The TOC content of organic-rich shales in Chang 7 member of the Yanchang Formation in Ordos Basin is abnormally high.The organic matter enrichment is essentially controlled by a combination of paleostructures

paleoclimate

paleosedimentary environments and events.We have analyzed these controlling factors and discussed their roles in the formation of organic-rich shales of the study area.The results show that the strong tectonic activity in the depositional period of Chang 7 member played a very important role in its enrichment of organic matter.Firstly

it led to a wide-scale subsi-dence of the whole basin and to an expansion of the lake water surface

producing more accommodation space.Furthermore

it induced multi-stage volcanic and submerged hydrothermal activities

and thereby delivered a large amount of nutrients to the lake.Sufficient growth space

rich nutrients supply and suitable climatic conditions have made aquatic orga-nisms flourish(the lacustrine primary productivity was extremely high)

which has provided a rich material basis for the formation of organic-rich shales.Although the oxydic-suboxydic bottom-water environment in Chang 7 sedimentary period is not ideal to the preservation of organic matter

a large number of organic matters are still preserved due to adequate influx

resulting in the enrichment of organic matter in the sediments.