Depocenter migration of the Ordos Basin in the late Triassic and its controls on shale distribution

The Ordos Basin has abundant conventional and unconventional oil and gas resources. Focusing on shale oil in the Ordos Basin, we studied the distribution, depositional features, and resource potential of shales in the upper Triassic Yanchang Formation based on the Ordos Basin development and depocenter migration. During the late Triassic, the Ordos Basin was a large cratonic sedimentary basin that bordered to the Hexi Corridor to the west, the southern North China block to the east, the Qilian and western Qinling orogenic zone to the south, and the foot of the Yin Mountains to the north. During deposition of the Yanchang Formation, its depocenter was not fixed. It migrated to the west before deposition of the Chang 7 oil layer and to the south after deposition of the Chang 7 oil layer. Controlled by the depocenter migration and relevant deep-lake facies, the Yanchang Formation mainly developed two sets of source rocks. The dark mudstone and shale in the Chang 9 oil layer is chiefly distributed in the south-central region of the basin, with thicknesses of 4–16 m and covers an area of approximately [Formula: see text]. The shales in the Chang 7 oil layer can be divided into two types, black oil shale and dark mudstone, and they are much thicker and more widespread than the dark mudstone in the Chang 9 oil layer. The black shale alone can be up to 60 m thick, and covers an area of more than [Formula: see text]. The shales in the Chang 7 and 9 oil layers were mainly formed in a deep-lake environment that produced high concentrations of organic matter and large hydrocarbon generation potential. According to preliminary estimates, the Chang 7 oil shale may contain [Formula: see text] of oil, thereby representing a huge resource potential with broad exploration prospectivity.