Sedimentary characteristics of gravelly braided rivers of the Lower Shihezi Formation in the Jin 72 well area, Dongsheng gas field

Braided river sandbodies are widely distributed oil and gas reservoirs in Chinese oil and gas fields, among which the study of sandy braided rivers is relatively mature, but the understanding of the sedimentary characteristics and sedimentary models of gravelly braided rivers is obviously insufficient, which brings many challenges to the effective development of oil and gas reservoirs. In this paper, the Lower Shihezi Formation in the Jin72 well area of the Dongsheng gas field, northern Ordos Basin is taken as the research object, and the lithofacies combination, sand body distribution pattern, and sedimentary model of different types of gravelly braided river sediments are studied by comprehensive use of core logging, well logging, and seismic data. Results show that there are three different types of gravel bed braided rivers that developed in the Lower Shihezi Formation reservoir. Sedimentary microfacies mainly include channel bars, braided channels, floodplains, point bars, and overflow banks. Intermittent and shallow gravel bed braided river deposits developed on the east side of the He 1 Member, characterized by thin single lithofacies, fast vertical change, large scale of braided channels and braided bars. The sand body is in the shape of wide, continuous sheets on the plane, and the longitudinal configuration is mainly cut and stacked with good connectivity. Perennial and deep-water gravel bed braided river deposits developed on the west side of the He 1 Member, characterized by thicker individual lithofacies, slower longitudinal changes, medium-scale braided channels and braided bars. The sand bodies are in the shape of wide, continuous sheets on the plane, contacted or cut and stacked in the vertical direction, and they have good connectivity. The He 2 and He 3 members developed conglomerate wandering river deposits in weak hydrodynamic conditions, characterized by thicker fine-grained lithofacies, smallscale braided channels and braided bars, and developed a "dual structure" similar to meandering rivers. The sand bodies are in the shape of narrow strips on the plane, mainly in independent configurations in the vertical direction, and they have poor connectivity. Finally, the sedimentary models of three different types of gravelly braided river were established by combining paleogeomorphology, provenance supply and paleoclimate, etc. This study can not only enrich the sedimentary theory of coarse-grained tight sandstone reservoirs but also provide effective guidance for the development of gravel bed braided river reservoirs around the world.