Main controlling factors for organic matter enrichment in Chang 7 member of the Yanchang Formation, Ordos Basin

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 subsidence 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 organisms 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. © 2019, OIL & GAS GEOLOGY Editorial Board. All right reserved.