Abstract:

Core observation and thin-section analysis reveal that the Ordovician weathering crust reservoir within the central uplift belt of Ordos basin is abundant in bitumen,which shows optical features of oil pyrolysis remnant——the direct evidence of the existence of ancient oil pools.The thermal maturity of bitumen is high,resulting in high bitumen reflectivities.For example,the reflectivity of bitumen from Xuntan-1 well ranges from 2.35% to 3.15%(1.85%～2.35% of vitrinite reflectance).Based on the bitumen filling and area coefficient,and well logging interpretation,the thickness of ancient oil polls is estimated to be 10～22 m.Analyses of TOC,organic geochemistry,and petrographic facies suggest that the mudstone in Pingliang Formation gradually thickens from east to west in the western margin depression of the basin,contains rich organic matters of saline organisms of lower form,and can serve as reliable source rocks for the ancient oil pools in the central uplift belt.Pool-forming history indicates that the burial depth and palaeogeotherm of Ordos basin were increased during the Middle Jurassic-Early Cretaceous time,resulting in the pyrolysis of ancient oil pools into ancient gas pools,and that the structural reversal of Ordos basin was caused by Yanshan Movement during the Later Cretaceous-Eogene time,resulting in the formation of the Shanbei slope and the destruction of the ancient oil pools.Part of oil and gas migrated and dissipated towards the shallower layers of Ordos basin,and part headed towards upper part of Shanbei slope,becoming one of the important sources for major gas fields in the central part of the basin.

Key words: bitumen, ancient oil pools, Pingliang formation shale, Ordovician, weathering crust, central uplift belt, Ordos basin