Sedimentary facies of isolated carbonate platform of the first to third members of the Lower Triassic Feixianguan Formation in the northeastern part of the Sichuan basin and other related aspects

Abstract：In most times of the Early Triassic Feixianguanian age (from phases 1 to 3), the northeastern Sichuan basin was in an environment of isolated carbonate platforms surrounded by a deep sea trough and formed by extensional faulting during the evolution of the South Qinling Mountain ocean. Concomitantly with the sea-level change, gradual closing of the Kaijiang-Liangping sea trough and eastward migration of the Chenkou-Exi sea trough, various isolated carbonate platform subfacies moved and were transformed, and finally the isolated carbonate platforms were transformed and amalgamated into a unifying large-scale land platform in northeastern Sichuan from the late part of the third phase to the early part of the fourth phase of the Feixianguanan age. On the basis of the regional geological setting and sedimentary characteristics (lithology character, paleontology, sedimentary structure and logging), the lagoon (including evaporative lagoon, restricted lagoon, semi-restricted lagoon and point beaches), platform-margin oolitic beach, slope and sea trough subfacies. A stereoscopic depositional model of isolated platforms was constructed and the distribution and evolutionary process of sedimentary facies from phases 1 to 3 of the Feixianguanan age were analyzed. Finally the authors analyzed the migration of the platform-margin oolitic beach facies belt that had conditions for the formation of good reservoirs in the late stage, inhomogeneity and log and seismic characteristics of the reservoirs and control of sedimentary facies on the distribution of oolitic beach gas reservoirs and made seismic lateral distribution forecast for the areas in platform-margin oolitic beach facies which are expected to become new breakthrough areas of gas exploration.