Microfacies analysis of braided river deposits based on seismic-to-well ties: A case study of the Jin-77 well block in the Dongsheng gas field, Ordos Basin

This study aims to identify the sedimentary characteristics of sequences in the 1st member of the Lower Shihezi Formation (hereafter referred to as the He 1 Member) in the Jin-77 well block within the Hangjinqi area,Dongsheng gas field, Ordos Basin. To this end, we investigate the log responses and seismic reflection signatures of lithofacies in the He 1 Member using data from core observations, well logs, and seismic data. In combination with the geophysical forward modeling results of sand bodies with various architectures, we summarize the comprehensive response patterns of sedimentary microfacies, log facies, and seismic facies of the He 1 Member. Accordingly, the sequence boundaries of the Lower Shihezi Formation and its overlying and underlying strata are delineated and the characteristics of their internal sedimentary microfacies are explored. What’s more, an integration and mutual constraining of well logs and seismic data is applied to determine the planar distribution of the sedimentary microfacies of the H1-2 sublayer using geophysical attribute-based stratal slicing technology. The results indicate the He 1 Member can be divided into four sublayers based on the high-frequency sequence framework and sand body distribution of the Lower Shihezi Formation. Sand bodies in the member are concentrated in the H1-1, H1-2, and H1-3 sublayers, with each sublayer measuring 15 ~ 20 m in thickness. A comprehensive analysis of lithologies, sedimentary structures,sedimentary facies markers, vertical lithologic assemblages, and sand body distribution characteristics reveals that the He 1 Member consists primarily of wandering braided-river deposit with limited storage space, sufficient supply, and unstable channels, and its main sedimentary microfacies include mid-channel bars, channels, and floodplains. The H1-2 sublayer, as the primary target interval, contains three or four nearly SN-trending braided river channels with widths ranging from 1 to 8 km. Among these channels, those in the north are identified as favorable sites for the development of mid-channel bar microfacies.