Migration and accumulation of crude oil in Upper Triassic tight sand reservoirs on the southwest margin of Ordos Basin, Central China: A case study of the Honghe Oilfield

The Upper Triassic tight sand oil in the Ordos Basin has gradually become a focus of research; however, compared to the centre of the basin, studies on the margin have been relatively weak. In this study, an integrated approach involving oil-source correlation, charge history, and pressure evolution is used to investigate the migration characteristics of each stage and to establish a migration and accumulation process model of the tight sand oil in the Yanchang Formation of Honghe Oilfield in combination with reservoir densification process. The results show that, based on the fluid inclusion analysis and authigenic illite K-Ar dating technique in combination with burial and thermal modellings, the accumulation periods were 145-130, 120-90, and 90-80Ma, which corresponds to the Cretaceous. According to the relative content of C-30-diahopane and drimane series compounds, the crude oil from Chang 8(T(3)ch(8)) and Chang 9(T(3)ch(9)) both originated from the oil shale of Chang 7(T(3)ch(7)) in the Upper Triassic Yanchang Formation. The migration characteristics and directions are comprehensively analyzed using the maturity parameters and key elements. In the early stage of the Early Cretaceous, with east-dipping strata, crude oil migrated from northeast to southwest. During the late Early Cretaceous, the middle-late Yanshanian movement promoted the development of strike-slip faults and fractures, and the oil migrated vertically downwards along them. Tectonic inversion changed the strata dip direction to west-dipping, and lateral migration was also changed to two directions of southwest and northeast. After the Late Cretaceous, the migration dynamics weakened due to strata uplift and cessation of hydrocarbon generation. Only slight adjustments to the crude oil occurred during the Himalayan movement; therefore, the current distribution pattern of the tight sand oil is still affected primarily by the stage of late Early Cretaceous. The migration and accumulation model of the Upper Triassic in the Honghe Oilfield, Ordos Basin, provides a theoretical basis for further exploration and is of general applicability to tight sand reservoirs in the margins of various basins.