## 5 Depositional Systems  
Integrating multiple geological controls including tectonic activity, sediment supply, and climatic conditions, this study identifies five depositional systems within the Dainan Formation of the Gaoyou Sag through systematic analysis of core facies, log facies, and seismic facies. These depositional systems are classified as deltaic, nearshore subaqueous fans, fan deltas, slump turbidite fans, and lacustrine systems. Their development and spatial distribution are fundamentally governed by the three-dimensional constraints of basin architecture, evolutionary stages of structural deformation, and sequence framework. Detailed facies mapping reveals significant structural control on depositional patterns. (1) The southern steep slope zone, characterized by intense faulting, predominantly hosts nearshore subaqueous fans and fan delta complexes. (2) The northern gentle slope domain features well-developed normal deltaic systems, indicating stable sedimentary conditions. (3) Littoral-shallow lacustrine facies encircle the basin margins, while deep and semi-deep lacustrine environments with slump turbidite fans are concentrated in central basin depocenters. This tripartite distribution demonstrates distinct zonal differentiation controlled by syndepositional fault activity and paleogeomorphological evolution [13]. Among the diverse depositional systems, deltaic facies constitute the principal reservoir sand bodies in the Dainan Formation of the Gaoyou Sag. The delta plain subfacies is dominated by conglomeratic sandstones exhibiting prominent scouring surfaces with sporadic clasts. Well-developed upward-fining sequences transition from medium- to fine-grained sandstones to silt-fine sandstones. These sandstones display massive bedding, trough cross-bedding, and parallel lamination, frequently interbedded by brown mudstone. The delta front subfacies consists predominantly of light gray to brown fine sandstones and siltstones intercalated with gray to dark gray mudstones. Basal scouring surfaces with well-rounded mud clasts are prevalent, accompanied by well-developed cross-bedding, parallel bedding, and climbing ripples. Progressing basinward, the prodelta environment is characterized by deep-water dark mudstones with intercalated slump-generated sandy mudstones or muddy sandstones that exhibit diagnostic soft-sediment deformation structures, including ball-pillow structures and convolute beddings (Figure 3). This vertical succession records distinct hydrodynamic energy gradients ranging from proximal high-energy fluvial-dominated settings to distal low-energy basinal environments. Figure 3: Sedimentary characteristics of delta in the Dainan Formation of the Gaoyou Sag. (**a**) Delta plain, well Y24, 2841.37m; (**b**) Delta front, well Y38, 3181.85m; (**c**) Prodelta, well L24, 2374.2 m. Star symbols indicate the depth locations corresponding to the core photographs in the right pane. The slump-generated turbidite fans are predominantly distributed within the central subsag zone of the Gaoyou Sag, developing along the hanging walls of the Hanliu and Zhenwu faults. These gravity-flow deposits exhibit distinctive characteristics: coarse-grained turbiditic sandstones and conglomerates are interbedded with deep to semi-deep lacustrine dark gray mudstones, forming fining-upward rhythmic successions. Well log responses show distinctive finger-shaped or serrated bell-shaped patterns on spontaneous potential (SP) curves, with individual sand bodies measuring 2-3 m in thickness. The deposits preserve classic gravity-driven sedimentary structures, including massive bedding, parallel lamination, and graded bedding as well as syndepositional deformation features such as scour-fill structures, ball-pillow configurations, loaded casts, and slump-induced contortions. The nearshorse svil, as a diagnostic depositional system in steep slope zones of faulted lacustrine basins, is preferentially developed along the hanging wall of the southern boundary fault in the Gaoyou Sag.