Patterns of Sedimentary Microfacies Combinations in Shallow-Water Delta Fronts and Their Reservoir Characteristic Differences: A Case Study from the Moxizhuang Area, Junggar Basin, China

Shallow-water delta front deposits represent important hydrocarbon reservoir bodies in continental lacustrine basins, and reservoir quality differentiation among varied sedimentary microfacies assemblages remains a core constraint on exploration evaluation. Taking the second member of the Jurassic Sangonghe Formation in the Moxizhuang area, central Junggar Basin, as a case study, this paper classifies sedimentary microfacies combination patterns and clarifies their genetic mechanisms of reservoir differentiation. Four patterns are identified through systematic core observation: superimposed subaqueous distributary channel (SC), truncated subaqueous distributary channel (TC), superimposed mouth bar (SMB), and upper channel and lower residual bar (CMB). Their formation conditions and reservoir features are further compared via hydrodynamic index calculation and blue epoxy-impregnated thin-section analysis. The results indicate that SC and TC patterns are widely developed in the study area, while SMB and CMB patterns are relatively limited. Hydrodynamic intensity decreases in the order of TC > CMB > SC > SMB, jointly controlled by base-level fluctuations and facies positions. SC reservoirs, dominated by rigid lithic fragments with good sorting, weak cementation, and intense dissolution, have the highest porosity and permeability with weak heterogeneity, constituting the most favorable reservoir type. TC reservoirs maintain relatively high porosity and permeability but show strong heterogeneity. SMB and CMB reservoirs, dominated by ductile lithic fragments with strong compaction and cementation, generally have poor petrophysical properties and are classified as unfavorable reservoirs. Rather than identifying entirely new microfacies architectural types, the core contribution of this study is the establishment of a quantitative genetic linkage between depositional hydrodynamic regimes, petrographic diagenesis, and reservoir quality differentiation in shallow-water delta fronts. These findings provide geological support for hydrocarbon exploration and favorable reservoir prediction in analogous lacustrine delta settings.