Sandy braided river deposits are widely observed in ancient stratigraphic records and modern sedimentary basins. Frequent channel migration shapes intricate internal architecture, while research on its influence on fine sedimentary architecture remains limited. This study utilizes DELFT3D to simulate and analyze the sedimentary processes and architecture of the channel bar in a sandy braided river. The results show that: (1) The channel bar consists of downstream accretion (DA), lateral accretion (LA), and aggradational accretion (AA). (2) Channel bars can be divided into three types: unit bars, compound bars, and complex compound bars. Unit bars are lobate-shaped and consist of multiple accretions, including DA in the core, AA layered above, and LA deposited on the sides. Compound bars result from the migration and amalgamation of multiple unit bars, characterized by a greater number of accretions. Compound bars can further be classified as DA-dominated or LA-dominated, depending on the dominant accretion type. Complex compound bars form from the merging of multiple unit bars and compound bars. In this paper, insights from a braided river on its sedimentary processes and evolution may help to improve river management and water resources research. The in-depth understanding of sedimentary architecture on channel bars can be utilized for detailed subsurface oil and gas reservoir characterization.

The Lower Shihezi Formation of the Daniudi gas field in the Ordos Basin is a typical reservoir of a braided river system in an alluvial plain, characterized by extensive braided river development, parallel development from the near source to the center of the basin, and frequent interweaving and cut stacking, as well as a complex deposition process that has seen frequent river channel changes. The braided river belt, braided channel, channel bar inside the river, and interlayer within the channel bar constitute a hierarchical and complicated architectural feature, which poses a great challenge to accurately characterize this type of reservoir for modeling. We proposed a hierarchical, level-by-level embedding, and progressive multiple-point geostatistical modeling strategy that is refined layer by layer according to a 5–3 level architectural unit hierarchy, with the modeling results of each level providing constraints for the next level modeling. The hierarchical geological model based on the combination of qualitative architectural anatomy and quantitative pre-architecture unit scale is critical in guiding the efficient development of the remaining gas in the braided river reservoir in Daniudi.

Abstract The sandy braided river depositional system developed in the Lower Shihezi Formation of the Daniudi gas field, Ordos Basin. It has the characteristics of frequent migration and oscillation of braided channels and large well spacing, making it challenging to portray the braided river sand body in this area, bringing uncertainty to the 3D geological modeling of the reservoir. This study takes the primary gas reservoir H1 member as an example. It establishes a quantitative geological knowledge database for the reservoir by statistically fitting the correlation equations between the braided channel and channel bar in the planes and profiles to reduce the uncertainty of reservoir modeling. This study combines the multisource and multiscale information from modern sedimentation, field outcrops, and tank simulation experiments of the braided river. From the data, the distribution intervals for the thickness and width of the braided channel sand body in the H1 member are 1–22 m and 7–320 m, respectively, and the thickness, width, and length distribution intervals of the sand body in the channel bar are 3–30 m, 80–1500 m, and 240–4200 m, respectively. A 3D training image is established using the object-based simulation method based on the H1 member’s well data and combining the quantitative parameters of various microfacies in the geological knowledge database. The multiple-point geostatistical modeling method is applied to establish a sedimentary microfacies model. The model’s uncertainty is reduced through multi-information fusion constraint modeling, providing a reliable basis for guiding the prediction of the remaining gas in the Daniudi gas field.