The thick clastic reservoir in the 3rd member of the Huagang Formation is a major gas pay zone and high-qua-lity reservoir in A gas field in the Xihu Sag

East China Sea Basin

but there still exists great dispute over its sedimentary microfacies.We studied the facies markers

microfacies types

sedimentary setting

and facies distribution of the target interval through integration of core

logging

seismic and laboratory data.The results show that the study area mainly deve-lops the braided river delta-front subfacies

which mainly consists of underwater distributary channels and interdistributary microfacies without sediments and facies markers indicating marine facies and transgression.Vertically

multi-stage underwater distributary channel sand bodies superimpose

featuring large thickness and great horizontal extension.Laterally

the underwater distributary channels are characterized by tree-like bifurcation and continuous distribution from northeast to southwest

with its provenance in the northeast.It is believed that a lacustrine-delta sedimentary system of continental setting exists in the study area and is typical traction current deposits without any transgression sequence.The underwater distributary channels microfacies of high-energy traction currents is advantageous for the development of prolific gas reservoirs in the study area.