Shallow braided river delta in the upper Huagang Formation in the central-northern part of Xihu Sag, East China Sea Basin

The sedimentary facies of H4 and H5 sand groups of the upper part of Huagang Formation in the central and northern Xihu Sag of the East China Sea Basin were redefined based on the analysis of core data and sand body distribution. New understanding is obtained that shallow water braided river delta was developed in the study area, showing large-scale sand bodies formed in the central and southern parts of the study area. Results suggest that: The western and eastern flanks of the study area are dominated by braided river delta plain facies, with a small area that dominated by braided river delta front facies. In the central and central-southern parts of the study area, the braided river delta plain and delta front were developed, which is mainly characterized by the inter-layer sediments of delta plain and delta front in a short interval. In the dry season, delta plain sediments developed in most of the area, and delta front sediments developed in the southern margin. In the flooding season, delta front sediments and a small amount of shallow lake sediments were developed. Under the guidance of the shallow braided river delta sedimentary model, a number of delta channel sand bodies and mouth bar sand bodies were recognized. This area is potential for oil and gas exploration, which has been confirmed by the latest exploration wells.