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.