Features of Mesozoic source rocks and oil-source correlation in southern Ordos Basin

Based on the studies of carbon isotopes and biomarkers,only one set of effective source rock was identified in Mesozoic in the southern Ordos Basin,that is,dark shale with big thickness deposited in lacustrine environment in the Upper Triassic Yanchang Formation.The source rocks mainly distributed in Qingyang,Zhengning,Zhiluo,Wuqi,Yanchi and Huanxian County.The dark shale in the seventh member of Yanchang Formation was the most favorable source rock in the study area,providing oil and gas for not only itself but also the Fuxian,Yan'an and Zhiluo Formations of Jurassic.With weak structural fluctuation and gentle stratigraphic dip angle (usually below 1°),hydrocarbon had migrated for short distance before accumulating in low-porosity and low-permeability reservoirs.Sand bodies were poorly connected and few fractures worked as vertical migration pathways.Hydrocarbon mainly accumulated in lacustrine depressions or neighboring areas.The results showed that the favorable places for prospecting in the southern Ordos Basin should be the lithologic traps close to the source rocks in Yanchang Formation.