Mid-channel bars in sandy braided rivers, boasting a large scale, high connectivity, and favorable physical properties, serve as a significant type of hydrocarbon reservoirs. Complex and variable hydrodynamic conditions endow mid-channel bars with multiple types and complex internal architectures, which constrain efficient oil and gas exploitation. This study aims to explore the sedimentary evolution pattern and architectural characteristics of mid-channel bars in sandy braided rivers, with a specific focus on the influence of the sedimentary process. To this end, we conduct the dynamic simulation and process analysis of the sedimentary evolution of sandy braided rivers using a sedimentary numerical simulation method based on the real-time solution in hydrodynamic fields. The results are as follows: (1) Mid-channel bars in sandy braided rivers evolve in five stages, namely the sequential formation and continuous conversion of lozenge-shaped bars, tongue-shaped bars, unit bars, composite bars, and reworked composite bars. These bars differ significantly in planar morphology, cross-sectional structure, and scale; (2) Interactions between water currents and mid-channel bars act as the predominant mechanism governing the sedimentary evolution of sandy braided rivers. Specifically, the constant changes in the convergence and divergence characteristics and distribution styles of water currents facilitate the formation, accretion, migration, and deformation of the mid-channel bars, which are under frequent and complex superimposition and cutting. In turn, the evolutionary dynamics of the mid-channel bars further induces the above-mentioned changes in water currents; (3) Three types of accretion stemming from progradation, lateral accretion, and aggradation occur within the mid-channel bars. In the process from the formation of lozenge-shaped bars to the emergence of reworked composite bars, the accretion within mid-channel bars evolves from an initial dominance of progradation to the coexistence of progradation and lateral accretion, culminating in a combination of all three accretion types. During the transitional phase, the length and width of the mid-channel bars experience a rapid increase, followed by a slow increase, and finally stabilize. As revealed by sedimentary records, reservoirs of the mid-channel bar microfacies terminating at different evolutionary stages differ significantly in planar distribution pattern, internal architectural characteristics, and scale.

Sandy braided river deposits are widely preserved in ancient stratigraphic records and act as a significant type of hydrocarbon reservoir. Due to the frequent and rapid migration of channels within the riverbed, the sedimentary architecture is highly complex. In this paper, a flume experiment was conducted to reveal the detailed depositional process and establish a fine sedimentary architecture model for sandy braided rivers. The result showed that (1) Three types of braid channels, including the lateral migration channel, the confluence channel, and the deep incised channel, were recognized based on geometry, scale, distribution, and spatial patterns; they are interconnected, forming a complex channel network. (2) Braid channels were characterized by lateral migration, abandonment, filling, and chute cutoff. Lateral migration of channels shaped the braid bars and dominated the formation, growth, and reworking of braid bars. (3) Controlled by the fast and frequent variations of the braid channel network, braid bars were continuously formed, reworked, reshaped, and composited of multiple accretions with different types, orientations, scales, and preservation degrees. Symmetrical and asymmetrical braid bars presented significantly different composition patterns. (4) Dominated by the continuous reworking of braid channels, temporary deposits were limited preserved, braid channel deposits account for 54.3 percent of the eventually preserved braided river deposits, and four types of amalgamate patterns were recognized. Braid bars were cut and limited preserved, only accounting for 45.7 percent of the eventually preserved braided river deposits. (5) During the experiment, only 28 percent of near-surface temporary deposits were eventually preserved in fragmented forms with the final experimental braided river; the shape, spatial patterns, and most of the deposits observed during the depositional process were largely reworked and poorly preserved. (6) The scale of eventually preserved braid bars and braid channels is significantly smaller than the temporary deposits from geomorphic observations. The aspect ratio of the eventually preserved braid bars and the width-to-depth ratio of the eventually preserved braid channel are also significantly different from that of the temporary ones measured from topography data.