SEQUENCE STRATIGRAPHIC ANALYSIS OF THE LOWER TRIASSIC FEIXIANGUAN FORMATION IN NORTHERN SICHUAN BASIN

The Sichuan Basin is China's largest natural gas producing area.In recent years,gas exploration achieved major breakthroughs in the eastern part of the basin.A number of oil and gas reservoirs,large and medium-sized gas fields,and natural gas prospects have been discovered in Permian and Triassic successions.Oil and gas reservoirs are mainly carbonate rocks of reef and beach facies.To explore reservoir rocks distribution,we applied sequence stratigraphy in the study of the Triassic Feixianguan Formation in the northern Sichuan Basin.On the basis of outcrop,core,logging,and seismic data,we have identified two sequences and four parasequence sets in the Feixianguan Formation.We recognized six sedimentary facies:evaporative platform facies,restricted platform facies,open platform facies,platform margin beach facies,slopes facies,and basin facies.Sequence division and identification were based on exposure surfaces and upward shallowing trends.Subaerial exposure surfaces are characterized by mudcracks,freshwater dissolution structures,dolomitization,dolomite and gypsum,and high gamma ray values.Shallow upward sequences can be upward-coarsening,upward-thickening,upward-fining,upward-shalining,upward-reddening,and upward-gamma-value-lowering.Stratigraphic correlation shows that in highstand systems tracts are characterized by late platform margin oolitic facies and intra-platform oolitic facies.Extensive dolomitization of platform margin oolitic facies led to greater porosity and permeability,hence the formation of favorable reservoirs.This research has implications for further exploration of the Sichuan Basin.