Research on Fluvial Depositional Evolution and Sandbody Distribution Patterns of the Sha21 Submember in Jinqian 5H Well Block, Tianfu Gas Field, Sichuan Basin

Located in the central Sichuan Basin, the Tianfu Gas Field exhibits multi-storied gas-bearing features within the Shaximiao Formation. However, the exploration and development of its tight gas reservoirs remain at a preliminary stage, necessitating enhanced understanding of the formation's depositional systems and sandstone distribution patterns.Therefore, this study focuses on the Sha2¹ submember in the Jinqian 5H well block of Tianfu Gas Field as the research target. Integrating regional geological background with 3D seismic data, drilling/logging data, and laboratory analyses, using a well-seismic combined approach,we systematically investigated the planar distribution and evolutionary characteristics of the depositional system in the study area. the study provides a reference case for sequence architecture analysis and reservoir prediction in similar fluvial sedimentary regions.

Research has shown that paleosoil geochemical analysis and mineral characteristics indicate that the Sha2¹ submember was in an arid to semi-arid climate environment. Field profiles and on-site core observations show that this period was a typical fluvial sedimentary structure,the entire sequence formed in a regressive depositional setting, accompanied by periodic fluctuations of the base level. Stratigraphically, the submember can be divided into four 4th-order sequences (medium-term cycles: MSC1-MSC4), corresponding to four sandstone units (F1 through F4) respectively. During the deposition of the F1 sand unit,the base level began to drop, and the source supply was relatively stable, mainly consisting of meandering rivers or low-sinuosity channels . During this stage, the accommodation space was relatively high, the channel incision was deep but lateral migration was limited, and the sand bodies showed relatively narrow (mostly <1km), consistent continuity and clear striped distribution characteristics. The F2 sandstone unit deposition period witnessed rapid base-level drop with markedly increased sediment supply and reduced accommodation space. During this phase, channels exhibited intense lateral migration and aggradation, resulting in significantly widened fluvial sandbodies (reaching 2-3 km in width) with extensive distribution and excellent continuity. During the F3 sandstone unit depositional period, the base-level drop slowed down and even experienced short-term rises while sediment supply decreased, leading to weakened fluvial energy. Local tectonic activity caused the migration of the channel system westward, forming relatively narrow (mostly <1km) yet still continuous channel sandbodies. During the deposition of the F4 sandstone unit, the base level had dropped to its lowest position with sharply reduced accommodation space, while fluvial energy significantly weakened and sediment supply further decreased. These conditions restricted sediment transport to distal areas, forcing lateral expansion and vertical stacking, resulting in proximal concentration of sandbodies with shortened lateral extension.

The sedimentary evolution in the study area was primarily controlled by three interrelated factors: (1) phased base-level changes within a regressive background, (2) fluctuations in sediment supply, and (3) the consequent dynamic feedback mechanisms affecting fluvial energy and channel morphology.The variations in channel width and morphology, along with their planar migration patterns, comprehensively document the fluvial system's response to changes in the accommodation-to-sediment supply ratio (A/S).

Keywords: Jinqian 5H well block; Sha2¹ Submember; sedimentary characteristics; sandstone distribution patterns