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.

In response to issues such as high water cuts and simultaneous gas-water production during the development of the J58 well block in the Ordos Basin, this study evaluates the influence of various reservoir factors on movable fluids based on pore-throat size classification in tight sandstone reservoirs. This helps to clarify the gas distribution pattern from a microscopic perspective. Taking 10 typical tight sandstone cores from the Shihezi Formation as examples, casting thin section observation, scanning electron microscopy (SEM), X-ray diffraction (XRD), high-pressure mercury intrusion (HPMI), and nuclear magnetic resonance (NMR) experiments were conducted. Using multifractal theory and NMR parameter-based pore-throat distribution transformation methods, the impact of reservoir parameters on the distribution of movable fluids within pore throats of different sizes was assessed. The results show that based on the shape and parameters of mercury intrusion curves, the pore structure can be divided into three types. Type Ⅰ shows a bimodal distribution of pore-throat sizes, with good physical properties and connectivity; Type Ⅱ shows an unimodal distribution dominated by medium-sized pores, with good sorting, but due to limited porethroat size, their physical properties are inferior to Type Ⅰ; Type Ⅲ have a pore-throat size distribution dominated by nanopores as the main peak and mesopores as the secondary peak, with the strongest heterogeneity in physical properties. According to the turning points in pore-throat size and fractal characteristic curves, the pore throats can be classified into mesopores (0.1~1 μm), micropores (0.01~0.1 μm), and nanopores (0.001~0.01 μm). Movable fluids are mainly found within mesopores and micropores, where the mesopores content plays a decisive role in the volume of movable fluids, while micropores, when in relatively high proportion, also have certain gas storage potential. Nanopores, however, have little impact on movable fluid distribution. The content of brittle minerals mainly affects the amount of movable fluid in mesopores, whereas clay mineral content has a negative impact on movable fluid content across all pore-throat sizes. The porosity contributed by different pore-throat sizes is positively correlated with movable fluid content; however, this correlation decreases as pore-throat size decreases due to the influence of reservoir connectivity. Permeability controls the distribution of movable fluids within pore throats of different sizes. Among pore-throat structure parameters, a higher fractal dimension negatively affects the distribution of movable fluids both overall and within porethroats of different sizes. Owing to the limitations imposed by differing contributions of pore-throat sizes to reservoir properties, the maximum mercury saturation parameter can only be used to characterize the distribution of movable fluids within mesopores.