The Tectonic-Sedimentary Evolution of the Yan’an Formation in the Ordos Basin and Its Petroleum Geological Significance

Utilizing well logging data, outcrop profiles, and previous research, this study analyzes the sedimentary and tectonic evolution of the Yan’an Formation in the Ordos Basin, correlating the resulting sedimentary facies with hydrocarbon reservoirs to establish the necessary connections. The study reveals that: (1) Vertically, the sediment grain size shows a pattern of coarser grains at the bottom and top, with finer grains in the middle. Horizontally, the grain size tends to become finer from the northern, western, and southern parts of the basin toward the central-western region. (2) Tectonic movements during the Yan’an period controlled the sedimentary environment. These tectonic activities, through uplift and subsidence, caused the Yan’an Formation to experience four stages of sedimentary environments: braided river, lake, delta, and meandering river. (3) The Yan’an Formation exhibits four types of reservoir sandbody stacking patterns—continuous superposition, intermittent superposition, interbedded sand-mud, and single sandbody types—with continuous and intermittent stacking being the most common. (4) The hydrocarbons in the Yan’an Formation originated from the Chang 7 Member of the Yanchang Formation and migrated into the Yan’an reservoirs. The oil is characterized by its low density, low viscosity, and low pour point, indicating it is a high-maturity, high-quality crude oil.