Sedimentary characteristics of transgressive fan delta of the 3rd Member of Eocene Liushagang Formation in eastern Wushi sag, Beibuwan Basin

The lithology of the 3rd Member of Liushagang Formation in eastern Wushi sag of Beibuwan Basin is relatively complex, with significant differences in sedimentary characteristics, rapid lateral changes and poor connectivity. The study of sediment sources and sedimentary facies is controversial, and the oil and gas exploration and reserves calculation have long been constrained by sedimentary facies study and its distribution regularity. In this paper, based on the core observation and fine description, combined with the logging, seismic and analytical data, the authors analyzed the sedimentary facies type and distribution characteristics of the 3rd Member of Liushagang Formation. It is concluded that the sediment sources of the 3rd Member of Liushagang Formation were from the high terrain in the north, and the transgressive fan delta was developed in this area, with larger sandbody thickness, narrow sedimentary facies, and fast sedimentary facies changes. The delta front can be further divided into four kinds of sedimentary microfacies, i.e., underwater distributary channel, channel dam, sandy sand, and shunt bay. Based on the sand-containing contours compiled by the combination of well and seismic survey, combined with the logging plane facies, seismic facies and seismic reflection structure, the sedimentary microfacies plane profile of the study area was drawn comprehensively, and the sedimentary characteristics of the 3rd Member of Liushagang Formation in the study area were determined. Finally, the fan delta deposition model in this area was established, which provides reference for oil and gas exploration and development and reserve calculation of the 3rd Member of Liushagang Formation in eastern Wushi sag, Beibuwan Basin.