This paper focuses on studying the upper member of the Xiaganchaigou Formation (E23) in the Late Oligocene of the Paleogene. The research objective is to deeply examine the provenance direction, sand body distribution law, and sedimentary microfacies development and evolution of the weak regressive shallow water delta in the expansion period of the E23 in the Nanbaxian area. To achieve this, a comprehensive analysis was conducted using fine core observation, drilling (logging) data, heavy mineral assemblage, ZTR index, sandstone clastic composition, grain size accumulation probability map, and sand- land ratio. The results show that: ① The provenance of the E23 sedimentary period in the Nanbaxian area mainly comes from the southern Qilian Mountains, which consist of intermediate- acid magmatic rocks and intermediate- high- grade metamorphic rocks. The provenance direction is almost northeast- southwest. The stable heavy minerals (magnetite, garnet, ilmenite, and zircon) have a high content (ranging from 81. 0% to 99. 9%), while the unstable heavy minerals (epidote and amphibole) show low content. ② The E23 sedimentary period in the Nanbaxian area is characterized by the development of a shallow delta- shallow lake sedimentary environment, primarily featuring underwater distributary channel sand bodies. The predominant lithology is lithic feldspathic sandstone, followed by feldspathic lithic sandstone. The composition maturity and sorting roundness are categorized as medium- poor. The particle size probability curve is mainly one jump and one suspension type, reflecting the influence of river action. Various sedimentary structures are present, including block bedding, normal sequence bedding, parallel bedding, and trough cross- bedding. ③ During the E23 sedimentary period, three NE- SW trending sand body enrichment zones were inherited in the Nanbaxian area (sand to ground ratio ranging from 35% to 65%). The degree of sand body development increased from the early to the middle stage, showing a progradation trend. However, the development of sand bodies weakened from the middle to the late stage, showing signs of weak retrogradation. ④ The shallow water delta front sedimentary subfacies were inherited in the Nanbaxian areaduring the E23 sedimentary period, with the continuous development of three sedimentary lobes. The underwater distributary channel microfacies are mainly developed, followed by the underwater interdistributary bay microfacies. The development scale of the shallow water delta front increases from the early to middle stage and decreases from the middle to late stage.