Geochemical Characteristics and Paleoenvironment Analysis of Shanxi Formation-Shiqianfeng Formation in Erdaogou Section, Southwest Margin of Ordos Basin

Significant breakthroughs have been achieved in the exploration of Upper Paleozoic oil and gas in the southwestern Ordos Basin. However, the sedimentary environment in this area is complex, and clarifying its evolutionary characteristics is of great significance for hydrocarbon exploration. Based on field geological investigation, X-ray analysis, elemental geochemical data, and regional tectonic setting, this study discusses the geochemical characteristics and paleoenvironmental evolution of clastic rocks from the Shanxi Formation to the Shiqianfeng Formation in the Erdaogou section on the southwestern margin of the basin. The results show that the Shanxi Formation experienced relatively strong chemical weathering, whereas the Shihezi and Shiqianfeng formations underwent moderate chemical weathering, with weathering intensity generally decreasing upward from the Shanxi Formation to the Shiqianfeng Formation. The Sr/Cu ratio indicates that the paleoclimate evolved from warm and humid conditions to a more arid setting: the Shanxi Formation corresponds to a warm-humid climate, the Lower Shihezi and Shiqianfeng formations to a semi-humid to semi-arid climate, and the Upper Shihezi Formation to an arid climate. Paleosalinity proxies suggest that the depositional water body was mainly freshwater to brackish, with locally elevated salinity during certain periods. Redox-sensitive element ratios indicate that the depositional environment was dominated by oxic conditions, with oxidation intensity gradually weakening upward, and the Shanxi Formation showing the strongest oxidation. In addition, paleowater depth generally shows a shallowing-upward trend. These results provide geochemical evidence for understanding sedimentary evolution and hydrocarbon exploration in the southwestern Ordos Basin.