feixianguan Formation.It is discovered that the vertical superimposition of platform-margin reef and open platform bioclastic shoal of the Changxing Formation and the platform-margin and open platform oolitic shoal in the first and second members of Feixianguan Formation resulted in the largest and thickest carbonate platform in Sichuan Basin and China.The Changxing Formation reef was formed on continental shelf during marine transgression of second-order sequence and is represented by moundy blank reflections on seismic profile.The Feixianguan Formation platform-margin and intra-platform oolitic shoal was formed at the early HST of second-order sequence and is represented by moundy chaotic-discrete strong reflections.The Changxing Formation reef and post-reef deposits provided a high energy shallow water environment for the formation of the successive Feixianguan Formation oolitic shoal.Carbonate sequence stratigraphic research shows that the Changxing Formation and the Feixianguan Formation can be divided into 5 third-order sequences

each of which can be subdivided into 2 to 3 fourth-order sequences.Integrated study of carbonate sedimentology and sequence stratigraphy reveals the evolution of Puguang carbonate platform.High-resolution sequence stratigraphic research of the carbonates based on drilling data reveals the symmetrical high-frequency cyclic features of the dolomite reservoirs of reef and shoal facies.Reservoir porosity increases along with the decreasing of gamma ray and resistivity and the coarsening of dolomite crystalline in each cycle.The building-up process of the platform and the characteristics of reservoir porosity distribution in the high-frequency sequences provide not only the necessary theoretical bases for further exploration of carbonate sequences of reef and shoal facies

but also a guide to gas development.They constitute an important part of the integrated research of exploration and development of carbonate oil/gas pools.