Abstract:

Exploration in vast offshore survey areas faces the challenge of limited single-well data. To reveal the lateral sedimentary microfacies variations still in controversy and vertical sedimentary evolution pattern of the Huagang Formation， we focus on the Huagang Formation in block X of the central anticlinal zone in the Xihu Sag based on a comprehensive analysis of available logs and seismic data. Using the seismic sedimentological interpretation technology， which allows for analyzing the evolutionary pattern of sedimentary facies with the assistance of paleogeomorphic reconstruction， we establish a high-resolution isochronous stratigraphic framework through the wavelet transform of logs and seismic data. Furthermore， we calibrate the seismic facies types based on single-well sedimentary facies and delineate the distribution of sedimentary facies using frequency-division spectral decomposition red， green， and blue （RGB） attributes. Key findings are as follows： （1） The wavelet transform results from logs and seismic data， demonstrating high consistency， allow for the division of the Huagang Formation of the study area into eight fourth-order cycles： three in the upper member （H1—H3） and five in the lower member （H4—H8）. Consequently， a high-resolution isochronous stratigraphic framework is formed； （2） A comprehensive analysis of core facies， logging facies， and seismic facies suggests that the Huagang Formation in the study area had a sedimentary environment of braided deltas. Frequency-division seismic amplitude attributes reveal that sandstone layers of varying thicknesses exhibit the most significant seismic amplitude responses in single-frequency volumes of 15 Hz， 30 Hz， and 45 Hz. Therefore， the frequency-division RGB blends can be used to effectively delineate the boundaries of different sedimentary bodies in a planar view； （3） In the study area， the H1 and H2 cycles generally consist of delta-front sediments， the H3 to H6 cycles feature a combination of delta-plain and delta-front sediments in a shallow-water setting， and the H7 and H8 cycles comprise braided delta plain sediments. The vertical sedimentary facies evolution of these sand beds is governed by sediment supply， relative lake level， and paleogeomorphology. Based on the isochronous stratigraphic framework established through time-frequency analyses of logs and seismic data， the distributions of sandstones with different thicknesses can be characterized through RGB blends of seismic attributes selected via correlation analysis. This approach enhances the identification accuracy of sedimentary body boundaries while diminishing the influence of researchers’ subjectivity， thus providing a reference for sedimentary facies characterization of other zones with sparse well patterns.

Key words: sedimentary evolution, stratigraphic framework, seismic sedimentology, Huagang Formation, Paleogene, Xihu Sag, East China Sea Shelf Basin （ECSSB）