Abstract: To address problems in studies of hydrocarbon migration and accumulation in ultra-low permeability reservoirs of terrestrial clastic rock, this paper studies sandstone oil reservoir in the lower part of Yanchang Formation in the western Ordos Basin by analyzing heterogeneity in structure, diagenetic and tightening sequence of its sedimentary architecture, thus clarifying the heterogeneity distribution pattern and crude oil charging characteristics of reservoir space. This study reveals that fluviolacustrine ultra-low permeability petroliferous reservoir exhibits strong heterogeneity; petroliferous sandstone is interbedded with non-petroliferous sandstone, and non-petroliferous sandstone can be divided into three types, i.e., calcareous sandstone, ductile grain-rich sandstone, and water-bearing sandstone. They differ dramatically in diagenetic sequence, where the formation of calcareous sandstone and ductile grain-rich sandstone are mostly related to medium-low grade architecture boundaries. In oil-bearing sandstone, three phases of bitumen can be identified, corresponding to oil charging in Late Jurassic, middle Early Cretaceous, and late Early Cretaceous strata, respectively. With such bitumen as time markers, diagenetic of oil-bearing sandstone can be divided into three phases, and correspondingly these three diagenetic phases have been identified in water-bearing sandstone. In tight sandstone with calcareous cements and ductile grain-rich rock fragments, diagenetic mainly occurs prior to the first phase of oil charging, thus forming barriers and interbeds within the reservoir, and different layers of incompletely insulated quasi-network structures; it even has controlling influences on the post fluid movement, leading to heterogeneity during telodiagenesis and hydrocarbon charging. Based on analyzing reservoir heterogeneity and sedimentary structure, diagenetic sequence and its relationship with hydrocarbon charging, and paleophysical property restoration, this paper proposes a model for oil accumulation in structurally heterogeneous ultra-low permeability sandstone.